# Pointer Container Library

## Conventions

There are a few design decisions that will affect how the classes are used. Besides these the classes are much like normal standard containers and provides almost the same interface. The new conventions are:

- Null pointers are not allowed by default
- All default iterators apply an extra layer of indirection
- All comparison operations are done on the pointed to objects and not at the pointer level
- The containers are neither Copy Constructible nor Assignable
- Stored elements are required to be Clonable for a subset of the operations
- Whenever objects are inserted into a container, they are cloned before insertion
- Whenever pointers are inserted into a container, ownership is transferred to the container
- Ownership can be transferred from a container on a per pointer basis
- Ownership can be transferred from a container to another container on a per iterator range basis
- A container can be cheaply returned from functions either by making a clone or by giving up ownership of the container
- Iterators are invalidated as in the corresponding standard container

# Null pointers are not allowed by default

If the user tries to insert the null pointer, the operation will throw a `bad_pointer` exception (see Example 1).

Use nullable to allow null pointers.

Please notice that all preconditions of the form

    x != 0;

are not active when the you have instantiated a container with `nullable<T>` as in

    boost::ptr_vector< boost::nullable<animal> > vec;
    vec.push_back( 0 ); // ok

# All default iterators apply an extra layer of indirection

This is done to make the containers easier and safer to use. It promotes a kind of pointer-less programming and the user of a class needs not worry about pointers except when allocating them (see Example 2). Iterators that provide access to the naked pointers are also provided since they might be useful in rare cases. For example, whenever `begin()` returns an iterator, `ptr_begin()` will return an iterator that allows one to iterate over the stored pointers.

# All comparison operations are done on the pointed to objects and not at the pointer level

For example, in `ptr_set<T>` the ordering is by default done by `boost::ptr_less<T>` which compares the indirected pointers. Similarly, `operator==()` for `container<Foo>` compares all objects with `operator==(const Foo&, const Foo&)`.

# The containers are neither Copy Constructible nor Assignable

This is because cloning a lot of pointers can be a very expensive operation; instead functions are provided to transfer ownership. If a deep-copy is needed anyway, every container has a `clone()` member function (see Example 3).

# Stored elements are required to be Clonable for a subset of the operations

This is because most polymorphic objects cannot be copied directly, but they can often be so by a use of a member function (see Example 4). Often it does not even make sense to clone an object in which case a large subset of the operations are still workable.

# Whenever objects are inserted into a container, they are cloned before insertion

This is necessary because all pointer containers take ownerships of stored objects (see Example 5).

# Whenever pointers are inserted into a container, ownership is transferred to the container

All containers take ownership of the stored pointers and therefore a container needs to have its own copies (see Example 5).

# Ownership can be transferred from a container on a per pointer basis

This can of course also be convenient. Whenever it happens, an `SmartContainer::auto_type` object is used to provide an exception-safe transfer (see Example 6).

# Ownership can be transferred from a container to another container on a per iterator range basis

This makes it possible to exchange data safely between different pointer containers without cloning the objects again (see Example 7).

# A container can be cheaply returned from functions either by making a clone or by giving up ownership of the container

Two special member functions, `clone()` and `release()`, both return an `auto_ptr<SmartContainer>` which can be assigned to another pointer container. This effectively reduces the cost of returning a container to one heap-allocation plus a call to `swap()` (see Example 3).

# Iterators are invalidated as in the corresponding standard container

Because the containers in this library wrap standard containers, the rules for invalidation of iterators are the same as the rules of the corresponding standard container.

For example, for both `boost::ptr_vector<T>` and `std::vector<U>` insertion and deletion only invalidates the deleted element and elements following it; all elements before the inserted/deleted element remain valid.

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copyright: | Thorsten Ottosen 2004-2005.