[/
 / Copyright (c) 2007 Ion Gaztanaga
 /
 / Distributed under the Boost Software License, Version 1.0. (See accompanying
 / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 /]

[library Boost.Intrusive
    [quickbook 1.3]
    [authors [Krzikalla, Olaf], [Gazta&ntilde;aga, Ion]]
    [copyright 2005 Olaf Krzikalla, 2006-2007 Ion Gazta&ntilde;aga]
    [id intrusive]
    [dirname intrusive]
    [purpose Intrusive containers]
    [license
        Distributed under the Boost Software License, Version 1.0.
        (See accompanying file LICENSE_1_0.txt or copy at
        [@http://www.boost.org/LICENSE_1_0.txt])
    ]
]

[section:introduction Introduction]

[section:introduction_presenting Presenting Boost.Intrusive]

[*Boost.Intrusive] is a library presenting some intrusive containers to
the world of C++. Intrusive containers are special containers
that offer [link intrusive.performance better performance]
and exception safety guarantees than non-intrusive containers (like STL containers). 

The performance benefits of intrusive containers makes them ideal as a building
block to efficiently construct complex containers like multi-index containers or
to design high performance code like memory allocation algorithms.

While intrusive containers were and are widely used in C, they
became more and more forgotten in C++ due to the presence of the standard
containers which don't support intrusive techniques.[*Boost.Intrusive] not only
reintroduces this technique to C++, but also encapsulates the implementation in
STL-like interfaces. Hence anyone familiar with standard containers can easily use
[*Boost.Intrusive].

[endsect]

[section:introduction_building_intrusive Building Boost.Intrusive]

There is no need to compile anything to use [*Boost.Intrusive], since it's
a header only library. Just include your Boost header directory in your
compiler include path.

[endsect]

[endsect]

[section:intrusive_vs_nontrusive Intrusive and non-intrusive containers]

[section:differences_intrusive_vs_nontrusive Differences between intrusive and non-intrusive containers]

The main difference between intrusive containers and non-intrusive containers is
that in C++ non-intrusive containers store [*copies] of values passed by the user.
Containers use the `Allocator` template parameter to allocate the stored values:

[c++]

   #include <list>
   #include <assert.h>

   int main()
   {
      std::list<MyClass> myclass_list;

      MyClass myclass(...);
      myclass_list.push_back(myclass);

      //The stored object is different from the original object
      assert(&myclass != &myclass_list.front());
      return 0;
   }


To store the newly allocated copy of `myclass`, the container needs additional
data: `std::list` usually allocates nodes that contain pointers to the
next and previous node and the value itself. Something similar to:

[c++]

   //A possible implementation of a std::list<MyClass> node
   class list_node
   {
      list_node *next;
      list_node *previous;
      MyClass    value; 
   };


On the other hand, an intrusive container does not store copies of passed objects,
but it stores the objects themselves. The additional data needed to insert the object
in the container must be provided by the object itself. For example, to insert `MyClass`
in an intrusive container that implements a linked list, `MyClass` must contain the
needed ['next] and ['previous] pointers:

[c++]

   class MyClass
   {
      MyClass *next;
      MyClass *previous;
      //Other members...
   };

   int main()
   {
      acme_intrusive_list<MyClass> list;

      MyClass myclass;
      list.push_back(myclass);

      //"myclass" object is stored in the list
      assert(&myclass == &list.front());
      return 0;
   }

As we can see, knowing which additional data the class should contain is not
an easy task. [*Boost.Intrusive] offers several intrusive containers and an easy
way to make user classes compatible with those containers.

[endsect]

[section:properties_of_intrusive Properties of Boost.Intrusive containers]

Semantically, a [*Boost.Intrusive] container is similar to an STL container 
holding pointers to objects. That is, if you have an intrusive list holding
objects of type `T`, then `std::list<T*>` would allow you to do quite the
same operations (maintaining and navigating a set of objects of type T and
types derived from it). 

A non-intrusive container has some limitations:

*  An object can only belong to one container: If you want to share an object
   between two containers, you either have to store multiple copies of those
   objects or you need to use containers of pointers: `std::list<Object*>`.

*  The use of dynamic allocation to create copies of passed values can be a performance
   and size bottleneck in some applications. Normally, dynamic allocation imposes
   a size overhead for each allocation to store bookeeping information and a
   synchronization to protected concurrent allocation from different threads.

*  Only copies of objects are stored in non-intrusive containers. Hence copy
   or move constructors and copy or move assignment operators are required. Non-copyable
   and non-movable objects can't be stored in non-intrusive containers.

*  It's not possible to store a derived object in a STL-container while
   retaining its original type.

Intrusive containers have some important advantages: 

*  Operating with intrusive containers doesn't invoke any memory management at all.
   The time and size overhead associated with dynamic memory can be minimized.

*  Iterating an Intrusive container needs less memory accesses than the semantically
   equivalent container of pointers: iteration is faster.

*  Intrusive containers offer better exception guarantees than non-intrusive containers.
   In some situation intrusives containers offer a no-throw guarantee that can't be 
   achieved with non-intrusive containers.

*  The computation of an iterator to an element from a pointer or reference to that element
   is a constant time operation (computing the position of `T*` in a `std::list<T*>` has
   linear complexity).

*  Intrusive containers offer predictability when inserting and erasing objects since no
   memory managed is done with intrusive containers. Memory management usually is not a predicable
   operation so complexity guarantees from non-intrusive containers are looser than the guarantees
   offered by intrusive containers.

Intrusive containers have also downsides:

*  Each type stored in an intrusive container needs additional memory holding the 
   maintenance information needed by the container. Hence, whenever a certain type shall
   be stored in an intrusive container [*you have to change the definition of that type]
   appropriately. Although this task is easy with [*Boost.Intrusive], touching the 
   definition of a type is sometimes a crucial issue.

*  In intrusive containers you don't store a copy of an object, [*but they rather the original object
   is linked with other objects in the container]. Objects don't need copy-constructors or assignment
   operators to be stored in intrusive containers. But you have to take care of possible side effects, 
   whenever you change the contents of an object (this is especially important for 
   associative containers).

*  The user [*has to manage the lifetime of inserted objects] independently from the
   containers.

*  Again you have to be [*careful]: in contrast to STL containers [*it's easy to render an
   iterator invalid] without touching the intrusive container directly, because the object
   can be disposed before is erased from the container.

*  [*Boost.Intrusive] containers are [*non-copyable and non-assignable]. Since intrusive
   containers don't have allocation capabilities, these operations have no sense. However,
   swapping can be used to implement move-capabilities. To ease the implementation of
   copy constructors and assignment operators of classes storing [*Boost.Intrusive]
   containers, [*Boost.Intrusive] offers special cloning functions. See
   [link intrusive.clone_from Cloning [*Boost.Intrusive] containers] section for more information.

*  Analyzing thread-safety of a program that uses containers is harder with intrusive containers, becuase
   the container might be modified indirectly without an explicitly call to a container member.

[table Summay of intrusive containers advantages and disadvantages
    [[Issue]                                                                     [Intrusive]                         [Non-intrusive]]
    [[Memory management]                                                         [External]                          [Internal through allocator]]
    [[Insertion/Erasure time]                                                    [Faster]                            [Slower]]
    [[Memory locality]                                                           [Better]                            [Worse]]
    [[Can hold non-copyable and non-movable objects by value]                    [Yes]                               [No]]
    [[Exception guarantees]                                                      [Better]                            [Worse]]
    [[Computation of iterator from value]                                        [Constant]                          [Non-constant]]
    [[Insertion/erasure predictability]                                          [High]                              [Low]]
    [[Memory use]                                                                [Minimal]                           [More than minimal]]
    [[Insert objects by value retaining polymorphic behavior]                    [Yes]                               [No (slicing)]]
    [[User must modify the definition of the values to insert]                   [Yes]                               [No]]
    [[Containers are copyable]                                                   [No]                                [Yes]]
    [[Inserted object's lifetime managed by]                                     [User (more complex)]               [Container (less complex)]]
    [[Container invariants can be broken without using the container]            [Easier]                            [Harder (only with containers of pointers)]]
    [[Thread-safety analysis]                                                    [Harder]                            [Easier]]
]

For a performance comparison between Intrusive and Non-intrusive containers see
[link intrusive.performance Performance] section.

[endsect]

[endsect]

[section:usage How to use Boost.Intrusive]

If you plan to insert a class in an intrusive container, you have to make some decisions 
influencing the class definition itself. Each class that will be used in an intrusive 
container needs some appropriate data members storing the information needed by the 
container. We will take a simple intrusive container, like an intrusive list
([classref boost::intrusive::list boost::intrusive::list]) for the following
examples, but all [*Boost.Intrusive] containers are very similar. To compile
the example using [classref boost::intrusive::list boost::intrusive::list],
just include:

[c++]

   #include <boost/intrusive/list.hpp>

Every class to be inserted in an intrusive container, needs to contain a hook that
will offer the necessary data and resources to be insertable in the container.
With [*Boost.Intrusive] you just choose the hook to be a public base class or
a public member of the class to be inserted.

[section:usage_base_hook Using base hooks]

For [classref boost::intrusive::list list], you can publicly derive from 
[classref boost::intrusive::list_base_hook list_base_hook].

[c++]

   template <class ...Options>
   class list_base_hook;

The class can take several options. [*Boost.Intrusive] classes receive arguments in the
form `option_name<option_value>`. You can specify the following options:   

*  [*`tag<class Tag>`]: this argument serves as a tag, so you can derive from more than one
   [classref boost::intrusive::list_base_hook list_base_hook] and hence put an object in
   multiple intrusive lists at the same time. An incomplete type can serve as a tag.
   If you specify two base hooks, you [*must] specify a different
   tag for each one. Example: `list_base_hook< tag<tag1> >`. If no tag is specified
   a default one will be used (more on default tags later).

*  [*`link_mode<link_mode_type LinkMode>`]: The second template argument controls the
   linking policy. [*Boost.Intrusive] currently supports
   3 modes: `normal_link`, `safe_link` and `auto_unlink`. By default, `safe_link`
   mode is used. More about these in sections
   [link intrusive.safe_hook Safe hooks] and [link intrusive.auto_unlink_hooks Auto-unlink hooks].
   Example: `list_base_hook< link_mode<auto_unlink> >`

*  [*`void_pointer<class VoidPointer>`]: this option is the pointer type to be used
   internally in the hook. The default value is `void *`, which means that raw pointers
   will be used in the hook. More about this in the section titled
   [link intrusive.using_smart_pointers Using smart pointers with Boost.Intrusive containers].
   Example: `list_base_hook< void_pointer< my_smart_ptr<void> >`

For the following examples, let's forget the options and use the default values:

[c++]

   #include <boost/intrusive/list.hpp>

   using namespace boost::intrusive;

   class Foo
      //Base hook with default tag, raw pointers and safe_link mode
      :  public list_base_hook<>
   { /**/ };

After that, we can define the intrusive list:

[c++]

   template <class T, class ...Options>
   class list;

`list` receives the type to be inserted in the container (`T`) as the first parameter
and optionally, the user can specify options. We have 3 option types:

*  [*`base_hook<class Hook>`] / [*`member_hook<class T, class Hook, Hook T::* PtrToMember>`] / 
   [*`value_traits<class ValueTraits>`]: All these options specify the relationship
   between the type `T` to be inserted in the list and the hook (since we can
   have several hooks in the same `T` type). `member_hook` will be explained
   a bit later and `value_traits` will be explained in the 
   [link intrusive.value_traits Containers with custom ValueTraits] section.
   [*If no option is specified, the container will be configured to use the base
   hook with the default tag].
   Some options configured for the hook (the type of the pointers, link mode...)
   will be propagated to the container.

*  [*`constant_time_size<bool Enabled>`]: Specifies if a constant time `size()`
   function is demanded for the container. This will instruct the intrusive
   container to store an additional member to keep track of the current size of the
   container. By default, contant-time size is activated.

*  [*`size_type<bool Enabled>`]: Specifies a type that can hold
   the size of the container. This type will be the type returned by `list.size()`
   and the type stored in the intrusive container if `constant_time_size<true>`
   is requested.
   The user normally will not need to change this type, but some
   containers can have a `size_type` that might be different from `std::size_t`
   (for example, STL-like containers, use the `size_type` defined by their allocator).
   [*Boost.Intrusive] can be used to implement such containers specifying the
   the type of the size. By default the type is `std::size_t`.

Example of a constant-time size intrusive list that will store Foo objects, using
the base hook with the default tag:

[c++]

   typedef list<Foo> FooList;

Example of a intrusive list with non constant-time size that will store Foo objects:

[c++]

   typedef list<Foo, constant_time_size<false> > FooList;

Remember that the user must specify the base hook if the base hook has no default tag
(e.g: if more than one base hook is used):

[c++]

   #include <boost/intrusive/list.hpp>

   using namespace boost::intrusive;
   
   struct my_tag;

   typedef list_base_hook< tag<my_tag> > BaseHook;
   class Foo   :  public BaseHook
   { /**/ };

   typedef list< Foo, base_hook<BaseHook> > FooList;

Once the list is defined, we can use it:

[c++]

   //An object to be inserted in the list
   Foo foo_object;
   FooList list;

   list.push_back(object);

   assert(&list.front() == &foo_object);

[endsect]

[section:usage_member_hook Using member hooks]

Sometimes an 'is-a' relationship between list hooks and the list value types
is not desirable. In this case, using a member hook as a data member instead of
'disturbing' the hierarchy might be the right way: you can add a public data
member `list_member_hook<...>` to your class.
This class can be configured with the same options as `list_base_hook`
except the option `tag`:

[c++]

   template <class ...Options>
   class list_member_hook;

Example:

[c++]

   #include <boost/intrusive/list.hpp>

   class Foo 
   {
      public:
      list_member_hook<> hook_;  
      //...
   };

When member hooks are used, the `member_hook` option is used to configure the
list:

[c++]

   //This option will configure "list" to use the member hook
   typedef member_hook<Foo, list_member_hook<>, &Foo::hook_> MemberHookOption;

   //This list will use the member hook
   typedef list<Foo, MemberHookOption> FooList;

Now we can use the container:

[c++]

   //An object to be inserted in the list
   Foo foo_object;
   FooList list;

   list.push_back(object);

   assert(&list.front() == &foo_object);

[endsect]

[section:usage_both_hooks Using both hooks]

You can insert the same object in several intrusive containers at the same time, just
using one hook for each container. This is a full example using base and member hooks:

[import ../example/doc_how_to_use.cpp]
[doc_how_to_use_code]

[endsect]

[section:usage_lifetime Object lifetime]

Even if the interface of [classref boost::intrusive::list list] is similar to
`std::list`, its usage is a bit different: You always have to keep in mind that 
you directly store objects in intrusive containers, not copies. The lifetime of a 
stored object is not bound to or managed by the container:

*  When the container gets destroyed before the object, the object is not destroyed,
   so you have to be careful to avoid resource leaks.
   
*  When the object is destroyed before the container, your program is likely to crash,
   because the container contains a pointer to an non-existing object.

[endsect]

[endsect]

[section:usage_when When to use?]

Intrusive containers can be used for highly optimized algorithms, where speed is a crucial
issue and...

*  additional memory management should be avoided.
*  the programmer needs to efficiently track the construction and destruction of objects.
*  exception safety, especially the no-throw guarantee, is needed.
*  the computation of an iterator to an element from a pointer or reference
   to that element should be a constant time operation.
*  it's important to achieve a well-known worst-time system response.
*  localization of data (e.g. for cache hit optimization) leads to measureable effects.

The last point is important if you have a lot of containers over a set of elements. E.g. if 
you have a vector of objects (say, `std::vector<Object>`) and you also have a list
storing a subset of those objects (`std::list<Object*>`), then operating on an Object
from the list iterator (`std::list<Object*>::iterator`) needs two steps:

*  Access from the iterator (usually on the stack) to the list node storing a pointer to `Object`.
*  Access from the pointer to `Object` to the Object stored in the vector.

While the objects themselves are tightly packed in the memory of the vector
(vector's memory is guaranteed to be contiguous), and form something
like a data block, list nodes can stay dispersed in the heap memory.
Hence depending on your system you can get a lot of cache misses. The same doesn't hold
for an intrusive list. Indeed, dereferencing an an iterator from an intrusive list is performed in
the same two steps as described above. But the list node is already embedded in the Object, so
the memory is directly tracked from the iterator to the Object. 

It's also possible to use intrusive containers when the objects to be stored can
have different or unknown size. This allows storing base and derived objects
in the same container as shown in the following example: 

[import ../example/doc_window.cpp]
[doc_window_code]

Due to certain properties of intrusive containers 
they are often more difficult to use than their STL-counterparts. That's why you
should avoid them in public interfaces of libraries. Classes to be stored in intrusive
containers must change their implementation to store the hook and this is not always
posible or desirable.

[endsect]

[section:concepts_summary Concept summary]

Here is a small summary of the basic concepts that will be used in the following
chapters:

[variablelist Brief Concepts Summary
[[Node Algorithms][A class containing typedefs and static functions that define 
   basic operations that can be applied to a groups of nodes. It's independent
   from the node definition, and it's configured taking a NodeTraits template
   parameter that describes the node.]]
[[Node Traits][A class that stores basic information and operations to insert a node in a group of nodes.]]
[[Hook][A class that a user must add as a base class or as a member to make the user class compatible with intrusive containers.]]
[[Intrusive Container][A class that stores user classes that have the needed hooks. It takes a ValueTraits template parameter as configuration information.]]
[[Pseudo-Intrusive Container][Similar to an intrusive container but a pseudo-intrusive container needs additional memory (e.g. an auxiliary array) to work.]]
[[Value Traits][A class containing typedefs and operations to obtain the node to be used by Node Algorithms from the user class and the inverse.]]
]

[endsect]

[section:presenting_containers Presenting Boost.Intrusive containers]

[*Boost.Intrusive] offers a wide range of intrusive containers:

*  [*slist]: An intrusive singly linked list. The size overhead is very small
   for user classes (usually the size of one pointer) but many operations have linear
   time complexity, so the user must be careful if he wants to avoid performance problems.

*  [*list]: A `std::list` like intrusive linked list. The size overhead is quite
   small for user classes (usually the size of two pointers). Many operations have
   constant time complexity.

*  [*set/multiset]: A `std::set/std::multiset` like intrusive associative containers.
   The size overhead is moderate for user classes (usually the size of three pointers).
   Many operations have logarithmic time complexity.

[*Boost.Intrusive] also offers pseudo-intrusive containers:

*  [*unordered_set/unordered_multiset]: A `std::tr1::unordered_set/std::tr1::unordered_multiset`
   like intrusive unordered associative containers.
   The size overhead is moderate for user classes (an average of two pointers per element).
   Many operations have an amortized constant time complexity.

Each of these intrusive containers can be configured with constant or linear time
size:

*  [*Linear time size]: The intrusive container doesn't hold a size member that it's
updated with every insertion/erasure. This implies that the `size()` function has not constant
time complexity. On the other hand, the container is smaller, and some operations, like
`splice()` taking a range of iterators in linked lists have constant time complexity
instead of linear complexity.

*  [*Constant time size]: The intrusive container holds a size member that it's updated
with every insertion/erasure. This implies that the `size()` function has constant time
complexity. On the other hand, increases the size of the container, and some operations,
like `splice()` taking a range of iterators, have linear time complexity in linked lists.

To make user classes compatible with these intrusive containers [*Boost.Intrusive]
offers two types of hooks for each container type:

*  [*Base hook]: The hook is stored as a public base class of the user class.

*  [*Member hook]: The hook is stored as a public member of the user class.

Apart from that, [*Boost.Intrusive] offers additional features:

*  [*Safe mode hooks]: Hook constructor initializes the internal data to a well-known
   safe state and intrusive containers check that state before inserting a value in the
   container. When erasing an element from the container, the container puts the hook
   in the safe state again. This allows a safer use mode and it can be used to detect
   programming errors. It implies an slight performance overhead in some operations
   and can convert some constant time operations in linear time operations.

*  [*Auto-unlink hooks]: The hook destructor removes the object from the container
   automatically and the user can safely unlink the object from the container without
   having any reference to the container.

*  [*Non-raw pointers]: If the user wants to use smart pointers instead of raw pointers, 
   [*Boost.Intrusive] hooks can
   be configured to use any type of pointers. This configuration information is also
   transmitted to the containers, so all the internal pointers used by intrusive containers
   configured with these hooks will be smart pointers. As an example,
   [*Boost.Interprocess] defines an smart pointer compatible with shared memory,
   called `offset_ptr`. [*Boost.Intrusive] can be configured to use this smart pointer
   to allow shared memory intrusive containers.

[endsect]

[section:safe_hook Safe hooks]

[section:features Features of the safe mode]

[*Boost.Intrusive] hooks can be configured to operate in safe-link mode.
The safe mode is activated by default, but it can be also explicitly activated:

[c++]

   //Configuring explicity the safe mode
   class Foo : public list_base_hook< link_mode<safe_link> >
   {};

Thanks to the safe-mode the user can detect without any external reference, if the object
is actually inserted in a container. Let's review the basic features of the safe-mode:

*  Hooks' constructor puts the hook in a well-known default state.

*  Hooks' destructor checks if the hook is in the well-known default state. If not,
   an assertion is raised.

*  Every time an object is being inserted in the intrusive container, the container
   checks if the hook is in the well-known default state. If not,
   an assertion is raised.

*  Every time an object is being erased from the intrusive container, the container
   puts the erased object in the well-known default state.

With these features, without any external reference the user can know if the object
has been inserted in a container calling the `is_linked()` member function.
If the object is not actually inserted 
in a container, the hook is in the default state and if it's inserted in a container, the
hook is not in the default state.

[endsect]

[section:configuring Configuring safe-mode assertions]

By default, all safe-mode assertions raised by [*Boost-Intrusive] hooks
and containers in are implemented using `BOOST_ASSERT`, which can be configured by
the user. See [@http://www.boost.org/libs/utility/assert.html] for more
information about `BOOST_ASSERT`.

`BOOST_ASSERT` is globally configured, so the user might
want to redefine intrusive safe-mode assertions without modifying the global
`BOOST_ASSERT`. This can be achieved redefining the following macros:

*  `BOOST_INTRUSIVE_SAFE_HOOK_DEFAULT_ASSERT`: This assertion will be
   used in insertion functions of the intrusive containers to check that
   the hook of the value to be inserted is default constructed.
*  `BOOST_INTRUSIVE_SAFE_HOOK_DESTRUCTOR_ASSERT`: This assertion will be
   used in hooks' destructors to check that the hook is in a default state.

If any of these macros is not redefined, the assertion will be defaul to `BOOST_ASSERT`.

[endsect]

[endsect]

[section:auto_unlink_hooks Auto-unlink hooks]

[section:auto_unlink_hooks_what What's an auto-unlink hook?]

[*Boost.Intrusive] offers additional hooks with unique features:

*  When the destructor of the hook is called, the hook checks if the node is inserted
   in a container. If so, the hook removes the node from the container.
*  The hook has a member function called `unlink()` that can be used to unlink the
   node from the container at any moment, without having any reference to the container,
   if the user want to do so.

These hooks have exactly the same size overhead as their analogue non auto-unlinking
hooks, but they have a restriction: they can only be used with
[link intrusive.presenting_containers non-constant time containers].
There is a reason for this: 

* Auto-unlink hooks don't store any reference to the container where they are inserted.
* Only containers with non constant-time `size()` allow removing an object from the container
  without using any reference to the container. 

This auto-unlink feature is useful in certain applications
but it must be used [*very carefuly]:

*  If several threads are using the same container the destructor of the auto-unlink
   hook will be called without any thread synchronization so removing the object is
   thread-unsafe.

*  Container contents change silently without modifying the container directly.
   This can lead to surprising effects.

These auto-unlink hooks have also safe-mode properties:

*  Hooks' constructors put the hook in a well-known default state.

*  Every time an object is being inserted in the intrusive container, the container
   checks if the hook is the well-known default state. If not,
   an assertion is raised.

*  Every time an object is being erased from the intrusive container, the container
   puts the erased object in the well-known default state.

[endsect]

[section:auto_unlink_hooks_example Auto-unlink hook example]

Let's see an example of an auto-unlink hook:

[import ../example/doc_auto_unlink.cpp]
[doc_auto_unlink_code]

[endsect]

[section:auto_unlink_and_constant_time Auto-unlink hooks and containers with constant-time `size()`]

As explained, [*Boost.Intrusive] auto-unlink hooks are incompatible with containers
that have constant-time `size()`, so if you try to define such container with an
auto-unlink hook's value_traits, you will get an static assertion:

[c++]
   
   #include <boost/intrusive/list.hpp>

   using boost::intrusive;

   struct MyTag;

   class MyClass : public list_base_hook< link_mode<auto_unlink> >
   {/**/};

   list <MyClass, constant_time_size<true> > bad_list;

   int main()
   {
      bad_list list;
      return 0;
   }

leads to an error similar to:

[pre
  error : use of undefined type 'boost::STATIC_ASSERTION_FAILURE<false>'
]

Pointing to code like this:

[c++]

   //Constant-time size is incompatible with auto-unlink hooks!
   BOOST_STATIC_ASSERT(!(constant_time_size && ((int)value_traits::link_mode == (int)auto_unlink)));

This way, there is no way to compile a program if you try to use auto-unlink hooks
in constant-time size containers.

[endsect]

[endsect]

[section:slist Intrusive singly linked list: slist]

[classref boost::intrusive::slist slist] is the simplest intrusive container of
[*Boost.Intrusive]: a singly linked list. The memory overhead
that imposes is 1 pointer per node. The size of an empty, non constant-time size 
[classref boost::intrusive::slist slist], is the size of 1 pointer. This
lightweight memory overhead comes with its drawbacks, though: many operations have
linear time complexity, even some that usually are constant time, like
[classref boost::intrusive::slist::swap swap]. [classref boost::intrusive::slist slist]
only provides forward iterators.

For most cases, a doubly linked list is preferrable because it offers more
constant-time functions with a slightly bigger overhead.
However, for some applications like
constructing more elaborated containers, singly linked lists are essential
because of their low size overhead.

[section:slist_hooks slist hooks]

Like the rest of [*Boost.Intrusive] containers,
[classref boost::intrusive::slist slist] has two hook types:

[c++]

   template <class ...Options>
   class slist_base_hook;

*  [classref boost::intrusive::slist_base_hook slist_base_hook]:
   the user class derives publicly from
   [classref boost::intrusive::slist_base_hook slist_base_hook] to make
   it [classref boost::intrusive::slist slist]-compatible.

[c++]

   template <class ...Options>
   class slist_member_hook;

*  [classref boost::intrusive::slist_member_hook slist_member_hook]:
   the user class contains a public 
   [classref boost::intrusive::slist_member_hook slist_member_hook] to make
   it [classref boost::intrusive::slist slist]-compatible.

[classref boost::intrusive::slist_base_hook slist_base_hook] and
[classref boost::intrusive::slist_member_hook slist_member_hook] receive the same options explained in
the section [link intrusive.usage How to use Boost.Intrusive]:

*  [*`tag<class Tag>`] (for base hooks only): This argument serves as a tag,
   so you can derive from more than one slist hook.
   Default: `tag<default_tag>`.

*  [*`link_mode<link_mode_type LinkMode>`]: The linking policy.
   Default: `link_mode<safe_link>`.

*  [*`void_pointer<class VoidPointer>`]: The pointer type to be used
   internally in the hook and propagated to the container.
   Default: `void_pointer<void*>`.

[endsect]

[section:slist_container slist container]

[c++]

   template <class T, class ...Options>
   class slist;

[classref boost::intrusive::slist slist] receives the same options explained in
the section [link intrusive.usage How to use Boost.Intrusive]:

*  [*`base_hook<class Hook>`] / [*`member_hook<class T, class Hook, Hook T::* PtrToMember>`] / 
   [*`value_traits<class ValueTraits>`]: To specify the hook type or value traits used
   to configure the container (to know about value traits go to the section
   titled [link intrusive.value_traits Containers with custom ValueTraits].

*  [*`constant_time_size<bool Enabled>`]: To activate the constant-time `size()` operation.
   Default: `constant_time_size<true>`

*  [*`size_type<bool Enabled>`]: To specify the type that will be used to store the size
   of the container. Default: `size_type<std::size_t>`

[endsect]

[section:slist_example Example]

Now let's see an small example using both hooks:

[import ../example/doc_slist.cpp]
[doc_slist_code]

[endsect]

[endsect]

[section:list Intrusive doubly linked list: list]

[classref boost::intrusive::list list] is a doubly linked list. The memory overhead
that imposes is 2 pointers per node. An empty, non constant-time size [classref boost::intrusive::list list]
has also the size of 2 pointers. [classref boost::intrusive::list list]
has many more constant-time operations than [classref boost::intrusive::slist slist]
and provides bidirectional iterator. It's recommendable to use use
[classref boost::intrusive::list list] instead of
[classref boost::intrusive::slist slist] if the size overhead is acceptable:

[section:list_hooks list hooks]

Like the rest of [*Boost.Intrusive] containers,
[classref boost::intrusive::list list] has two hook types:

[c++]

   template <class ...Options>
   class list_base_hook;

*  [classref boost::intrusive::list_base_hook list_base_hook]: the user class
   derives publicly from [classref boost::intrusive::list_base_hook list_base_hook]
   to make it [classref boost::intrusive::list list]-compatible.

[c++]

   template <class ...Options>
   class list_member_hook;

*  [classref boost::intrusive::list_member_hook list_member_hook]: 
   the user class contains a public 
   [classref boost::intrusive::list_member_hook list_member_hook] to make
   it [classref boost::intrusive::list list]-compatible.

[classref boost::intrusive::list_base_hook list_base_hook] and
[classref boost::intrusive::list_member_hook list_member_hook] receive
the same options explained in the section
[link intrusive.usage How to use Boost.Intrusive]:

*  [*`tag<class Tag>`] (for base hooks only): This argument serves as a tag,
   so you can derive from more than one list hook.
   Default: `tag<default_tag>`.

*  [*`link_mode<link_mode_type LinkMode>`]: The linking policy.
   Default: `link_mode<safe_link>`.

*  [*`void_pointer<class VoidPointer>`]: The pointer type to be used
   internally in the hook and propagated to the container.
   Default: `void_pointer<void*>`.

[endsect]

[section:list_container list container]

[c++]

   template <class T, class ...Options>
   class list;

[classref boost::intrusive::list list] receives the same options explained in
the section [link intrusive.usage How to use Boost.Intrusive]:

*  [*`base_hook<class Hook>`] / [*`member_hook<class T, class Hook, Hook T::* PtrToMember>`] / 
   [*`value_traits<class ValueTraits>`]: To specify the hook type or value traits used
   to configure the container (to know about value traits go to the section
   titled [link intrusive.value_traits Containers with custom ValueTraits].

*  [*`constant_time_size<bool Enabled>`]: To activate the constant-time `size()` operation.
   Default: `constant_time_size<true>`

*  [*`size_type<bool Enabled>`]: To specify the type that will be used to store the size
   of the container. Default: `size_type<std::size_t>`

[endsect]

[section:list_example Example]

Now let's see an small example using both hooks:

[import ../example/doc_list.cpp]
[doc_list_code]

[endsect]

[endsect]

[section:set_multiset Intrusive associative containers: set, multiset, rbtree]

[*Boost.Intrusive] also offers associative containers that can be very useful
when creating more complex associative containers, like containers maintaining
one or more indices with different sorting semantics. Boost.Intrusive associative 
containers, like most STL associative container implemenatations are based on
red-black trees.

The memory overhead of these containers is usually 3 pointers and a bit (with
alignment issues, this means 3 pointers and an integer).
This size can be reduced to 3 pointers if pointers have even alignment
(which is usually true in most systems).

An empty, non constant-time size [classref boost::intrusive::set set],
[classref boost::intrusive::multiset multiset] or
[classref boost::intrusive::rbtree rbtree]
has also the size of 3 pointers and an integer (3 pointers when optimized for size).
These containers have logarithmic complexity in many
operations like
searches, insertions, erasures, etc... [classref boost::intrusive::set set] and
[classref boost::intrusive::multiset multiset] are the
intrusive equivalents of standard `std::set` and `std::multiset` containers.

[classref boost::intrusive::rbtree rbtree] is a superset of 
[classref boost::intrusive::set set] and
[classref boost::intrusive::multiset multiset] containers that offers
functions to insert unique and multiple keys.

[section:set_multiset_hooks set, multiset and rbtree hooks]

[classref boost::intrusive::set set],
[classref boost::intrusive::multiset multiset] and
[classref boost::intrusive::rbtree rbtree] share the same hooks.
This is an advantage, because the same
user type can be inserted first in a [classref boost::intrusive::multiset multiset]
and after that in [classref boost::intrusive::set set] without
changing the definition of the user class.

[c++]

   template <class ...Options>
   class set_base_hook;

*  [classref boost::intrusive::set_base_hook set_base_hook]:
   the user class derives publicly from
   [classref boost::intrusive::set_base_hook set_base_hook] to make
   it [classref boost::intrusive::set set]/[classref boost::intrusive::multiset multiset]-compatible.

[c++]

   template <class ...Options>
   class set_member_hook;

*  [classref boost::intrusive::set_member_hook set_member_hook]:
   the user class contains a public
   [classref boost::intrusive::set_member_hook set_member_hook] to make
   it [classref boost::intrusive::set set]/[classref boost::intrusive::multiset multiset]-compatible.

[classref boost::intrusive::set_base_hook set_base_hook] and
[classref boost::intrusive::set_member_hook set_member_hook] receive
the same options explained in the section
[link intrusive.usage How to use Boost.Intrusive] plus a size optimization option:

*  [*`tag<class Tag>`] (for base hooks only): This argument serves as a tag,
   so you can derive from more than one base hook.
   Default: `tag<default_tag>`.

*  [*`link_mode<link_mode_type LinkMode>`]: The linking policy.
   Default: `link_mode<safe_link>`.

*  [*`void_pointer<class VoidPointer>`]: The pointer type to be used
   internally in the hook and propagated to the container.
   Default: `void_pointer<void*>`.

*  [*`optimize_size<bool Enable>`]: The hook will be optimized for size
   instead of speed. The hook will embed the color bit of the red-black
   tree node in the parent pointer if pointer alignment is even.
   Optimizing the size will reduce speed performance a bit since masking
   operations will be needed to access parent pointer and color attributes.
   Default: `optimize_size<false>`.

[endsect]

[section:set_multiset_containers set, multiset and rbtree containers]

[c++]

   template <class T, class ...Options>
   class set;

   template <class T, class ...Options>
   class multiset;

   template <class T, class ...Options>
   class rbtree;

These containers receive the same options explained in the section
[link intrusive.usage How to use Boost.Intrusive]:

*  [*`base_hook<class Hook>`] / [*`member_hook<class T, class Hook, Hook T::* PtrToMember>`] / 
   [*`value_traits<class ValueTraits>`]: To specify the hook type or value traits used
   to configure the container (to know about value traits go to the section
   titled [link intrusive.value_traits Containers with custom ValueTraits].

*  [*`constant_time_size<bool Enabled>`]: To activate the constant-time `size()` operation.
   Default: `constant_time_size<true>`

*  [*`size_type<bool Enabled>`]: To specify the type that will be used to store the size
   of the container. Default: `size_type<std::size_t>`

And they also can receive an additional option:

*  [*`compare<class Compare>`]: Comparison function for the objects to be inserted
   in containers. The comparison functor must induce a strict weak ordering.
   Default: `compare< std::less<T> >`

[endsect]

[section:set_multiset_example Example]

Now let's see an small example using both hooks and both containers:

[import ../example/doc_set.cpp]
[doc_set_code]

[endsect]

[endsect]

[section:unordered_set_unordered_multiset Pseudo-Intrusive unordered associative containers: unordered_set, unordered_multiset]

[*Boost.Intrusive] also offers hashed containers that can be very useful to implement
fast-lookup containers.  These containers
([classref boost::intrusive::unordered_set unordered_set] and [classref boost::intrusive::unordered_multiset unordered_multiset])
are pseudo-intrusive containers: they need additional memory apart from the hook
stored in the `value_type`. This additional
memory must be passed in the constructor of the container.

Unlike C++ TR1 unordered associative containers (which are also hashed containers),
the contents of these pseudo-intrusive containers are not rehashed to maintain a
load factor: that would require memory management and intrusive containers don't
implement any memory management at all. However, the user can request an explicit
rehashing passing a new bucket array.
This also offers an additional guarantee over TR1 unordered associative containers:
[*iterators are not invalidated when inserting an element] in the container. 

As with TR1 unordered associative containers, rehashing invalidates iterators,
changes ordering between elements and changes which buckets elements appear in,
but does not invalidate pointers or references to elements. 

Apart from expected hash and equality function objects, [*Boost.Intrusive] unordered
associative containers' constructors take an argument specifying an auxiliary
bucket vector to be used by the container.

[section:unordered_set_unordered_multiset_performance unordered_set and unordered_multiset performance notes]

The size overhead for a hashed container is moderate: 1 pointer per value plus
a bucket array per container. The size of an element of the bucket array
is usually one pointer. To obtain a good performance hashed container,
the bucket length is usually the same as the number of elements that the
container contains, so a well-balanced hashed container (`bucket_count()` is 
equal to `size()` ) will have an equivalent overhead of two pointers per element.

An empty, non constant-time size [classref boost::intrusive::unordered_set unordered_set] or
[classref boost::intrusive::unordered_multiset unordered_multiset]
has also the size of `bucket_count()` pointers. 

Insertions, erasures, and searches, have amortized constant-time complexity in
hashed containers. However, some worst-case guarantees are linear. See
[classref boost::intrusive::unordered_set unordered_set] or
[classref boost::intrusive::unordered_multiset unordered_multiset] for complexity guarantees
of each operation.

[*Be careful with non constant-time size hashed containers]: some operations, like
`empty()`, have linear complexity, unlike other [*Boost.Intrusive] containers.

[endsect]

[section:unordered_set_unordered_multiset_hooks unordered_set and unordered_multiset hooks]

[classref boost::intrusive::unordered_set unordered_set] and [classref boost::intrusive::unordered_multiset unordered_multiset] share the same hooks. This is an advantage, because the same
user type can be inserted first in a [classref boost::intrusive::unordered_multiset unordered_multiset] and after that in [classref boost::intrusive::unordered_set unordered_set] without
changing the definition of the user class.

[c++]

   template <class ...Options>
   class unordered_set_base_hook;

*  [classref boost::intrusive::unordered_set_base_hook unordered_set_base_hook]:
   the user class derives publicly from
   [classref boost::intrusive::unordered_set_base_hook unordered_set_base_hook] to make
   it [classref boost::intrusive::unordered_set unordered_set]/[classref boost::intrusive::unordered_multiset unordered_multiset]-compatible.

[c++]

   template <class ...Options>
   class unordered_set_member_hook;

*  [classref boost::intrusive::unordered_set_member_hook unordered_set_member_hook]:
   the user class contains a public
   [classref boost::intrusive::unordered_set_member_hook unordered_set_member_hook] to make
   it [classref boost::intrusive::unordered_set unordered_set]/[classref boost::intrusive::unordered_multiset unordered_multiset]-compatible.

[classref boost::intrusive::unordered_set_base_hook unordered_set_base_hook] and
[classref boost::intrusive::unordered_set_member_hook unordered_set_member_hook] receive
the same options explained in the section
[link intrusive.usage How to use Boost.Intrusive]:

*  [*`tag<class Tag>`] (for base hooks only): This argument serves as a tag,
   so you can derive from more than one base hook.
   Default: `tag<default_tag>`.

*  [*`link_mode<link_mode_type LinkMode>`]: The linking policy.
   Default: `link_mode<safe_link>`.

*  [*`void_pointer<class VoidPointer>`]: The pointer type to be used
   internally in the hook and propagated to the container.
   Default: `void_pointer<void*>`.

Apart from them, these hooks offer additional options:

*  [*`store_hash<bool Enabled>`]: This option reserves additional space in
   the hook to store the hash value of the object once it's introduced in the
   container. When this option is used, the unordered container will store
   the calculated hash value in the hook and rehashing operations won't need
   to recalculate the hash of the value.
   This option will improve the perfomance of unordered containers when
   rehashing is frequent or hashing the value is a slow operation.
   Default: `store_hash<false>`.

[endsect]

[section:unordered_set_unordered_multiset_containers unordered_set and unordered_multiset containers]

[c++]

   template<class T, class ...Options>
   class unordered_set;

   template<class T, class ...Options>
   class unordered_multiset;

As mentioned, unordered containers need an auxiliary array to work. [*Boost.Intrusive]
unordered containers receive this auxiliary array packed in a type called `bucket_traits`
(which can be also customized by a container option). All unordered containers receive
a `bucket_traits` object in their constructors. The default `bucket_traits` class
is initialized with a pointer to an array of buckets and its size:

[c++]

   #include <boost/intrusive/unordered_set.hpp>

   using namespace boost::intrusive;

   struct MyClass : public unordered_set_base_hook<>
   {};

   typedef unordered_set<MyClass>::bucket_type     bucket_type;
   typedef unordered_set<MyClass>::bucket_traits   bucket_traits;

   int main()
   {
      bucket_type buckets[100];
      unordered_set<MyClass> uset(bucket_traits(buckets, 100));
      return 0;
   }

Each hashed container needs [*its own bucket traits], that is, [*its own
bucket vector]. Two hashed containers
[*can't] share the same `bucket_type` elements. The bucket array [*must] be
destroyed [*after] the container using it is destroyed, otherwise, the result
is undefined.

[classref boost::intrusive::unordered_set unordered_set] and
[classref boost::intrusive::unordered_multiset unordered_multiset]
receive the same options explained in the section
[link intrusive.usage How to use Boost.Intrusive]:

*  [*`base_hook<class Hook>`] / [*`member_hook<class T, class Hook, Hook T::* PtrToMember>`] / 
   [*`value_traits<class ValueTraits>`]: To specify the hook type or value traits used
   to configure the container (to know about value traits go to the section
   titled [link intrusive.value_traits Containers with custom ValueTraits].

*  [*`constant_time_size<bool Enabled>`]: To activate the constant-time `size()` operation.
   Default: `constant_time_size<true>`

*  [*`size_type<bool Enabled>`]: To specify the type that will be used to store the size
   of the container. Default: `size_type<std::size_t>`

And they also can receive two additional options:

*  [*`equal<class Equal>`]: Equality function for the objects to be inserted
   in containers. Default: `equal< std::equal_to<T> >`

*  [*`hash<class Hash>`]: Hash function to be used in the container.
   Default: `hash< boost::hash<T> >`

*  [*`bucket_traits<class BucketTraits>`]: A type that wraps the bucket vector to
   be used by the unordered container. Default: a type initialized by the address
   and size of a bucket array and stores both variables internally.

*  [*`power_2_buckets<bool Enabled>`]: The user guarantees that only bucket arrays
   with power of two length will be used. The container will then use masks instead of modulo
   operations to obtain the bucket number from the hash value. Masks are faster than
   modulo operations and for some applications modulo operations can impose
   a considerable overhead. In debug mode an assertion will be raised if the user
   provides a bucket length that is not power of two.
   Default: `constant_time_size<false>`.

[endsect]

[section:unordered_set_unordered_multiset_example Example]

Now let's see an small example using both hooks and both containers:

[import ../example/doc_unordered_set.cpp]
[doc_unordered_set_code]

[endsect]

[section:custom_bucket_traits Custom bucket traits]

Instead of using the default `bucket_traits` class to store the bucket array, a user
can define his own class to store the bucket array using the [*['bucket_traits<>]]
option. A user-defined bucket-traits must fulfill the following interface:

[c++]

   class my_bucket_traits
   {
      bucket_ptr        bucket_begin();
      const_bucket_ptr  bucket_begin() const;
      std::size_t bucket_count() const;
   };


The following bucket traits just stores a pointer to the bucket
array but the size is a compile-time constant. Note the use of the auxiliary
[classref boost::intrusive::unordered_bucket unordered_bucket] and 
[classref boost::intrusive::unordered_bucket_ptr unordered_bucket_ptr]
utilities to obtain the type of the bucket and its pointer before defining
the unordered container:

[import ../example/doc_bucket_traits.cpp]
[doc_bucket_traits]

[endsect]

[endsect]

[section:splay_set_multiset Intrusive splay tree based associative containers: splay_set, splay_multiset and , splay_tree]

C++ associative containers are usually based on red-black tree implementations (e.g.: STL,
Boost.Intrusive associative containers). However, there are other interesting data
structures that offer some advantages (and also disadvantages).

Splay trees are self-adjusting binary search trees used tipically in caches, memory
allocators and other applications, because splay trees have a "caching effect": recently
accessed elements have better access times that elements accessed less frequently.
For more information on splay trees see [@http://en.wikipedia.org/wiki/Splay_tree Wikipedia entry].

[*Boost.Intrusive] offers 3 containers based on splay trees:
[classref boost::intrusive::splay_set splay_set],
[classref boost::intrusive::splay_multiset splay_multiset] and
[classref boost::intrusive::splaytree splaytree]. The first two are similar to
[classref boost::intrusive::set set] or
[classref boost::intrusive::multiset multiset] and the latter is a generalization
that offers functions both to insert unique and multiple keys.

The memory overhead of these containers with Boost.Intrusive hooks is usually 3 pointers.
An empty, non constant-time size splay container has also the size of 3 pointers.

[section:splay_set_multiset_disadvantages Advantages and disadvantages of splay tree based containers]

Splay tree based intrusive containers have logarithmic complexity in many
operations like searches, insertions, erasures, etc... but if some elements are
more frequently accessed than others, splay trees perform faster searches than equivalent
balanced binary trees (such as red-black trees).

The caching effect offered by splay trees comes with a cost: the tree must be
rebalanced when a element is searched. This disallows const versions of search
functions like `find()`, `lower_bound()`, `upper_bound()`, `equal_range()`,
`count()`...

Because of this, splay-tree based associative containers are not drop-in
replacements of [classref boost::intrusive::set set]/
[classref boost::intrusive::splay_set splay_set].

Apart from this, if element searches are randomized, the tree will be rebalanced
without taking advantage of the cache effect, so splay trees can offer worse
performance than other balanced trees for some search patterns.

[endsect]

[section:splay_set_multiset_hooks splay_set, splay_multiset and splaytree hooks]

[classref boost::intrusive::splay_set splay_set],
[classref boost::intrusive::splay_multiset splay_multiset] and
[classref boost::intrusive::splaytree splaytree]
share the same hooks.

[c++]

   template <class ...Options>
   class splay_set_base_hook;

*  [classref boost::intrusive::splay_set_base_hook splay_set_base_hook]:
   the user class derives publicly from this class to make
   it compatible with splay tree based containers.

[c++]

   template <class ...Options>
   class splay_set_member_hook;

*  [classref boost::intrusive::set_member_hook set_member_hook]:
   the user class contains a public member of this class to make
   it compatible with splay tree based containers.

[classref boost::intrusive::splay_set_base_hook splay_set_base_hook] and
[classref boost::intrusive::splay_set_member_hook splay_set_member_hook] receive
the same options explained in the section
[link intrusive.usage How to use Boost.Intrusive]:

*  [*`tag<class Tag>`] (for base hooks only): This argument serves as a tag,
   so you can derive from more than one base hook.
   Default: `tag<default_tag>`.

*  [*`link_mode<link_mode_type LinkMode>`]: The linking policy.
   Default: `link_mode<safe_link>`.

*  [*`void_pointer<class VoidPointer>`]: The pointer type to be used
   internally in the hook and propagated to the container.
   Default: `void_pointer<void*>`.

[endsect]

[section:set_multiset_containers splay_set, splay_multiset and splaytree containers]

[c++]

   template <class T, class ...Options>
   class splay_set;

   template <class T, class ...Options>
   class splay_multiset;

   template <class T, class ...Options>
   class splaytree;

These containers receive the same options explained in the section
[link intrusive.usage How to use Boost.Intrusive]:

*  [*`base_hook<class Hook>`] / [*`member_hook<class T, class Hook, Hook T::* PtrToMember>`] / 
   [*`value_traits<class ValueTraits>`]: To specify the hook type or value traits used
   to configure the container (to know about value traits go to the section
   titled [link intrusive.value_traits Containers with custom ValueTraits].

*  [*`constant_time_size<bool Enabled>`]: To activate the constant-time `size()` operation.
   Default: `constant_time_size<true>`

*  [*`size_type<bool Enabled>`]: To specify the type that will be used to store the size
   of the container. Default: `size_type<std::size_t>`

And they also can receive an additional option:

*  [*`compare<class Compare>`]: Comparison function for the objects to be inserted
   in containers. The comparison functor must induce a strict weak ordering.
   Default: `compare< std::less<T> >`

[endsect]

[section:splay_set_bst_hook Splay trees with BST hooks]

Intrusive splay containers can also use plain binary search tree hooks
[classref boost::intrusive::bs_set_base_hook bs_set_base_hook] and
[classref boost::intrusive::bs_set_base_hook bs_set_base_hook].
These hooks can be used by other intrusive containers like
intrusive scapegoat containers
[classref boost::intrusive::sg_set sg_set] and
[classref boost::intrusive::sg_multiset sg_multiset] so a programmer
might prefer using a binary search tree hook so that the same type
can be introduced in some situations in an splay container but that
can also be introduced in other compatible containers as well when
the hook is not being used in an splay container.

[classref boost::intrusive::bs_set_base_hook bs_set_base_hook] and
[classref boost::intrusive::bs_set_base_hook bs_set_member_hook] admit
the same options as [classref boost::intrusive::splay_set_base_hook splay_set_base_hook].

[endsect]

[section:splay_set_multiset_example Example]

Now let's see an small example using both splay hooks,
binary search tree hooks and
[classref boost::intrusive::splay_set splay_set]/
[classref boost::intrusive::splay_multiset splay_multiset]
containers:

[import ../example/doc_splay_set.cpp]
[doc_splay_set_code]

[endsect]

[endsect]

[section:avl_set_multiset Intrusive avl tree based associative containers: avl_set, avl_multiset and avltree]

Similar to red-black trees, AVL trees are balanced binary trees. 
AVL trees are often compared with red-black trees because they support the same set of operations
and because red-black trees also take O(log n) time for the basic operations. 
AVL trees are more rigidly balanced than Red-Black trees, leading to slower insertion and
removal but faster retrieval, so AVL trees perform better
than red-black trees for lookup-intensive applications. 

[*Boost.Intrusive] offers 3 containers based on avl trees:
[classref boost::intrusive::avl_set avl_set],
[classref boost::intrusive::avl_multiset avl_multiset] and
[classref boost::intrusive::avltree avltree]. The first two are similar to
[classref boost::intrusive::set set] or
[classref boost::intrusive::multiset multiset] and the latter is a generalization
that offers functions both to insert unique and multiple keys.

The memory overhead of these containers with Boost.Intrusive hooks is usually 3
pointers and 2 bits (due to alignment, this usually means 3 pointers plus an integer).
This size can be reduced to 3 pointers if pointers have 4 byte alignment
(which is usually true in 32 bit systems).

An empty, non constant-time size [classref boost::intrusive::avl_set avl_set],
[classref boost::intrusive::avl_multiset avl_multiset] or
[classref boost::intrusive::avltree avltree]
has also the size of 3 pointers and an integer (3 pointers when optimized for size).

[section:avl_set_multiset_hooks avl_set, avl_multiset and avltree hooks]

[classref boost::intrusive::avl_set avl_set],
[classref boost::intrusive::avl_multiset avl_multiset] and
[classref boost::intrusive::avltree avltree]
share the same hooks.

[c++]

   template <class ...Options>
   class avl_set_base_hook;

*  [classref boost::intrusive::avl_set_base_hook avl_set_base_hook]:
   the user class derives publicly from this class to make
   it compatible with avl tree based containers.

[c++]

   template <class ...Options>
   class avl_set_member_hook;

*  [classref boost::intrusive::set_member_hook set_member_hook]:
   the user class contains a public member of this class to make
   it compatible with avl tree based containers.

[classref boost::intrusive::avl_set_base_hook avl_set_base_hook] and
[classref boost::intrusive::avl_set_member_hook avl_set_member_hook] receive
the same options explained in the section
[link intrusive.usage How to use Boost.Intrusive] plus an option to optimize
the size of the node:

*  [*`tag<class Tag>`] (for base hooks only): This argument serves as a tag,
   so you can derive from more than one base hook.
   Default: `tag<default_tag>`.

*  [*`link_mode<link_mode_type LinkMode>`]: The linking policy.
   Default: `link_mode<safe_link>`.

*  [*`void_pointer<class VoidPointer>`]: The pointer type to be used
   internally in the hook and propagated to the container.
   Default: `void_pointer<void*>`.

*  [*`optimize_size<bool Enable>`]: The hook will be optimized for size
   instead of speed. The hook will embed the balance bits of the AVL
   tree node in the parent pointer if pointer alignment is multiple of 4.
   Optimizing the size will reduce speed performance a bit since masking
   operations will be needed to access parent pointer and balance factor attributes.
   Default: `optimize_size<false>`.

[endsect]

[section:set_multiset_containers avl_set, avl_multiset and avltree containers]

[c++]

   template <class T, class ...Options>
   class avl_set;

   template <class T, class ...Options>
   class avl_multiset;

   template <class T, class ...Options>
   class avltree;

These containers receive the same options explained in the section
[link intrusive.usage How to use Boost.Intrusive]:

*  [*`base_hook<class Hook>`] / [*`member_hook<class T, class Hook, Hook T::* PtrToMember>`] / 
   [*`value_traits<class ValueTraits>`]: To specify the hook type or value traits used
   to configure the container (to know about value traits go to the section
   titled [link intrusive.value_traits Containers with custom ValueTraits].

*  [*`constant_time_size<bool Enabled>`]: To activate the constant-time `size()` operation.
   Default: `constant_time_size<true>`

*  [*`size_type<bool Enabled>`]: To specify the type that will be used to store the size
   of the container. Default: `size_type<std::size_t>`

And they also can receive an additional option:

*  [*`compare<class Compare>`]: Comparison function for the objects to be inserted
   in containers. The comparison functor must induce a strict weak ordering.
   Default: `compare< std::less<T> >`

[endsect]

[section:avl_set_multiset_example Example]

Now let's see an small example using both hooks and
[classref boost::intrusive::avl_set avl_set]/
[classref boost::intrusive::avl_multiset avl_multiset]
containers:

[import ../example/doc_avl_set.cpp]
[doc_avl_set_code]

[endsect]

[endsect]


[section:sg_set_multiset Intrusive scapegoat tree based associative containers: sg_set, sg_multiset and sgtree]

A scapegoat tree is a self-balancing binary search tree, that provides worst-case O(log n)
lookup time, and O(log n) amortized insertion and deletion time.
Unlike other self-balancing binary search trees that provide worst case O(log n) lookup
time, scapegoat trees have no additional per-node overhead compared to a regular binary
search tree.

A binary search tree is said to be weight balanced if half the nodes are on the left
of the root, and half on the right. An α-height-balanced tree is defined with defined
with the following equation:

[*['height(tree) <= log1/α(tree.size())]]

*  [*['α == 1]]: A tree forming a linked list is considered balanced.
*  [*['α == 0.5]]: Only a perfectly balanced binary is considered balanced.

Scapegoat trees are loosely ['α-height-balanced] so:

[*['height(tree) <= log1/α(tree.size()) + 1]]

Scapegoat trees support any α between 0.5 and 1. If α is higher, the tree is rebalanced
less often, obtaining quicker insertions but slower searches. Lower
α values improve search times. Scapegoat-trees implemented in [*Boost.Intrusive] offer the possibility of
[*changing α at run-time] taking advantage of the flexibility of scapegoat trees.
For more information on scapegoat trees see [@http://en.wikipedia.org/wiki/Scapegoat_tree Wikipedia entry].

Scapegoat trees also have downsides:

*  They need additional storage of data on the
   root (the size of the tree, for example) to achieve logarithmic complexity operations
   so it's not possible to offer `auto_unlink` hooks. The size of an empty scapegoat
   tree is also considerably increased.

*  The operations needed to determine if the tree is unbalanced require floating-point
   operations like ['log1/α]. If the system has no floating point operations (like some
   embedded systems), scapegoat tree operations might become slow.

[*Boost.Intrusive] offers 3 containers based on scapegoat trees:
[classref boost::intrusive::sg_set sg_set],
[classref boost::intrusive::sg_multiset sg_multiset] and
[classref boost::intrusive::sgtree sgtree]. The first two are similar to
[classref boost::intrusive::set set] or
[classref boost::intrusive::multiset multiset] and the latter is a generalization
that offers functions both to insert unique and multiple keys.

The memory overhead of these containers with Boost.Intrusive hooks is 3
pointers.

An empty, [classref boost::intrusive::sg_set sg_set],
[classref boost::intrusive::sg_multiset sg_multiset] or
[classref boost::intrusive::sgtree sgtree]
has also the size of 3 pointers, two integers and two floating point values
(equivalent to the size of 7 pointers on most systems).

[section:sg_set_multiset_hooks Using binary search tree hooks: bs_set_base_hook and bs_set_member_hook]

[classref boost::intrusive::sg_set sg_set],
[classref boost::intrusive::sg_multiset sg_multiset] and
[classref boost::intrusive::sgtree sgtree] don't use their
own hooks but plain binary search tree hooks. This has many advantages
since binary search tree hooks can also be used to insert values in
splay containers.

[c++]

   template <class ...Options>
   class bs_set_base_hook;

*  [classref boost::intrusive::bs_set_base_hook bs_set_base_hook]:
   the user class derives publicly from this class to make
   it compatible with scapegoat tree based containers.

[c++]

   template <class ...Options>
   class bs_set_member_hook;

*  [classref boost::intrusive::set_member_hook set_member_hook]:
   the user class contains a public member of this class to make
   it compatible with scapegoat tree based containers.

[classref boost::intrusive::bs_set_base_hook bs_set_base_hook] and
[classref boost::intrusive::bs_set_member_hook bs_set_member_hook] receive
the same options explained in the section
[link intrusive.usage How to use Boost.Intrusive] plus an option to optimize
the size of the node:

*  [*`tag<class Tag>`] (for base hooks only): This argument serves as a tag,
   so you can derive from more than one base hook.
   Default: `tag<default_tag>`.

*  [*`link_mode<link_mode_type LinkMode>`]: The linking policy.
   Default: `link_mode<safe_link>`.

*  [*`void_pointer<class VoidPointer>`]: The pointer type to be used
   internally in the hook and propagated to the container.
   Default: `void_pointer<void*>`.

[endsect]

[section:sg_set_multiset_containers sg_set, sg_multiset and sgtree containers]

[c++]

   template <class T, class ...Options>
   class sg_set;

   template <class T, class ...Options>
   class sg_multiset;

   template <class T, class ...Options>
   class sgtree;

These containers receive the same options explained in the section
[link intrusive.usage How to use Boost.Intrusive]:

*  [*`base_hook<class Hook>`] / [*`member_hook<class T, class Hook, Hook T::* PtrToMember>`] / 
   [*`value_traits<class ValueTraits>`]: To specify the hook type or value traits used
   to configure the container (to know about value traits go to the section
   titled [link intrusive.value_traits Containers with custom ValueTraits].

*  [*`size_type<bool Enabled>`]: To specify the type that will be used to store the size
   of the container. Default: `size_type<std::size_t>`

And they also can receive additional options:

*  [*`compare<class Compare>`]: Comparison function for the objects to be inserted
   in containers. The comparison functor must induce a strict weak ordering.
   Default: `compare< std::less<T> >`

*  [*`floating_point<bool Enable>`]: 
   When this option is deactivated, the scapegoat tree loses the ability to change
   the balance factor α at run-time, but the size of an empty container is reduced
   and no floating point operations are performed, normally increasing container
   performance. The fixed α factor that is used when this option is activated
   is ['1/sqrt(2) ~ 0,70711]. Default: `floating_point<true>`

[endsect]

[section:sg_set_multiset_example Example]

Now let's see an small example using both hooks and
[classref boost::intrusive::sg_set sg_set]/
[classref boost::intrusive::sg_multiset sg_multiset]
containers:

[import ../example/doc_sg_set.cpp]
[doc_sg_set_code]

[endsect]

[endsect]




















[section:advanced_lookups_insertions Advanced lookup and insertion functions for associative containers]

[section:advanced_lookups Advanced lookups]

[*Boost.Intrusive] associative containers offer the same interface as STL associative
containers. However, STL and TR1 ordered and unordered simple associative containers 
(`std::set`, `std::multiset`, `std::tr1::unordered_set` and `std::tr1::unordered_multiset`)
have some inefficiencies caused by the interface: the user can only operate with `value_type`
objects. When using these containers we must use `iterator find(const value_type &value)`
to find a value. The same happens in other functions
like `equal_range`, `lower_bound`, `upper_bound`...

However, sometimes the object to be searched it's quite expensive to construct:

[import ../example/doc_assoc_optimized_code.cpp]
[doc_assoc_optimized_code_normal_find]

`Expensive` is an expensive object to construct. If "key" c-string is quite long
`Expensive` has to construct a `std::string` using heap memory. Like
`Expensive`, many times the only member taking part in ordering issues is just
a small part of the class. For example, with `Expensive`, only the internal
`std::string` is needed to compare the object. 

In both containers, if we call `get_from_set/get_from_unordered_set` in a loop, we might get a performance penalty,
because we are forced to create a whole `Expensive` object to be able to find an
equivalent one.

Sometimes this interface limitation is severe, because
we [*might not have enough information to construct the object] but we might
[*have enough information to find the object]. In this case, a name it's enough
to search `Expensive` in the container but constructing an `Expensive`
might require more information that the user might not have.

To solve this, [classref boost::intrusive::set set]/[classref boost::intrusive::multiset multiset]
offer alternative functions, which take any type comparable with the value and a 
functor that should be compatible with the 
ordering function of the associative container.
[classref boost::intrusive::unordered_set unordered_set]/[classref boost::intrusive::unordered_multiset unordered_multiset]
offers functions that take any key type and compatible hash and equality functions. Now, let's see the
optimized search function:

[doc_assoc_optimized_code_optimized_find]

This new arbitrary key overload is also available for other functions taking
values as arguments:

*  equal_range
*  lower_bound
*  upper_bound
*  count
*  find
*  erase

Check [classref boost::intrusive::set set],
[classref boost::intrusive::multiset multiset],
[classref boost::intrusive::unordered_set unordered_set],
[classref boost::intrusive::unordered_multiset unordered_multiset]
references to know more about those functions.

[endsect]

[section:advanced_insertions Advanced insertions]

A similar issue happens with insertions in simple ordered and unordered associative
containers with unique keys (`std::set` and `std::tr1::unordered_set`). In these containers,
if a value is already present, the value to be inserted is discarded. With expensive
values, if the value is already present, we can suffer efficiency problems.

[classref boost::intrusive::set set] and [classref boost::intrusive::unordered_set unordered_set]
have insertion functions to check efficiently, without
constructing the value, if a value is present or not and if it's not present, a
function to insert it immediately without any further lookup.
For example, using the same `Expensive` class,
this function can be inefficient:

[doc_assoc_optimized_code_normal_insert]
 
If the object is already present, we are constructing an `Expensive` that
will be discarded, and this is a waste of resources. Instead of that, let's use
`insert_check` and `insert_commit` functions:

[doc_assoc_optimized_code_optimized_insert]

`insert_check` is similar to a normal `insert` but:

*  `insert_check` can be used with arbitrary keys
*  if the insertion is possible (there is no equivalent value) collects all the needed information
in an `insert_commit_data` structure, so that `insert_commit`:
   *   [*does not execute] further comparisons
   *   can be executed with [*constant-time complexity]
   *   has [*no-throw guarantee].

These functions must be used with care, since
no other insertion or erasure must be executed between an `insert_check` and an `insert_commit`
pair. Otherwise, the behaviour is undefined.
`insert_check` and `insert_commit` will come handy
for developers programming efficient non-intrusive associative containers.
See [classref boost::intrusive::set set]
and [classref boost::intrusive::unordered_set unordered_set] reference for more information about
`insert_check` and `insert_commit`.

With multiple ordered and unordered associative containers
([classref boost::intrusive::multiset multiset] and 
[classref boost::intrusive::unordered_multiset unordered_multiset]) there's
no need for these advanced insertion functions, since insertions are always succesful.

[endsect]

For more information about advanced lookup and insertion functions see
[classref boost::intrusive::set set],
[classref boost::intrusive::multiset multiset],
[classref boost::intrusive::unordered_set unordered_set] and
[classref boost::intrusive::unordered_multiset unordered_multiset] references.

[endsect]

[section:erasing_and_disposing Erasing and disposing values from Boost.Intrusive containers]

One of the most tedious tasks when using intrusive containers is the management of the erased elements.
When using STL containers, the container itself unlinks and destroys the contained elements, but with
intrusive containers, the user must explicitly destroy the object after erasing an element from the container.
This makes STL-like functions erasing multiple objects unhelpful: the user can't destroy every erased element.
For example, let's take the function `remove_if` from [classref boost::intrusive::list list]:

[c++]

   template<class Pred>
   void remove_if(Pred pred);

How can the user destroy the elements (say, using `operator delete`) that will be erased according
to the predicate? [*Boost.Intrusive] containers offer additional functions that take a function
object that will be called after the element has been erased from the container. For example,
[classref boost::intrusive::list list] offers:

[c++]

   template<class Pred, class Disposer>
   void remove_and_dispose_if(Pred pred, Disposer disposer)

With this function the user can efficiently remove and destroy elements if the disposer
function destroys an object: `remove_and_dispose_if` 
will call "disposer" function object for every removed element. [classref boost::intrusive::list list] offers
more functions taking a disposer function object as argument, like `erase_and_dispose`, `clear_and_dispose`,
`remove_and_dispose`... 

Note that the disposing function does not need to just destroy the object. It can
implement any other operation like inserting the remove object in another container.
Let's see an small example:

[import ../example/doc_erasing_and_disposing.cpp]
[doc_erasing_and_disposing]

All [*Boost.Intrusive] containers offer these "erase + dispose" additional members for all functions
that erase an element from the container.



[endsect]

[section:clone_from Cloning [*Boost.Intrusive] containers]

As previously mentioned, [*Boost.Intrusive] containers are [*non-copyable and non-assignable], because
intrusive containers don't allocate memory at all. To implement a copy-constructor or assignment operator,
the user must clone one by one all the elements of the container and insert them in another intrusive container.
However, cloning by hand is usually more inefficient than a member cloning function and an specialized cloning
function can offer more guarantees than the manual cloning (better exception safety guarantees, for example).

To ease the implementation of copy constructors and assignment operators of classes containing [*Boost.Intrusive]
containers, all [*Boost.Intrusive] containers offer an special cloning function called `clone_from`.

Apart from the container to be cloned, `clone_from` takes two function objects as arguments. For example, the
`clone_from` member function of [classref boost::intrusive::list list]:

[c++]

   template <class Cloner, class Disposer>
   void clone_from(const list &src, Cloner cloner, Disposer disposer);
   
This function will make `*this` a clone of `src`. Let's explain the arguments:

*   The first parameter is the list to be cloned.
*   The second parameter is a function object that will clone `value_type` objects and
   return a pointer to the clone. It must implement the following function:
   `pointer operator()(const value_type &)`.
*   The second parameter is a function object that will dispose `value_type` objects. It's used first
   to empty the container before cloning and to dispose the elements if an exception is thrown.
   
The cloning function works as follows:

*   First clears and disposes all the elements from *this using the disposer function object.
*   After that starts cloning all the elements of the source container using the cloner function object.
*   If any operation in the cloning function (for example, the cloner function object) throws,
   all the constructed elements are disposed using the disposer function object.


Here's an example of `clone_from`:

[import ../example/doc_clone_from.cpp]
[doc_clone_from]

[endsect]

[section:using_smart_pointers Using smart pointers with Boost.Intrusive containers]

[*Boost.Intrusive] hooks can be configured to use other pointers than raw pointers.
When a [*Boost.Intrusive] hook is configured with an smart pointer as an argument,
this pointer configuration is passed to the containers. For example, if the following
hook is configured with an smart pointer (for example, an offset pointer from
[*Boost.Interprocess]):

[import ../example/doc_offset_ptr.cpp]
[doc_offset_ptr_0]

Any intrusive list constructed using this hook will be ready for shared memory,
because the intrusive list will also use offset pointers internally. For example,
we can create an intrusive list in shared memory combining [*Boost.Interprocess]
and [*Boost.Intrusive]:

[doc_offset_ptr_1]

[section:smart_pointers_requirements Requirements for smart pointers compatible with Boost.Intrusive]

Not every smart pointer is compatible with [*Boost.Intrusive], the smart pointer must
have the following features:

*  It must support the same operations as a raw pointer, except casting.
*  It must be convertible to a raw pointer and constructible from a raw pointer.
*  It must have the same ownership semantics as a raw pointer. This means that
   resource management smart pointers (like `boost::shared_ptr`) can't be used.

The conversion from the smart pointer to a raw pointer must be implemented following
Boost smart pointer `detail::get_pointer()` function. This function will be found using
ADL. For example, for `boost::interprocess::offset_ptr` `detail::get_pointer` is defined
as follows:

[c++]

   template<class T>
   T * detail::get_pointer(boost::interprocess::offset_ptr<T> const & p)
   {  return p.get();   }

[endsect]

[endsect]

[section:obtaining_iterators_from_values Obtaining iterators from values]

[*Boost.Intrusive] offers another useful feature that's not present in STL
containers: it's possible to obtain an iterator to a value from the value itself.
This feature is implemented in [*Boost.Intrusive] containers by a 
function called `iterator_to`:

[c++]

   iterator iterator_to(reference value);
   const_iterator iterator_to(const_reference value);

For [*Boost.Intrusive] containers that have local iterators, like unordered
associative containers, we can also obtain local iterators:

[c++]

   local_iterator local_iterator_to(reference value);
   const_local_iterator local_iterator_to(const_reference value) const;

For most [*Boost.Intrusive] containers
([classref boost::intrusive::list list],
[classref boost::intrusive::slist slist],
[classref boost::intrusive::set set],
[classref boost::intrusive::multiset multiset]) we have an alternative 
static `s_iterator_to` function.

For unordered associative containers
([classref boost::intrusive::unordered_set unordered_set],
[classref boost::intrusive::multiset multiset]),
`iterator_to` has no static alternative function.
On the other hand, `local_iterator_to` functions
have their `s_local_iterator_to` static alternatives.

Alternative static functions are available under certain circunstances
explained in the [link intrusive.value_traits.stateful_value_traits Stateful value traits] section,
but the programmer uses hooks provided by [*Boost.Intrusive], those functions
will be available.

Let's see an small function that shows the use of `iterator_to` and 
`local_iterator_to`:

[import ../example/doc_iterator_from_value.cpp]
[doc_iterator_from_value]

[endsect]

[section:concepts Concepts explained]

This section will expand the explanation of previously presented basic concepts
before explaining the customization options of [*Boost.Intrusive].

*  [*Node Algorithms]: A set of static functions that implement basic operations
   on a group of nodes: initialize a node, link_mode_type a node to a group of nodes,
   unlink a node from another group of nodes... For example, a circular
   singly linked list is a group of nodes, where each node has a pointer to the
   next node. [*Node Algorithms] just require a [*NodeTraits]
   template parameter and they can work with any [*NodeTraits] class that fulfills
   the needed interface. As an example, here is a class that implements algorithms
   to manage a group of nodes forming a circular singly linked list:

[c++]

   template<class NodeTraits>
   struct my_slist_algorithms
   {
      typedef typename NodeTraits::node_ptr       node_ptr;
      typedef typename NodeTraits::const_node_ptr const_node_ptr;

      //Get the previous node of "this_node"
      static node_ptr get_prev_node(node_ptr this_node)
      {
         node_ptr p = this_node;
         while (this_node != NodeTraits::get_next(p))
            p = NodeTraits::get_next(p);
         return p;
      }

      // number of elements in the group of nodes containing "this_node"
      static std::size_t count(const_node_ptr this_node) 
      {
         std::size_t result = 0;
         const_node_ptr p = this_node;
         do{
            p = NodeTraits::get_next(p);
            ++result;
         } while (p != this_node);
         return result;
      }

      // More operations
      // ...
   };

*  [*Node Traits]: A class that encapsulates the basic information and
   operations on a node that forms a group of nodes:
   the type of the node, a function to obtain the pointer to the next node... 
   [*Node Traits] are the configuration information [*Node Algorithms]
   need. Each type of [*Node Algorithms] expects an interface that compatible
   [*Node Traits] must implement.
   As an example, this is the definition of a [*Node Traits] class that
   is compatible with the previously presented `my_slist_algorithms`:

[c++]

   struct my_slist_node_traits
   {
      //The type of the node
      struct node
      {
         node *next_;
      };      
     
      typedef node *       node_ptr;
      typedef const node * const_node_ptr;

      //A function to obtain a pointer to the next node
      static node_ptr get_next(const_node_ptr n)
      {  return n->next_;  }  

      //A function to set the pointer to the next node
      static void set_next(node_ptr n, node_ptr next)
      {  n->next_ = next;  }  
   };


*  [*Hook]: A class that the user must add as a base class or as a member to his own
   class to make that class insertable in an intrusive container. Usually the hook
   contains a node object, that will be used to form the group of nodes:
   For example, the following class is a [*Hook] that the user can add as a base class,
   to make the user class compatible with a singly linked list container:

[c++]

   class my_slist_base_hook
         //This hook contains a node, that will be used
         //to link the user object in the group of nodes
      : private my_slist_node_traits::node
   {
      typedef my_slist_node_traits::node_ptr       node_ptr;
      typedef my_slist_node_traits::const_node_ptr const_node_ptr;

      //Converts the generic node to the hook
      static my_slist_base_hook *to_hook_ptr(node_ptr p)
      {  return static_cast<my_slist_base_hook*>(p); }

      //Returns the generic node stored by this hook
      node_ptr to_node_ptr()
      { return static_cast<node *const>(this); }

      // More operations
      // ...
   };

   //To make MyClass compatible with an intrusive singly linked list
   //derive our class from the hook.
   class MyClass
      :  public my_slist_base_hook
   {
      void set(int value);
      int get() const;

      private:
      int value_;
   };

*  [*Intrusive Container]: A container that offers an STL-like interface to store
   user objects. An intrusive container can be templatized to store different
   value types that use different hooks. An intrusive container is also more elaborated
   than a group of nodes: it can store the number of elements to achieve constant-time
   size information, it can offer debugging facilities...
   For example, an [classref boost::intrusive::slist slist] container 
   (intrusive singly linked list) should
   be able to hold `MyClass` objects that might have decided to store the hook
   as a base class or as a member. Internally, the container will use [*Node Algorithms]
   to implement its operations, and an intrusive container is configured using
   a template parameter called [*ValueTraits]. [*ValueTraits] will contain
   the information to convert user classes in nodes compatible with [*Node Algorithms].
   For example, this a possible [classref boost::intrusive::slist slist] implementation:

[c++]

   template<class ValueTraits, ...>
   class slist
   {
      public:
      typedef typename ValueTraits::value_type value_type;

      //More typedefs and functions
      // ...

      //Insert the value as the first element of the list
      void push_front (reference value) 
      {
         node_ptr to_insert(ValueTraits::to_node_ptr(value));
         circular_list_algorithms::link_after(to_insert, get_root_node()); 
      }

      // More operations
      // ...
   };

*  [*Pseudo-Intrusive Container]: A pseudo-intrusive container is similar to an
   intrusive container, but apart from the values to be inserted in the container,
   it needs additional memory (for example, auxiliary arrays or indexes).

*  [*Value Traits]: As we can see, to make our classes intrusive-friendly we add
   a simple hook as a member or base class. The hook contains a generic node
   that will be inserted in a group of nodes. [*Node Algorithms] just work
   with nodes and don't know anything about user classes. On the other
   hand, an intrusive container needs to know how to obtain a node from a user class,
   and also the inverse operation.   
   So we can define [*ValueTraits] as the glue between user classes and nodes
   required by [*Node Algorithms].
   Let's see a possible implementation of a value traits class that glues MyClass
   and the node stored in the hook:

[c++]

   struct my_slist_derivation_value_traits
   {
      public:
      typedef slist_node_traits           node_traits;
      typedef MyClass                     value_type;
      typedef node_traits::node_ptr       node_ptr;
      typedef value_type*                 pointer;
      //...

      //Converts user's value to a generic node
      static node_ptr to_node_ptr(reference value)
      { return static_cast<slist_base_hook &>(value).to_node_ptr(); }

      //Converts a generic node into user's value
      static value_type *to_value_ptr(node_traits::node *n) 
      { static_cast<value_type*>(slist_base_hook::to_hook_ptr(n)); }

      // More operations
      // ...
   };

[endsect]

[section:node_algorithms Node algorithms with custom NodeTraits]

As explained in the [link intrusive.concepts Concepts] section, [*Boost.Intrusive]
containers are implemented using node algorithms that work on generic nodes.

Sometimes, the use of intrusive containers is expensive for some environments
and the programmer might want to avoid all the template instantiations
related to [*Boost.Intrusive] containers. However, the user can still benefit
from [*Boost.Intrusive] using the node algorithms, because some of those algorithms,
like red-black tree algorithms, are not trivial to write.

All node algorithm classes are
templatized by a `NodeTraits` class. This class encapsulates the needed internal
type declarations and operations to make a node compatible with node algorithms.
Each type of node algorithms has its own requirements:

[section:circular_slist_algorithms Intrusive singly linked list algorithms]

These algorithms are static
members of the [classref boost::intrusive::circular_slist_algorithms circular_slist_algorithms] class:

[c++]

   template<class NodeTraits>
   struct circular_slist_algorithms;

An empty list is formed by a node whose pointer to the next node points
to itself. [classref boost::intrusive::circular_slist_algorithms circular_slist_algorithms]
is configured with a NodeTraits class, which encapsulates 
the information about the node to be manipulated. NodeTraits must support the
following interface:

[*Typedefs]:

*  `node`: The type of the node that forms the circular list

*  `node_ptr`: The type of a pointer to a node (usually node*)

*  `const_node_ptr`: The type of a pointer to a const node (usually const node*)

[*Static functions]:

*  `static node_ptr get_next(const_node_ptr n);`:
   Returns a pointer to the next node stored in "n".

*  `static void set_next(node_ptr n, node_ptr next);`:
   Sets the pointer to the next node stored in "n" to "next".

Once we have a node traits configuration we can use [*Boost.Intrusive] algorithms
with our nodes:

[import ../example/doc_slist_algorithms.cpp]
[doc_slist_algorithms_code]

For a complete list of functions see
[classref boost::intrusive::circular_slist_algorithms circular_slist_algorithms reference].

[endsect]

[section:circular_list_algorithms Intrusive doubly linked list algorithms]

These algorithms are static
members of the [classref boost::intrusive::circular_list_algorithms circular_list_algorithms] class:

[c++]

   template<class NodeTraits>
   struct circular_list_algorithms;

An empty list is formed by a node whose pointer to the next node points
to itself. [classref boost::intrusive::circular_list_algorithms circular_list_algorithms]
is configured with a NodeTraits class, which encapsulates 
the information about the node to be manipulated. NodeTraits must support the
following interface:

[*Typedefs]:

*  `node`: The type of the node that forms the circular list

*  `node_ptr`: The type of a pointer to a node (usually node*)

*  `const_node_ptr`: The type of a pointer to a const node (usually const node*)

[*Static functions]:

*  `static node_ptr get_next(const_node_ptr n);`:
   Returns a pointer to the next node stored in "n".

*  `static void set_next(node_ptr n, node_ptr next);`:
   Sets the pointer to the next node stored in "n" to "next".

*  `static node_ptr get_previous(const_node_ptr n);`:
   Returns a pointer to the previous node stored in "n".

*  `static void set_previous(node_ptr n, node_ptr prev);`:
   Sets the pointer to the previous node stored in "n" to "prev".

Once we have a node traits configuration we can use [*Boost.Intrusive] algorithms
with our nodes:

[import ../example/doc_list_algorithms.cpp]
[doc_list_algorithms_code]

For a complete list of functions see
[classref boost::intrusive::circular_list_algorithms circular_list_algorithms reference].

[endsect]

[section:rbtree_algorithms Intrusive red-black tree algorithms]

These algorithms are static
members of the [classref boost::intrusive::rbtree_algorithms rbtree_algorithms] class:

[c++]

   template<class NodeTraits>
   struct rbtree_algorithms;


An empty tree is formed by a node whose pointer to the parent node is null,
the pointers to the left and right nodes to itself and whose color is red.
[classref boost::intrusive::rbtree_algorithms rbtree_algorithms]
is configured with a NodeTraits class, which encapsulates 
the information about the node to be manipulated. NodeTraits must support the
following interface:

[*Typedefs]:

*  `node`: The type of the node that forms the circular rbtree

*  `node_ptr`: The type of a pointer to a node (usually node*)

*  `const_node_ptr`: The type of a pointer to a const node (usually const node*)

*  `color`: The type that can store the color of a node

[*Static functions]:

*  `static node_ptr get_parent(const_node_ptr n);`:
   Returns a pointer to the parent node stored in "n".

*  `static void set_parent(node_ptr n, node_ptr p);`:
   Sets the pointer to the parent node stored in "n" to "p".

*  `static node_ptr get_left(const_node_ptr n);`:
   Returns a pointer to the left node stored in "n".

*  `static void set_left(node_ptr n, node_ptr l);`:
   Sets the pointer to the left node stored in "n" to "l".

*  `static node_ptr get_right(const_node_ptr n);`:
   Returns a pointer to the right node stored in "n".

*  `static void set_right(node_ptr n, node_ptr r);`:
   Sets the pointer to the right node stored in "n" to "r".

*  `static color get_color(const_node_ptr n);`:
   Returns the color stored in "n".

*  `static void set_color(node_ptr n, color c);`:
   Sets the color stored in "n" to "c".

*  `static color black();`:
   Returns a value representing the black color.

*  `static color red();`:
   Returns a value representing the red color.

Once we have a node traits configuration we can use [*Boost.Intrusive] algorithms
with our nodes:

[import ../example/doc_rbtree_algorithms.cpp]
[doc_rbtree_algorithms_code]

For a complete list of functions see
[classref boost::intrusive::rbtree_algorithms rbtree_algorithms reference].

[endsect]

[section:splaytree_algorithms Intrusive splay tree algorithms]

These algorithms are static
members of the [classref boost::intrusive::splaytree_algorithms splaytree_algorithms] class:

[c++]

   template<class NodeTraits>
   struct splaytree_algorithms;


An empty tree is formed by a node whose pointer to the parent node is null,
the pointers to the left and right nodes to itself.
[classref boost::intrusive::splaytree_algorithms splaytree_algorithms]
is configured with a NodeTraits class, which encapsulates 
the information about the node to be manipulated. NodeTraits must support the
following interface:

[*Typedefs]:

*  `node`: The type of the node that forms the circular splaytree

*  `node_ptr`: The type of a pointer to a node (usually node*)

*  `const_node_ptr`: The type of a pointer to a const node (usually const node*)

[*Static functions]:

*  `static node_ptr get_parent(const_node_ptr n);`:
   Returns a pointer to the parent node stored in "n".

*  `static void set_parent(node_ptr n, node_ptr p);`:
   Sets the pointer to the parent node stored in "n" to "p".

*  `static node_ptr get_left(const_node_ptr n);`:
   Returns a pointer to the left node stored in "n".

*  `static void set_left(node_ptr n, node_ptr l);`:
   Sets the pointer to the left node stored in "n" to "l".

*  `static node_ptr get_right(const_node_ptr n);`:
   Returns a pointer to the right node stored in "n".

*  `static void set_right(node_ptr n, node_ptr r);`:
   Sets the pointer to the right node stored in "n" to "r".

Once we have a node traits configuration we can use [*Boost.Intrusive] algorithms
with our nodes:

[import ../example/doc_splaytree_algorithms.cpp]
[doc_splaytree_algorithms_code]

For a complete list of functions see
[classref boost::intrusive::splaytree_algorithms splaytree_algorithms reference].

[endsect]

[section:avltree_algorithms Intrusive avl tree algorithms]

[classref boost::intrusive::avltree_algorithms avltree_algorithms] have the same
interface as [classref boost::intrusive::rbtree_algorithms rbtree_algorithms].

[c++]

   template<class NodeTraits>
   struct avltree_algorithms;

[classref boost::intrusive::avltree_algorithms avltree_algorithms]
is configured with a NodeTraits class, which encapsulates 
the information about the node to be manipulated. NodeTraits must support the
following interface:

[*Typedefs]:

*  `node`: The type of the node that forms the circular avltree

*  `node_ptr`: The type of a pointer to a node (usually node*)

*  `const_node_ptr`: The type of a pointer to a const node (usually const node*)

*  `balance`: A type that can represent 3 balance types (usually an integer)

[*Static functions]:

*  `static node_ptr get_parent(const_node_ptr n);`:
   Returns a pointer to the parent node stored in "n".

*  `static void set_parent(node_ptr n, node_ptr p);`:
   Sets the pointer to the parent node stored in "n" to "p".

*  `static node_ptr get_left(const_node_ptr n);`:
   Returns a pointer to the left node stored in "n".

*  `static void set_left(node_ptr n, node_ptr l);`:
   Sets the pointer to the left node stored in "n" to "l".

*  `static node_ptr get_right(const_node_ptr n);`:
   Returns a pointer to the right node stored in "n".

*  `static void set_right(node_ptr n, node_ptr r);`:
   Sets the pointer to the right node stored in "n" to "r".

*  `static balance get_balance(const_node_ptr n);`:
   Returns the balance factor stored in "n".

*  `static void set_balance(node_ptr n, balance b);`:
   Sets the balance factor stored in "n" to "b".

*  `static balance negative();`:
   Returns a value representing a negative balance factor.

*  `static balance zero();`:
   Returns a value representing a zero balance factor.

*  `static balance positive();`:
   Returns a value representing a positive balance factor.

Once we have a node traits configuration we can use [*Boost.Intrusive] algorithms
with our nodes:

[import ../example/doc_avltree_algorithms.cpp]
[doc_avltree_algorithms_code]

For a complete list of functions see
[classref boost::intrusive::avltree_algorithms avltree_algorithms reference].

[endsect]

[endsect]

[section:value_traits Containers with custom ValueTraits]

As explained in the [link intrusive.concepts Concepts] section, [*Boost.Intrusive]
containers need a `ValueTraits` class to perform transformations between nodes and
user values. `ValueTraits` can be explicitly configured (using the `value_traits<>` option)
or implicitly configured (using hooks and their `base_hook<>`/`member_hook<>` options).
`ValueTraits` contains
all the information to glue the `value_type` of the containers and the node to be
used in node algorithms, since these types can be different. Apart from this,
`ValueTraits` also store information about the link policy of the values to be inserted.

Instead of using [*Boost.Intrusive] predefined hooks
a user might want to develop customized containers, for example, using nodes that are
optimized for an specific
application or that are compatible with a a legacy ABI. A user might want
to have only two additional pointers in his class and insert the class in a doubly
linked list sometimes and in a singly linked list in other situations. You can't
achieve this using [*Boost.Intrusive] predefined hooks. Now, instead of using
`base_hook<...>` or `member_hook<...>` options the user will specify the
`value_traits<...>` options. Let's see how we can do this:

[section:value_traits_interface ValueTraits interface]

`ValueTraits` have the following interface:

[c++]

   #include <boost/pointer_to_other.hpp>
   #include <boost/intrusive/link_mode.hpp>

   struct my_value_traits
   {
      typedef implementation_defined                                    node_traits;
      typedef implementation_defined                                    value_type;
      typedef node_traits::node_ptr                                     node_ptr;
      typedef node_traits::const_node_ptr                               const_node_ptr;
      typedef boost::pointer_to_other<node_ptr, value_type>::type       pointer;
      typedef boost::pointer_to_other<node_ptr, const value_type>::type const_pointer;

      static const link_mode_type link_mode = some_linking_policy;

      static node_ptr       to_node_ptr    (value_type &value);
      static const_node_ptr to_node_ptr    (const value_type &value);
      static pointer        to_value_ptr   (node_ptr n);
      static const_pointer  to_value_ptr   (const_node_ptr n);
   };

Let's explain each type and function:

*  [*['node_traits]]: The node configuration that it's needed by node algorithms.
   These node traits and algorithms are
   described in the previous chapter: [link intrusive.node_algorithms Nodes Algorithms].

   *  If my_value_traits is meant to be used with [classref boost::intrusive::slist slist],
      `node_traits` should follow
      the interface needed by [classref boost::intrusive::circular_slist_algorithms circular_slist_algorithms].

   *  If my_value_traits is meant to be used with [classref boost::intrusive::list list],
      `node_traits` should follow
      the interface needed by [classref boost::intrusive::circular_list_algorithms circular_list_algorithms].

   *  If my_value_traits is meant to be used with [classref boost::intrusive::set set]/[classref boost::intrusive::multiset multiset],
      `node_traits` should follow
      the interface needed by [classref boost::intrusive::rbtree_algorithms rbtree_algorithms].

   *  If my_value_traits is meant to be used with [classref boost::intrusive::unordered_set unordered_set]/
      [classref boost::intrusive::unordered_multiset unordered_multiset], `node_traits`
      should follow the interface needed by [classref boost::intrusive::circular_slist_algorithms circular_slist_algorithms].

*  [*['node_ptr]]: A typedef for `node_traits::node_ptr`.

*  [*['const_node_ptr]]: A typedef for `node_traits::const_node_ptr`.
   
*  [*['value_type]]: The type that the user wants to insert in the container. This type can be
   the same as `node_traits::node` but it can be different (for example, `node_traits::node`
   can be a member type of `value_type`). If `value_type` and `node_traits::node` are the
   same type, the `to_node_ptr` and `to_value_ptr` functions are trivial.
   
*  [*['pointer]]: The type of a pointer to a `value_type`. It must be the same pointer type
   as `node_ptr`: If `node_ptr` is `node *` `pointer` must be `value_type*`. If
   `node_ptr` is `smart_ptr<node_traits::node>`, `pointer` must be `smart_ptr<value_type>`.
   This can be generically achieved using `boost::pointer_to_other` utility from [*Boost SmartPointers]
   defined in `<boost/pointer_to_other.hpp>`.
   
*  [*['const_pointer]]: The type of a pointer to a `const value_type`. It must be the same pointer type
   as `node_ptr`: If `node_ptr` is `node *` `const_pointer` must be `const value_type*`. If
   `node_ptr` is `smart_ptr<node_traits::node>`, `const_pointer` must be `smart_ptr<const value_type>`
   This can be generically achieved using `boost::pointer_to_other` utility from [*Boost SmartPointers]
   defined in `<boost/pointer_to_other.hpp>`.
     
*  [*['link_mode]]: Indicates that `value_traits` needs some additional work or checks from the
   container. The types are enumerations defined in the `link_mode.hpp` header.
   These are the possible types:

   *  [*`normal_link`]: If this linking policy is specified in a `ValueTraits` class
      as the link, containers
      configured with such `ValueTraits` won't set the hooks
      of the erased values to a default state. Containers also won't
      check that the hooks of the new values are default initialized.
      normal_link,

   *  [*`safe_link`]: If this linking policy is specified in a `ValueTraits` class
      as the link, containers
      configured with such `ValueTraits` will set the hooks
      of the erased values to a default state. Containers also will
      check that the hooks of the new values are default initialized.

   *  [*`auto_unlink`]: Same as "safe_link" but containers with
      constant-time size features won't be
      compatible with `ValueTraits` configured with this policy.
      Containers also know that the a value can be silently erased from
      the container without using any function provided by the containers.

*  [*['static node_ptr to_node_ptr (value_type &value)]] and
   [*['static const_node_ptr to_node_ptr (const value_type &value)]]:
   These function take a reference to a value_type and return a pointer to the node
   to be used with node algorithms.

*  [*['static pointer to_value_ptr (node_ptr n)]] and
   [*['static const_pointer to_value_ptr (const_node_ptr n)]]:
   These function take a pointer to a node and return a pointer to the value
   that contains the node.

[endsect]

[section:value_traits_example Custom ValueTraits example]

Let's define our own `value_traits` class to be able to use [*Boost.Intrusive]
containers with an old C structure whose definition can't be changed. 
That legacy type has two pointers that can be used to build singly and doubly linked
lists: in singly linked lists we only need a pointer, whereas in doubly
linked lists, we need two pointers. Since we only have two pointers, we can't insert
the object in a singly and doubly linked list at the same time. 
This is the definition of the old node:

[import ../example/doc_value_traits.cpp]
[doc_value_traits_code_legacy]

Now we have to define a NodeTraits class that will implement the functions/typedefs
that will make the legacy node compatible with [*Boost.Intrusive] algorithms. After that,
we'll define a ValueTraits class that will configure [*Boost.Intrusive] containers:

[doc_value_traits_value_traits]

Defining a value traits class that just defines `value_type` as 
`legacy_node_traits::node` is a common approach when defining customized
intrusive containers, so [*Boost.Intrusive] offers a templatized
[classref boost::intrusive::trivial_value_traits trivial_value_traits] class
that does exactly what we want:

[c++]
   
   #include <boost/intrusive/trivial_value_traits.hpp>

   //Now we can define legacy_value_traits just with a single line
   using namespace boost::intrusive;
   typedef trivial_value_traits<legacy_node_traits, normal_link> legacy_value_traits;
   

Now we can just define the containers that will store the legacy abi objects and write
a little test:

[doc_value_traits_test]

As seen, several key elements of [*Boost.Intrusive] can be reused with custom user types,
if the user does not want to use provided [*Boost.Intrusive] facilities.

[endsect]

[section:reusing_node_algorithms Reusing node algorithms for different values]

In the previous example, `legacy_node_traits::node` type and
`legacy_value_traits::value_type` are the same type, but this is not necessary. It's possible
to have several `ValueTraits` defining the same `node_traits` type (and thus, the same `node_traits::node`).
This reduces the number of node algorithm instantiations, but 
now `ValueTraits::to_node_ptr` and `ValueTraits::to_value_ptr` functions need to offer
conversions between both types. Let's see an small example:

First, we'll define the node to be used in the algorithms. For a linked list,
we just need a node that stores two pointers:

[import ../example/doc_advanced_value_traits.cpp]
[doc_advanced_value_traits_code]

Now we'll define two different types that will be inserted in intrusive lists and
we'll define a templatized `ValueTraits` that will work for both types:

[doc_advanced_value_traits_value_traits]

Now define two containers. Both containers will instantiate the same list algorithms
(`circular_list_algorithms<simple_node_traits>`),
due to the fact that the value traits used to define the containers provide the same `node_traits` type:

[doc_advanced_value_traits_containers]

All [*Boost.Intrusive] containers using predefined hooks use this technique to minimize code size:
all the possible [classref boost::intrusive::list list] containers
created with predefined hooks that define the same `VoidPointer` type
share the same list algorithms.

[endsect]

[section:simplifying_value_traits Simplifying value traits definition]

The previous example can be further simplified using
[classref boost::intrusive::derivation_value_traits derivation_value_traits]
class to define a value traits class with a value that stores the
`simple_node` as a base class:

[c++]

   #include <boost/intrusive/derivation_value_traits.hpp>

   //value_1, value_2, simple_node and simple_node_traits are defined
   //as in the previous example...
   //...

   using namespace boost::intrusive;

   //Now define the needed value traits using 
   typedef derivation_value_traits<value_1, simple_node_traits, normal_link> ValueTraits1;
   typedef derivation_value_traits<value_2, simple_node_traits, normal_link> ValueTraits2;

   //Now define the containers
   typedef list <value1, value_traits<ValueTraits1> > Value1List;
   typedef list <value2, value_traits<ValueTraits2> > Value2List;

We can even choose to store `simple_node` as a member of `value_1` and `value_2`
classes and use [classref boost::intrusive::member_value_traits member_value_traits]
to define the needed value traits classes:

[import ../example/doc_advanced_value_traits2.cpp]
[doc_advanced_value_traits2_value_traits]

[endsect]

[section:stateful_value_traits Stateful value traits]

Until now all shown custom value traits are stateless, that is, [*the transformation between nodes
and values is implemented in terms of static functions]. It's possible to use [*stateful] value traits
so that we can even separate nodes and values and [*avoid modifying types to insert nodes].
[*Boost.Intrusive] differentiates between stateful and stateless value traits checking if the ValueTraits
class is empty:

*  If the class is empty, a [*stateless] value traits is assumed.
   Node <-> Value transformations must be static functions.
*  If the class is not empty, a [*stateful] value traits is assumed.
   Node <-> Value transformations must be non-static functions.

Using stateful value traits it's possible to create containers of non-copyable/moveble objects [*without modifying]
the definition of the class to be inserted. This interesting property is achieved without using global variables
(stateless value traits could use global variables to achieve the same property), so:

*  [*Thread-safety guarantees]: Better thread-safety guarantees can be achieved with stateful
   value traits, since accessing to global resources might require syncronization primitives that
   can be avoided when using the internal state.
*  [*Flexibility]: A stateful value traits type can be configured at run-time.
*  [*Run-time polimorphism]: A value traits might implement node <-> value
   transformations as virtual functions. A single container type could be
   configured at run-time to use different node <-> value relatioships.

Stateful value traits have many advantages but also some downsides:

*  [*Performance]: Value traits operations should be very efficient since they are basic operations used by containers.
   [*A heavy node <-> value transformation can downgrade intrusive containers' performance].
*  [*Exception guarantees]: The stateful ValueTraits must maintain no-throw guarantees, otherwise, the
   container invariants won't be preserved.
*  [*Static functions]: Some static functions offered by intrusive containers are not
   available because node <-> value transformations are not static.
*  [*Bigger iterators]: The size of some iterators is increased because the iterator
   needs to store a pointer to the stateful value traits to implement node to value
   tranformations (e.g. `operator*()` and `operator->()`).

An easy and useful example of stateful value traits is when an array of values can be indirectly introduced
in a list guaranteeing no additional allocation apart from the initial resource reservation:

[import ../example/doc_stateful_value_traits.cpp]
[doc_stateful_value_traits]

[endsect]

[endsect]

[section:thread_safety Thread safety guarantees]

Intrusive containers have similar same thread-safety guarantees than STL containers.

*  Several threads can have read or write access to different instances is safe as long as inserted
   objects are different.
*  Concurrent read-only access to the same container is safe.

Some Intrusive hooks (auto-unlink hooks, for example) modify containers without
having a reference to them: this is considered a write access to the container.

Other functions, like checking if an objects is already inserted in a containers using the `is_linked()`
member of safe hooks is a read-access to the container without having a reference to them, so no other
thread should have write access (direct or indirect) to that container.

Since the same object can be inserted in several containers at the same time using different hooks,
the thread safety of [*Boost.Intrusive] is related to the containers and also the object whose lifetime
is manually managed by the user.

As we can see, the analysis of the thread-safety of a program using [*Boost.Intrusive] is harder
than with non-intrusive containers.

To analyze the thread-safety, take in care the following points:

*  Auto-unlink hook's destructor and `unlink()` functions modify the container indirectly.
*  Safe mode and auto-unlink hook's `is_linked()` function is a read access to the container.
*  Inserting an object in several containers that will be modified by different threads has no thread-safety
   guarantee, although in most platforms it will be thread-safe without locking.

[endsect]

[section:obtaining_same_type_reducing_space Obtaining the same types and reducing symbol length]

The flexible option specification mechanism used by [*Boost.Intrusive] for hooks and containers
has also a couple of downsides:

*  If a user specifies the same options in different order or specifies some options and lefts the
   rest as defaults the type of the created container/hook will be different. Sometimes
   this is annoying, because two programmers specifying the same options might end with incompatible
   types. For example, the following two lists, although they're using the same options, have not
   the same type:

[c++]

   #include <boost/intrusive/list.hpp>

   using namespace boost::intrusive;

   //Explicitly specify constant-time size and size type
   typedef list<T, constant_time_size<true>, size_type<std::size_t> List1;

   //Implicitly specify constant-time size and size type
   typedef list<T> List2;
   
*  Option specifiers lead to long template symbols for classes and functions. Option specifiers themselves
   are verbose and without variadic templates, several default template parameters are assigned for
   non-specified options. Object and debugging information files can grow and compilation times
   might suffer a bit if long names are produced.

To solve these issues [*Boost.Intrusive] offers some helper metafunctions that that reduce symbol lengths
and create the same type if the same options (either explicitly or implicitly) are used. This also
improves compilation times. All containers and hooks have their respective `make_xxx` versions.
Previous shown example can be rewritten like this to obtain the same list type:

[c++]

  #include <boost/intrusive/list.hpp>

   using namespace boost::intrusive;

   #include <boost/intrusive/list.hpp>

   using namespace boost::intrusive;

   //Explicitly specify constant-time size and size type
   typedef make_list<T, constant_time_size<true>, size_type<std::size_t>::type List1;

   //Implicitly specify constant-time size and size type
   typedef make_list<T>::type List2;

Produced symbol lengths and compilation times are usually shorter and object/debug files are smaller.
If you are a programmer concerned with file sizes and compilation times, this option is your choice.

[endsect]

[section:design_notes Design Notes]

When designing [*Boost.Intrusive] the following guidelines have been taken into account:

[section: Boost.Intrusive in performance sensitive environments]

[*Boost.Intrusive] should be a valuable tool in performance sensitive environments,
and following this guideline, [*Boost.Intrusive] has been designed to offer well
known complexity guarantees. Apart from that, some options, like optional 
constant-time, have been designed to offer faster complexity guarantees in some
functions, like `slist::splice`.

The advanced lookup and insertion functions for associative containers, taking
an arbitrary key type and predicates, were designed to avoid unnecessary object
constructions.

[endsect]

[section: Boost.Intrusive in space constrained environments]

[*Boost.Intrusive] should be useful in space constrained environments,
and following this guideline [*Boost.Intrusive] separates node algorithms
and intrusive containers to avoid instantiating node algorithms for each
user type. For example, a single class of red-black algorithms will be instantiated
to implement all set and multiset containers using raw pointers. This way,
[*Boost.Intrusive] wants to avoid any code size overhead associated with templates.

Apart from that, [*Boost.Intrusive] implements some size improvements: for example,
red-black trees embed the color bit in the parent pointer lower bit, if nodes
are two-byte aligned. The possibility to avoid constant-time size operations can
save some size on containers, and this extra size optimization is noticeable
when the container is empty or contains few values.

[endsect]

[section: Boost.Intrusive as basic building block]

[*Boost.Intrusive] should be a basic building block to build more complex containers
and this guideline has motivated many design decisions. For example, the possibility
to have more than one hook per user type opens the possibility to implement multi-index
containers on top of [*Boost.Intrusive].

[*Boost.Intrusive] containers implement advanced functions taking function objects
as arguments (`clone_from`, `erase_and_dispose`, `insert_check`...). These
functions come handy when implementing non-intrusive containers
(for example, STL-like containers) on top of intrusive containers.

[endsect]

[section: Extending Boost.Intrusive]

[*Boost.Intrusive] offers a wide range of containers but also allows the
construction of custom containers reusing [*Boost.Intrusive] elements.
The programer might want to use node algorithms directly or
build special hooks that take advantage of its application environment.

For example, the programmer can use can customize parts of [*Boost.Intrusive]
to manage old data structures whose definition can't be changed.

[endsect]

[endsect]

[section:performance Performance]

[*Boost.Intrusive] containers offer speed improvements comparing to non-intrusive containers,
basically because:

*  We can minimize memory allocation/deallocation calls.
*  We obtain better memory locality.

This section will show some performance tests comparing some operations on 
`boost::intrusive::list` and `std::list`:

*  Insertions using `push_back` and container destruction will show the
   overhead associated with memory allocation/deallocation.
*  `reverse` member function will show the advantages of the compact
   memory representation that can be achieved with intrusive containers.
*  `sort` and `write access` tests will show the advantage of intrusive containers
   minimizing the memory accesses when comparing them with containers of pointers.

Given an object of type `T`, [classref boost::intrusive::list boost::intrusive::list<T>]
can replace `std::list<T>` to avoid memory allocation overhead,
or it can replace `std::list<T*>` when the user wants to obtain containers with
polymorphic values or wants to share values between several containers.
Because of this versatility, the performance tests will be executed for 6 different
list types:

*  3 intrusive lists holding a class named `itest_class`,
   each one with a different linking policy (`normal_link`, `safe_link`, `auto_unlink`).
   The `itest_class` objects will be tightly packed in a `std::vector<itest_class>` object.

*  `std::list<test_class>`, where `test_class` is exactly the same as `itest_class`,
   but without the intrusive hook.

*  `std::list<test_class*>`. The list will contain pointers to `test_class` objects
   tightly packed in a `std::vector<test_class>` object. We'll call this configuration ['compact pointer list]

*  `std::list<test_class*>`. The list will contain pointers to `test_class` objects owned by a
   `std::list<test_class>` object. We'll call this configuration ['disperse pointer list].

Both `test_class` and `itest_class` are templatized classes that can be configured with
a boolean to increase the size of the object. This way, the tests can be executed with
small and big objects. Here is the first part of the testing code, which shows
the definitions of `test_class` and `itest_class` classes, and some other
utilities:

[import ../perf/perf_list.cpp]
[perf_list_value_type]

As we can see, `test_class` is a very simple class holding an `int`. `itest_class`
is just a class that has a base hook ([classref boost::intrusive::list_base_hook list_base_hook])
and also derives from `test_class`.

`func_ptr_adaptor` is just a functor adaptor to convert function objects taking
`test_list` objects to funtion objects taking pointers to them.

You can find the full test code code in the 
[@../../perf/perf_list.cpp perf_list.cpp] source file.

[section:performance_results_push_back Back insertion and destruction]

The first test will measure the benefits we can obtain with intrusive containers
avoiding memory allocations and deallocations . All the objects to be
inserted in intrusive containers are allocated in a single allocation call,
whereas `std::list` will need to allocate memory for every and deallocate it
for every erasure (or container destruction).

Let's compare the code to be executed for each container type for different insertion tests:

[perf_list_push_back_intrusive]

For intrusive containers, all the values are created in a vector and after that
inserted in the intrusive list.

[perf_list_push_back_stdlist]

For a standard list, elements are pushed back using push_back().

[perf_list_push_back_stdptrlist]

For a standard compact pointer list, elements are created in a vector and pushed back
in the pointer list using push_back().

[perf_list_push_back_disperse_stdptrlist]

For a ['disperse pointer list], elements are created in a list and pushed back
in the pointer list using push_back().

These are the times in microseconds for each case, and the normalized time:

[table Back insertion + destruction times for Visual C++ 7.1 / Windows XP
    [[Container]                       [Time in us/iteration (small object / big object)] [Normalized time (small object / big object)]]
    [[`normal_link` intrusive list]     [5000 / 22500]                        [1 / 1]]
    [[`safe_link` intrusive list]       [7812 / 32187]                        [1.56 / 1.43]]
    [[`auto_unlink` intrusive list]     [10156 / 41562]                       [2.03 / 1.84]]
    [[Standard list]                    [76875 / 97500]                       [5.37 / 4.33]]
    [[Standard compact pointer list]    [76406 / 86718]                      [15.28 / 3.85]]
    [[Standard disperse pointer list]  [146562 / 175625]                     [29.31 / 7.80]]
]

[table Back insertion + destruction times for GCC 4.1.1 / MinGW over Windows XP
    [[Container]                       [Time in us/iteration (small object / big object)] [Normalized time (small object / big object)]]
    [[`normal_link` intrusive list]     [4375 / 22187]                  [1 / 1]]
    [[`safe_link` intrusive list]       [7812 / 32812]                  [1.78 / 1.47]]
    [[`auto_unlink` intrusive list]     [10468 / 42031]                 [2.39 / 1.89]]
    [[Standard list]                    [81250 / 98125]                [18.57 / 4.42]]
    [[Standard compact pointer list]    [83750 / 94218]                [19.14 / 4.24]]
    [[Standard disperse pointer list]  [155625 / 175625]                [35.57 / 7.91]]
]

[table Back insertion + destruction times for GCC 4.1.2 / Linux Kernel 2.6.18 (OpenSuse 10.2)
    [[Container]                       [Time in us/iteration (small object / big object)] [Normalized time (small object / big object)]]
    [[`normal_link` intrusive list]     [4792 / 20495]                  [1 / 1]]
    [[`safe_link` intrusive list]       [7709 / 30803]                  [1.60 / 1.5]]
    [[`auto_unlink` intrusive list]     [10180 / 41183]                 [2.12 / 2.0]]
    [[Standard list]                    [17031 / 32586]                 [3.55 / 1.58]]
    [[Standard compact pointer list]    [27221 / 34823]                 [5.68 / 1.69]]
    [[Standard disperse pointer list]  [102272 / 60056]                [21.34 / 2.93]]
]

The results are logical: intrusive lists just need one allocation. The destruction
time of the `normal_link` intrusive container is trivial (complexity: `O(1)`),
whereas `safe_link` and `auto_unlink` intrusive containers need to put the hook of
erased values' in the default state (complexity: `O(NumElements)`). That's why
`normal_link` intrusive list shines in this test.

Non-intrusive containers need to make much more allocations and that's why they are
lagging behind. The `disperse pointer list` needs to make `NumElements*2` allocations,
so the result is not surprising.

Linux test shows that standard containers perform very well against intrusive containers
with big objects. Nearly the same GCC version in MinGW performs worse, so maybe the
a good memory allocator is the reason for these excelent results.

[endsect]

[section:performance_results_reversing Reversing]

The next test measures the time needed to complete calls to the member function `reverse()`.
Values (`test_class` and `itest_class`) and lists are created like explained in the
previous section.

Note that for pointer lists, `reverse` [*does not need to access `test_class` values
stored in another list or vector],
since this function just needs to adjust internal pointers, so in theory, all tested
lists need to perform the same operations.

These are the results:

[table Reverse times for Visual C++ 7.1 / Windows XP
    [[Container]                       [Time in us/iteration (small object / big object)] [Normalized time (small object / big object) (small object / big object)]]
    [[`normal_link` intrusive list]     [2656 / 10625]                 [1 / 1.83]]
    [[`safe_link` intrusive list]       [2812 / 10937]                 [1.05 / 1.89]]
    [[`auto_unlink` intrusive list]     [2710 / 10781]                 [1.02 / 1.86]]
    [[Standard list]                    [5781 / 14531]                 [2.17 / 2.51]]
    [[Standard compact pointer list]    [5781 / 5781]                  [2.17 / 1]]
    [[Standard disperse pointer list]  [10781 / 15781]                 [4.05 / 2.72]]
]

[table Reverse times for GCC 4.1.1 / MinGW over Windows XP
    [[Container]                       [Time in us/iteration (small object / big object)] [Normalized time (small object / big object)]]
    [[`normal_link` intrusive list]     [2656 / 10781]                 [1 / 2.22]]
    [[`safe_link` intrusive list]       [2656 / 10781]                 [1 / 2.22]]
    [[`auto_unlink` intrusive list]     [2812 / 10781]                 [1.02 / 2.22]]
    [[Standard list]                    [4843 / 12500]                 [1.82 / 2.58]]
    [[Standard compact pointer list]    [4843 / 4843]                  [1.82 / 1]]
    [[Standard disperse pointer list]   [9218 / 12968]                 [3.47 / 2.67]]
]

[table Reverse times for GCC 4.1.2 / Linux Kernel 2.6.18 (OpenSuse 10.2)
    [[Container]                       [Time in us/iteration (small object / big object)] [Normalized time (small object / big object)]]
    [[`normal_link` intrusive list]     [2742 / 10847]                 [1 / 3.41]]
    [[`safe_link` intrusive list]       [2742 / 10847]                 [1 / 3.41]]
    [[`auto_unlink` intrusive list]     [2742 / 11027]                 [1 / 3.47]]
    [[Standard list]                    [3184 / 10942]                 [1.16 / 3.44]]
    [[Standard compact pointer list]    [3207 / 3176]                  [1.16 / 1]]
    [[Standard disperse pointer list]   [5814 / 13381]                 [2.12 / 4.21]]
]

For small objects the results show that the compact storage of values in intrusive
containers improve locality and reversing is faster than with standard containers,
whose values might be dispersed in memory because each value is independently
allocated. Note that the dispersed pointer list (a list of pointers to values
allocated in another list) suffers more because nodes of the pointer list
might be more dispersed, since allocations from both lists are interleaved
in the code:

[c++]

   //Object list (holding `test_class`)
   stdlist objects;

   //Pointer list (holding `test_class` pointers)
   stdptrlist l;

   for(int i = 0; i < NumElements; ++i){
      //Allocation from the object list
      objects.push_back(stdlist::value_type(i));
      //Allocation from the pointer list
      l.push_back(&objects.back());
   }

For big values the compact pointer list wins because when reversing doesn't need access
to the values stored in another container. Since all the allocations for nodes of
this pointer list are likely to be near (since there is no other allocation in the
process until the pointer list is created) locality is better than with intrusive
containers. The dispersed pointer list, like with small values, has poor locality.

[endsect]

[section:performance_results_sorting Sorting]

The next test measures the time needed to complete calls the member function
`sort(Pred pred)`. Values (`test_class` and `itest_class`) and lists are created like explained in the
first section. The values will be sorted in ascending and descenting order each
iteration. For example, if ['l] is a list:

[c++]

   for(int i = 0; i < NumIter; ++i){
      if(!(i % 2))
         l.sort(std::greater<stdlist::value_type>());
      else
         l.sort(std::less<stdlist::value_type>());
   }

For a pointer list, the function object will be adapted using `func_ptr_adaptor`:

[c++]

   for(int i = 0; i < NumIter; ++i){
      if(!(i % 2))
         l.sort(func_ptr_adaptor<std::greater<stdlist::value_type> >());
      else
         l.sort(func_ptr_adaptor<std::less<stdlist::value_type> >());
   }

Note that for pointer lists, `sort` will take a function object that [*will access
`test_class` values stored in another list or vector], so pointer lists will suffer
an extra indirection: they will need to access the `test_class` values stored in
another container to compare to elements.

These are the results:

[table Sort times for Visual C++ 7.1 / Windows XP
    [[Container]                       [Time in us/iteration (small object / big object)] [Normalized time (small object / big object)]]
    [[`normal_link` intrusive list]     [16093 / 38906]                [1 / 1]]
    [[`safe_link` intrusive list]       [16093 / 39062]                [1 / 1]]
    [[`auto_unlink` intrusive list]     [16093 / 38906]                [1 / 1]]
    [[Standard list]                    [32343 / 56406]                [2.0 / 1.44]]
    [[Standard compact pointer list]    [33593 / 46093]                [2.08 / 1.18]]
    [[Standard disperse pointer list]   [46875 / 68593]                [2.91 / 1.76]]
]

[table Sort times for GCC 4.1.1 / MinGW over Windows XP
    [[Container]                       [Time in us/iteration (small object / big object)] [Normalized time (small object / big object)]]
    [[`normal_link` intrusive list]     [15000 / 39218]                [1 / 1]]
    [[`safe_link` intrusive list]       [15156 / 39531]                [1.01 / 1.01]]
    [[`auto_unlink` intrusive list]     [15156 / 39531]                [1.01 / 1.01]]
    [[Standard list]                    [34218 / 56875]                [2.28 / 1.45]]
    [[Standard compact pointer list]    [35468 / 49218]                [2.36 / 1.25]]
    [[Standard disperse pointer list]   [47656 / 70156]                [3.17 / 1.78]]
]

[table Sort times for GCC 4.1.2 / Linux Kernel 2.6.18 (OpenSuse 10.2)
    [[Container]                       [Time in us/iteration (small object / big object)] [Normalized time (small object / big object)]]
    [[`normal_link` intrusive list]     [18003 / 40795]                [1 / 1]]
    [[`safe_link` intrusive list]       [18003 / 41017]                [1 / 1]]
    [[`auto_unlink` intrusive list]     [18230 / 40941]                [1.01 / 1]]
    [[Standard list]                    [26273 / 49643]                [1.45 / 1.21]]
    [[Standard compact pointer list]    [28540 / 43172]                [1.58 / 1.05]]
    [[Standard disperse pointer list]   [35077 / 57638]                [1.94 / 1.41]]
]

The results show that intrusive containers are faster than standard
containers. We can see that the pointer
list holding pointers to values stored in a vector is quite fast, so the extra
indirection that needs to access the value is minimized because all the values
are tightly stored, improving cache. The disperse list, on the other hand, is
slower because the indirection to access to values stored in the object list is
more expensive than the access to values stored in a vector.

[endsect]

[section:performance_results_write_access Write access]

The next test measures the time needed to iterate all the elements of a list, and
increment the value of the internal `i_` member:

[c++]

   stdlist::iterator it(l.begin()), end(l.end());
   for(; it != end; ++it)
      ++(it->i_);

Values (`test_class` and `itest_class`) and lists are created like explained in
the first section. Note that for pointer lists, the iteration will suffer
an extra indirection: they will need to access the `test_class` values stored in
another container:

[c++]

   stdptrlist::iterator it(l.begin()), end(l.end());
   for(; it != end; ++it)
      ++((*it)->i_);

These are the results:

[table Write access times for Visual C++ 7.1 / Windows XP
    [[Container]                       [Time in us/iteration (small object / big object)] [Normalized time (small object / big object)]]
    [[`normal_link` intrusive list]     [2031 / 8125]                 [1 / 1]]
    [[`safe_link` intrusive list]       [2031 / 8281]                 [1 / 1.01]]
    [[`auto_unlink` intrusive list]     [2031 / 8281]                 [1 / 1.01]]
    [[Standard list]                    [4218 / 10000]                [2.07 / 1.23]]
    [[Standard compact pointer list]    [4062 / 8437]                 [2.0 / 1.03]]
    [[Standard disperse pointer list]   [8593 / 13125]                [4.23 / 1.61]]
]

[table Write access times for GCC 4.1.1 / MinGW over Windows XP
    [[Container]                       [Time in us/iteration (small object / big object)] [Normalized time (small object / big object)]]
    [[`normal_link` intrusive list]     [2343 / 8281]                 [1 / 1]]
    [[`safe_link` intrusive list]       [2500 / 8281]                 [1.06 / 1]]
    [[`auto_unlink` intrusive list]     [2500 / 8281]                 [1.06 / 1]]
    [[Standard list]                    [4218 / 10781]                [1.8 / 1.3]]
    [[Standard compact pointer list]    [3906 / 8281]                 [1.66 / 1]]
    [[Standard disperse pointer list]   [8281 / 13750]                [3.53 / 1.66]]
]

[table Write access times for GCC 4.1.2 / Linux Kernel 2.6.18 (OpenSuse 10.2)
    [[Container]                       [Time in us/iteration (small object / big object)] [Normalized time (small object / big object)]]
    [[`normal_link` intrusive list]     [2286 / 8468]                 [1 / 1.1]]
    [[`safe_link` intrusive list]       [2381 / 8412]                 [1.04 / 1.09]]
    [[`auto_unlink` intrusive list]     [2301 / 8437]                 [1.01 / 1.1]]
    [[Standard list]                    [3044 / 9061]                 [1.33 / 1.18]]
    [[Standard compact pointer list]    [2755 / 7660]                 [1.20 / 1]]
    [[Standard disperse pointer list]   [6118 / 12453]                [2.67 / 1.62]]
]

Like with the read access test, the results show that intrusive containers outperform
all other containers if the values are tightly packed in a vector.
The disperse list is again the slowest one.

[endsect]

[section:performance_results_conclusions Conclusions]

Intrusive containers can offer performance benefits that can not be achieved with
equivalent non-intrusive containers. Memory locality improvements are noticeable
when objects to be inserted are small. Minimizing memory allocation/deallocation calls is also
an important factor and intrusive containers make this simple if the user allocates
all the objects to be inserted in intrusive containers in containers like `std::vector` or `std::deque`.

[endsect]

[endsect]

[section:disabling_exceptions Disabling exceptions support]

[*Boost.Intrusive] might be useful in environments where exceptions are not available
or recommendable (like embedded or real-time systems). [*Boost.Intrusive] uses the
global Boost mechanism to disable exception handling, so that if the compiler
configuration disables exceptions, `BOOST_NO_EXCEPTIONS` is defined and exception
handling is disabled.

This mechanism is a global mechanism to disable exceptions. If for any reason,
the user wants to disable exception handling [*only] in [*Boost.Intrusive],
`BOOST_INTRUSIVE_DISABLE_EXCEPTION_HANDLING` can be defined to disable
exception handling in the library.

[endsect]

[section:tested_compilers Tested compilers]

[*Boost.Intrusive] has been tested in the following compilers/platforms:

*  Visual 7.1/WinXP
*  Visual 8.0/WinXP
*  GCC 4.1.1/MinGW
*  GCC 3.4.4/Cygwin
*  Intel 9.1/WinXP
*  GCC 4.1.2/Linux
*  GCC 3.4.3/Solaris 11
*  GCC 4.0/Mac Os 10.4.1
*  SunCC 5.8/Solaris 11

[endsect]

[section:references References]

* SGI's [@http://www.sgi.com/tech/stl/ STL Programmer's Guide].
   [*Boost.Intrusive] is based on STL concepts and interface.

* Dr. Dobb's, September 1, 2005: [@http://www.ddj.com/architect/184402007 ['Implementing Splay Trees in C++] ].
   [*Boost.Intrusive] splay containers code is based on this article.

* Olaf's original intrusive container library: [@http://freenet-homepage.de/turtle++/intrusive.html ['STL-like intrusive containers] ].

[endsect]

[section:acknowledgments Acknowledgements]

[*Olaf Krzikalla] would like to thank:

*  [*Markus Schaaf] for pointing out the possibility and the advantages of the derivation 
approach. 

*  [*Udo Steinbach] for encouragements to present this work for boost, a lot of fixes and
helpful discussions. 

*  [*Jaap Suter] for the initial hint, which eventually lead to the member value_traits.

[*Ion Gaztanaga] would like to thank:

*  [*Olaf Krzikalla] for the permission to continue his great work.

*  [*Joaquín M. López Muñoz] for his thorough review, help, and ideas.

*  [*Cory Nelson], [*Daniel James], [*Dave Harris], [*Guillaume Melquiond],
   [*Henri Bavestrello], [*Hervé Brönnimann], [*Kai Brüning], [*Kevin Sopp],
   [*Paul Rose], [*Pavel Vozelinek], [*Howard Hinnant], [*Olaf Krzikalla],
   [*Samuel Debionne], [*Stjepan Rajko], [*Thorsten Ottosen], [*Tobias Schwinger],
   [*Tom Brinkman] and [*Steven Watanabe]
   for their comments and reviews in the Boost.Intrusive formal review.

*  Thanks to of [*Julienne Walker] and [*The EC Team] ([@http://eternallyconfuzzled.com])
   for their great algorithms.

*  Thanks to [*Daniel K. O.] for his AVL tree rebalancing code.

*  Thanks to [*Ralf Mattethat] for his splay tree article and code.

*  Special thanks to [*Steven Watanabe] and [*Tobias Schwinger] for their
   invaluable suggestions and improvements.

[endsect]

[xinclude autodoc.xml]

[section:license_notices License notices]

Most of the internal implementation of red-black trees is based on that of SGI STL stl_tree.h file:

['Copyright (c) 1996,1997 Silicon Graphics Computer Systems, Inc. 
Permission to use, copy, modify, distribute and sell this software and its documentation for any purpose is hereby granted without fee, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation. Silicon Graphics makes no representations about the suitability of this software for any purpose. It is provided "as is" without express or implied warranty.]

['Copyright (c) 1994 Hewlett-Packard Company 
Permission to use, copy, modify, distribute and sell this software and its documentation for any purpose is hereby granted without fee, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation. Hewlett-Packard Company makes no representations about the suitability of this software for any purpose. It is provided "as is" without express or implied warranty.]

The tree destruction algorithm is based on Julienne Walker and The EC Team code:

['This code is in the public domain. Anyone may
use it or change it in any way that they see
fit. The author assumes no responsibility for 
damages incurred through use of the original
code or any variations thereof.]

['It is requested, but not required, that due
credit is given to the original author and
anyone who has modified the code through
a header comment, such as this one.]

[endsect]
