| # Assignment Library

Copyright © 2003-2004 Thorsten Ottosen

Use, modification and distribution is subject to the Boost Software License, Version 1.0 (see http://www.boost.org/LICENSE_1_0.txt).

### Table of Contents

- Introduction
- Tutorial
  - Function `operator+=()`
  - Function `operator()()`
  - Function `list_of()`
  - Function `map_list_of()`
  - Function `tuple_list_of()`
  - Functions `repeat()` and `repeat_fun()`
  - Functions `ref_list_of()` and `cref_list_of()`
  - A "complicated" example
  - Functions `ptr_push_back(), ptr_push_front()` and `ptr_insert()`
  - Function `ptr_list_of()`
- Reference
  - Headers
  - Standard containers
  - Functions `list_of()` and `map_list_of()`
  - Functions `repeat()` and `repeat_fun()`
  - Class `list_inserter`
  - Function `make_list_inserter()`
  - Customizing argument list sizes
- Exceptions and exception-safety
- Extending the library
- Supported libraries
- Examples

- Portability
- History and Acknowledgment
- References

## Introduction

There appear to be few practical uses of `operator,()`.
Bjarne Stroustrup, The Design and Evolution of C++

The purpose of this library is to make it easy to fill containers with data by overloading `operator,()` and `operator()()`. These two operators make it possible to construct lists of values that are then copied into a container:

- A comma-separated list:

      vector<int> v;
      v += 1,2,3,4,5,6,7,8,9;

- A parenthesis-separated list:

      map<string,int> m;
      insert( m )( "Bar", 1 )( "Foo", 2 );

These lists are particularly useful in learning, testing, and prototyping situations, but can also be handy otherwise. The library comes with predefined operators for the containers of the standard library, but most functionality will work with any standard compliant container. The library also makes it possible to extend user defined types so for example a member function can be called for a list of values instead of its normal arguments.

## Tutorial

Within two minutes you should be able to use this library. The main components are explained in these sections:

- Function `operator+=()`
- Function `operator()()`
- Function `list_of()`
- Function `map_list_of()`
- Function `tuple_list_of()`
- Functions `repeat()` and `repeat_fun()`
- Functions `ref_list_of()` and `cref_list_of()`
- A "complicated" example
- Functions `ptr_push_back(), ptr_push_front()` and `ptr_insert()`
- Function `ptr_list_of()`

The two first functions are used for adding elements after a container object has been created whereas the next two is used when we need to initialize an object.

### Function `operator+=()`

To fill a vector (or any standard container) with values using `operator+=()` you write

>     #include <boost/assign/std/vector.hpp> // for 'operator+=()'
>     #include <boost/assert.hpp>;
>     using namespace std;
>     using namespace boost::assign; // bring 'operator+=()' into scope

>     {
>         vector<int> values;
>         values += 1,2,3,4,5,6,7,8,9; // insert values at the end of the container
>         BOOST_ASSERT( values.size() == 9 );
>         BOOST_ASSERT( values[0] == 1 );
>         BOOST_ASSERT( values[8] == 9 );
>     }

Here we only stuffed constants into the container, but the list can consists of arbitrary expressions as long as the result of each expression is convertible to the `value_type` of the container.

### Function `operator()()`

We do not call `operator()()` directly, but instead we call a function that returns a proxy-object that defines `operator()()`. The function that returns the proxy object is always named after the member function that is used to copy the values in the list into the container. So to fill a map with pairs of values you write

>     #include <boost/assign/list_inserter.hpp> // for 'insert()'
>     #include <boost/assert.hpp>
>     #include <string>
>     using namespace std;
>     using namespace boost::assign; // bring 'insert()' into scope

>     {
>         map<string,int> months;
>         insert( months )
>             ( "january",   31 )( "february", 28 )
>             ( "march",     31 )( "april",    30 )
>             ( "may",       31 )( "june",     30 )
>             ( "july",      31 )( "august",   31 )
>             ( "september", 30 )( "october",  31 )
>             ( "november",  30 )( "december", 31 );
>         BOOST_ASSERT( months.size() == 12 );
>         BOOST_ASSERT( months["january"] == 31 );
>     }

Note that `operator()()` is much more handy when we need to construct objects using several arguments (up to five arguments are supported by default, but the limit can be customized). This is also true for sequences:

>     #include <boost/assign/list_inserter.hpp> // for 'push_front()'
>     #include <boost/assert.hpp>
>     #include <string>
>     #include <utility>
>     using namespace std;
>     using namespace boost::assign; // bring 'push_front()' into scope

>     {
>         typedef pair< string,string > str_pair;
>         deque<str_pair> deq;
>         push_front( deq )( "foo", "bar")( "boo", "far" );
>         BOOST_ASSERT( deq.size() == 2 );
>         BOOST_ASSERT( deq.front().first == "boo" );
>         BOOST_ASSERT( deq.back().second == "bar" );
>     }

Besides `push_front()` we could also have used `push_back()` if the container has a corresponding member function. Empty parentheses can be used to insert default-constructed objects, for example, `push_front( deq )()()` will insert two default-constructed `str_pair` objects.

If `operator()()` is too cumbersome to use with eg. `push_front()`we can also say

>     deque<int> di;
>     push_front( di ) = 1,2,3,4,5,6,7,8,9;
>     BOOST_ASSERT( di.size() == 9 );
>     BOOST_ASSERT( di[0] == 9 );

Just to make it perfectly clear, the code above is not restricted to the standard containers, but will work with all standard compliant containers with the right member function. It is only `operator+=()` that has been restricted to the standard containers.

### Function `list_of()`

But what if we need to initialize a container? This is where `list_of()` comes into play. With `list_of()` we can create anonymous lists that automatically converts to any container:

>     #include <boost/assign/list_of.hpp> // for 'list_of()'
>     #include <boost/assert.hpp>
>     #include <list>
>     #include <stack>
>     #include <string>
>     using namespace std;
>     using namespace boost::assign; // bring 'list_of()' into scope

>     {
>         const list<int> primes = list_of(1)(2)(3)(5)(7)(11);
>         BOOST_ASSERT( primes.size() == 6 );
>         BOOST_ASSERT( primes.back() == 11 );
>         BOOST_ASSERT( primes.front() == 1 );

>         const stack<string> names = list_of( "Mr. Foo" )( "Mr. Bar")( "Mrs. FooBar" ).to_adapter();
>         const stack<string> names2 = (list_of( "Mr. Foo" ), "Mr. Bar", "Mrs. FooBar" ).to_adapter();
>         BOOST_ASSERT( names.size() == 3 );
>         BOOST_ASSERT( names.top() == "Mrs. FooBar" );
>     }

If we need to initialize a container adapter, we need to help the compiler a little by calling `to_adapter()`. As the second example also shows, we can use a comma-separated list with `list_of()` if we add parenthesis around the entire right hand side. It is worth noticing that the first argument of `list_of()` determines the type of the anonymous list. In case of the stack, the anonymous list consists of `const char*` objects which are then converted to a stack of `string` objects. The conversion is always possible as long as the conversion between the stored types is possible.

Please notice that `list_of()` can even convert to a `boost::array<T,sz>` and see also the list of supported libraries .

### Function `map_list_of()`

This function is defined for pure convenience when working with maps. Its usage is simple:

>     #include <boost/assign/list_of.hpp> // for 'map_list_of()'
>     #include <boost/assert.hpp>
>     #include <map>
>     using namespace std;
>     using namespace boost::assign; // bring 'map_list_of()' into scope

>     {
>         map<int,int> next = map_list_of(1,2)(2,3)(3,4)(4,5)(5,6);
>         BOOST_ASSERT( next.size() == 5 );
>         BOOST_ASSERT( next[ 1 ] == 2 );
>         BOOST_ASSERT( next[ 5 ] == 6 );

>         // or we can use 'list_of()' by specifying what type
>         // the list consists of
>         next = list_of< pair<int,int> >(6,7)(7,8)(8,9);
>         BOOST_ASSERT( next.size() == 3 );
>         BOOST_ASSERT( next[ 6 ] == 7 );
>         BOOST_ASSERT( next[ 8 ] == 9 );
>     }

The function `pair_list_of()` may also be used.

### Function `tuple_list_of()`

If you are working with tuples, it might be convenient to use `tuple_list_of()`:

>     #include <boost/assign/list_of.hpp>
>     #include <vector>

>     using namespace std;
>     using namespace boost::assign;

>     {
>         typedef boost::tuple<int,std::string,int> tuple;

>         vector<tuple> v = tuple_list_of( 1, "foo", 2 )( 3, "bar", 4 );
>         BOOST_CHECK( v.size() == 2 );
>         BOOST_CHECK( boost::get<0>( v[1] ) ==  3 );
>     }

### Functions `repeat()` and `repeat_fun()`

Sometimes it is too irritating to repeat the same value many times. This is where `repeat()` can be handy:

>     #include <boost/assign/list_of.hpp>
>     #include <boost/assign/std/vector.hpp>
>     #include <boost/assert.hpp>

>     using namespace std;
>     using namespace boost::assign;

>     {
>         vector<int> v;
>         v += 1,2,3,repeat(10,4),5,6,7,8,9;
>         // v = [1,2,3,4,4,4,4,4,4,4,4,4,4,5,6,7,8,9]
>         BOOST_ASSERT( v.size() == 3 + 10 + 5 );

>         v = list_of(1).repeat(5,2)(3);
>         // v = [1,2,2,2,2,2,3]
>         BOOST_ASSERT( v.size() == 1 + 5 + 1 );

>         push_back( v )(1).repeat(1,2)(3);
>         // v = old v + [1,2,3]
>         BOOST_ASSERT( v.size() == 10 );
>     }

As we can see, then the first argument to `repeat()` is the number of times to repeat the second argument.

A more general list can be constructed with `repeat_fun()`:

>     #include <boost/assign/std/vector.hpp>
>     #include <boost/assert.hpp>
>     #include <cstdlib> // for 'rand()'

>     using namespace std;
>     using namespace boost::assign;

>     template< class T >
>     struct next
>     {
>         T seed;
>         next( T seed ) : seed(seed)
>         { }

>         T operator()()
>         {
>             return seed++;
>         }
>     };

>     {
>         vector<int> v;
>         v += 1,2,repeat_fun(4,&rand),4;
>         // v = [1,2,?,?,?,?,4]
>         BOOST_ASSERT( v.size() == 7 );

>         push_back( v ).repeat_fun(4,next<int>(0))(4).repeat_fun(4,next<int>(5));
>         // v = old v + [0,1,2,3,4,5,6,7,8]
>         BOOST_ASSERT( v.size() == 16 );
>     }

The only requirement of the second argument to `repeat_fun()` is that it is a nullary function.

### Functions `ref_list_of()` and `cref_list_of()`

When you need to create an anonymous range of values and speed is essential, these two functions provide what you want.

>     #include <boost/assign/list_of.hpp>
>     #include <algorithm>

>     //
>     // Define Range algorithm
>     //
>     template< class Range >
>     typename Range::const_iterator max_element( const Range& r )
>     {
>         return std::max_element( r.begin(), r.end() );
>     }

>     using namespace boost::assign;

>     {
>         int a=1,b=5,c=3,d=4,e=2,f=9,g=0,h=7;
>         int& max = *max_element( ref_list_of<8>(a)(b)(c)(d)(e)(f)(g)(h) );
>         BOOST_CHECK_EQUAL( max, f );
>         max = 8;
>         BOOST_CHECK_EQUAL( f, 8 );
>         const int& const_max = *max_element(cref_list_of<8>(a)(b)(c)(d)(e)(f)(g)(h) );
>         BOOST_CHECK_EQUAL( max, const_max );
>     }

You can only use lvalues with `ref_list_of()` while `cref_list_of()` accepts rvalues too. Do not worry about not specifying exactly the right size; the extra space used is minimal and there is no runtime overhead associated with it. You may also use these functions instead of `list_of()` if speed is essential.

### A "complicated" example

As a last example, let us assume that we need to keep track of the result of soccer matches. A team will get one point if it wins and zero otherwise. If there has been played three games in each group, the code might look like this:

>     #include <boost/assign/list_of.hpp>
>     #include <boost/assign/list_inserter.hpp>
>     #include <boost/assert.hpp>
>     #include <string>
>     #include <vector>

>     using namespace std;
>     using namespace boost::assign;

>     {
>         typedef vector<int>                   score_type;
>         typedef map<string,score_type>        team_score_map;
>         typedef pair<string,score_type>       score_pair;

>         team_score_map group1, group2;

>         //
>         // method 1: using 'insert()'
>         //
>         insert( group1 )( "Denmark", list_of(1)(1) )
>                         ( "Germany", list_of(0)(0) )
>                         ( "England", list_of(0)(1) );
>         BOOST_ASSERT( group1.size() == 3 );
>         BOOST_ASSERT( group1[ "Denmark" ][1] == 1 );
>         BOOST_ASSERT( group1[ "Germany" ][0] == 0 );

>         //
>         // method 2: using 'list_of()'
>         //
>         group2 = list_of< score_pair >
>                             ( "Norway",  list_of(1)(0) )
>                             ( "USA",     list_of(0)(0) )
>                             ( "Andorra", list_of(1)(1) );
>         BOOST_ASSERT( group2.size() == 3 );
>         BOOST_ASSERT( group2[ "Norway" ][0] == 1 );
>         BOOST_ASSERT( group2[ "USA" ][0] == 0 );
>     }

In the first example, notice how the result of `list_of()` can be converted automatically to a `vector<int>` because `insert()` knows it expects a `vector<int>`. In the second example we can see that `list_of()` is somewhat less intelligent since here it needs to be told explicitly what arguments to expect. (In the future it might be possible to introduce a more intelligent conversion layer in `list_of()`.)

### Functions `ptr_push_back(), ptr_push_front()` and `ptr_insert()`

For use with Boost.Pointer Container a few special exception-safe functions are provided. Using these function you do not need to call `new` manually:

>     #include <boost/assign/ptr_list_inserter.hpp>
>     #include <boost/ptr_container/ptr_deque.hpp>
>     #include <boost/ptr_container/ptr_set.hpp>

>     //
>     // Example class
>     //
>     struct Foo
>     {
>         int i;

>         Foo() : i(0)
>         { }
>         Foo( int i ) : i(i)
>         { }
>         Foo( int i, int ) : i(i)
>         { }
>         Foo( const char*, int i, int ) : i(i)
>         { }
>     };

>     //
>     // Required by ptr_set<Foo>
>     //
>     inline bool operator<( Foo l, Foo r )
>     {
>         return l.i < r.i;
>     }

>     using namespace boost;
>     using namespace boost::assign;

>     {
>         ptr_deque<Foo> deq;
>         ptr_push_back( deq )()();
>         BOOST_CHECK( deq.size() == 2u );
>         ptr_push_front( deq )( 3 )( 42, 42 )( "foo", 42, 42 );
>         BOOST_CHECK( deq.size() == 5 );

>         ptr_set<Foo> a_set;
>         ptr_insert( a_set )()( 1 )( 2, 2 )( "foo", 3, 3 );
>         BOOST_CHECK( a_set.size() == 4 );
>         ptr_insert( a_set )()()()();
>         BOOST_CHECK( a_set.size() == 4 ); // duplicates not inserted
>     }

### Function `ptr_list_of()`

Just like you can use `list_of()` to initialize containers, you can use `ptr_list_of()` to initialize a pointer container. Here is a small example:

>     #include <boost/assign/ptr_list_of.hpp>
>     #include <boost/ptr_container/ptr_deque.hpp>

>     using namespace boost;
>     using namespace boost::assign;

>     {
>         ptr_deque<Foo> deq;
>         deq = ptr_list_of<Foo>( 42 )()()( 3, 3 )( "foo", 2, 1 ).to_container( deq );
>         BOOST_CHECK( deq.size() == 5 );
>     }

Notice the trailing `.to_container(deq)` which helps many poor compilers to figure out the conversion (a few get it right). Notice also that pointer maps are not supported.

That is all; now you are ready to use this library.

## Reference

It is worth noticing the way the library is implemented. A free-standing function (eg. `push_back()` or `operator+=()`) returns a proxy object which is responsible for the insertion or the assignment. The proxy object does the insertion or assignment by overloading `operator,()` and `operator()()` and by calling the "insert" function from within those operators. The "insert" function is typically stored in the proxy object by using `boost::function`.

Often overloading of `operator,()` is discouraged because it can lead to surprising results, but the approach taken in this library is safe since the user never deals with objects that have overloaded `operator,()` directly. However, you should be aware of this:

The expressions in a comma-separated list no longer follow the rules of the built-in comma-operator. This means that the order of evaluation of expressions in a comma-separated list is undefined like when one specifies a list of function arguments.

Most of the code in this document use `int` in the examples, but of course it works for arbitrary types as long as they are Copy Constructible. The inserted data need not be constant data, but can be variables or data returned from functions; the only requirement is that the type of the data is convertible to the type stored in the container.

All forwarding is done by passing objects by `const` reference. Originally arguments were passed by value (and still is in `tuple_list_of()`). One thing to remember is that references can be passed by using `boost::ref`.

Everything is put in namespace `boost::assign`.

More details can be found below:

- Headers
- Standard containers
- Functions `list_of()` and `map_list_of()`
- Functions `repeat()` and `repeat_fun()`
- Class `list_inserter`
- Function `make_list_inserter()`
- Customizing argument list sizes

### Headers

An overview of the headers in this library is given below. Please notice `<boost/assign/list_inserter.hpp>` is included for each header that defines `operator+=()`.

Header | Includes
--- | ---
<boost/assign.hpp> | everything except support for pointer containers
<boost/assign/list_of.hpp> | `list_of()`, map_list_of(), tuple_list_of(), ref_list_of() and `cref_list_of()`
<boost/assign/std.hpp> | `operator+=()` for all standard containers (see below)
<boost/assign/std/deque.hpp> | `operator+=()` for `std::deque`, `<deque>`
<boost/assign/std/list.hpp> | `operator+=()` for `std::list`, `<list>`
<boost/assign/std/map.hpp> | `operator+=()` for `std::map` and `std::multimap` , `<map>`
<boost/assign/std/queue.hpp> | `operator+=()` for `std::queue` and `std::priority_queue`, `<queue>`
<boost/assign/std/set.hpp> | `operator+=()` for `std::set` and `std::multiset`, `<set>`
<boost/assign/std/slist.hpp> | `operator+=()` for `std::slist` if the class is available , `<slist>`
<boost/assign/std/stack.hpp> | `operator+=()` for `std::stack`, `<stack>`
<boost/assign/std/vector.hpp> | `operator+=()` for `std::vector`, `<vector>`
<boost/assign/assignment_exception.hpp> | Class `assignment_exception` which might be thrown by the proxy returned by `list_of()`
<boost/assign/list_inserter.hpp> | Functions `make_list_inserter()`, `push_back()`, `push_front()`,`insert()`, `push()` and class `list_inserter` which is the back-bone of this entire library.
<boost/assign/ptr_list_inserter.hpp> | Functions `ptr_push_back()`, `ptr_push_front()` and `ptr_insert()`
<boost/assign/ptr_list_of.hpp> | Function `ptr_list_of()`

### Standard containers

In the following three dots (...) will mean implementation defined. `operator+=()` returns a proxy that forwards calls to either `push_back()`,`insert()`, or `push()` depending on which operation the container supports.

Synopsis

>     namespace boost
>     {
>     namespace assign
>     {
>         template< class V, class A, class V2 >
>         list_inserter< ... >    operator+=( std::deque<V,A>& c, V2 v );

>         template< class V, class A, class V2 >
>         list_inserter< ... >    operator+=( std::list<V,A>& c, V2 v );

>         template< class K, class V, class C, class A, class P >
>         list_inserter< ... >    operator+=( std::map<K,V,C,A>& m, const P& p );

>         template< class K, class V, class C, class A, class P >
>         list_inserter< ... >    operator+=( std::multimap<K,V,C,A>& m, const P& p );

>         template< class V, class C, class V2 >
>         list_inserter< ... >    operator+=( std::queue<V,C>& c, V2 v );

>         template< class V, class C, class V2 >
>         list_inserter< ... >    operator+=( std::priority_queue<V,C>& c, V2 v );

>         template< class K, class C, class A, class K2 >
>         list_inserter< ... > operator+=( std::set<K,C,A>& c, K2 k );

>         template< class K, class C, class A, class K2 >
>         list_inserter< ... > operator+=( std::multiset<K,C,A>& c, K2 k );

>         #ifdef BOOST_HAS_SLIST

>         template< class V, class A, class V2 >
>         list_inserter< ... >    operator+=( std::slist<V,A>& c, V2 v );

>         #endif

>         template< class V, class C, class V2 >
>         list_inserter< ... >    operator+=( std::stack<V,C>& c, V2 v );

>         template< class V, class A, class V2 >
>         list_inserter< ... >    operator+=( std::vector<V,A>& c, V2 v );

>     } // namespace 'assign'
>     } // namespace 'boost'

Note that the extra template argument `V2` etc. is necessary to allow for types convertible to `V`.

### Functions `list_of()` and `map_list_of()`

These two functions are used to construct anonymous list which can be converted to any standard container and `boost::array<T,sz>.` The object returned by the two functions is guaranteed to have the interface described below.

Synopsis

>     namespace boost
>     {
>     namespace assign
>     {
>         template< class T >
>         class Implementation-defined
>         {
>         public:
>             const_iterator  begin() const;
>             const_iterator  end() const;

>             template< class U >
>             Implementation-defined& operator,( U u );

>             // inserts default-constructed object
>             Implementation-defined& operator()();

>             template< class U >
>             Implementation-defined& operator()( U u );

>             template< class U, class U2 >
>             Implementation-defined& operator()( U u, U2 u2 );

>             //
>             // and similarly up to 5 arguments
>             //

>             //
>             // Convert to a 'Container'. 'Container' must have a constructor
>             // which takes two iterators.
>             //
>             template< class Container >
>             operator Container() const;

>             //
>             // Convert to a container adapter like 'std::stack<>'.
>             //
>             Convertible-to-adapter to_adapter() const;

>             //
>             //
>             // Convert to eg. 'boost::array<T,std::size_t>'. If the
>             // assigned variable is too small,
>             // a assignment_exception is thrown.
>             // If the assigned variable it is too big, the rest of the
>             // values are  default-constructed.
>             //
>             template< template <class,std::size_t> class Array, class U, std::size_t sz >
>             operator Array<U,sz>() const;
>         };

>         template< class T >
>         Implementation-defined   list_of();

>         template< class T >
>         Implementation-defined   list_of( T t );

>         template< class T, class U, class U2 >
>         Implementation-defined   list_of( U u, U2 u2 );

>         template< class T, class U, class U2, class U3 >
>         Implementation-defined   list_of( U u, U2 u2, U3 u3 );

>         template< class T, class U, class U2, class U3, class U4 >
>         Implementation-defined   list_of( U u, U2 u2, U3 u3, U4 u4 );

>         template< class T, class U, class U2, class U3, class U4, class U5 >
>         Implementation-defined   list_of( U u, U2 u2, U3 u3, U4 u4, U5 u5 );

>         template< class Key, class T >
>         Implementation-defined   map_list_of( Key k, T t )
>         {
>             return list_of< std::pair<Key,T> >()( k, t );
>         }
>     } // namespace 'assign'
>     } // namespace 'boost'

### Functions `repeat()` and `repeat_fun()`

These two function exist both as free-standing functions and as member functions of the object returned by `list_of()` and of `list_inserter`. The free-standing versions are used to create a hook for `operator,()` so we can call the functions in the middle of a comma-list. The member functions are used when we need to call the functions in the middle of a parenthesis-list. In both cases we have that

- the return type is always implementation defined for both functions,
- the argument list of `repeat()` is `(std::size_t,T)`, and
- the argument list of `repeat_fun()` is `(std::size_t,Nullary_function)`

### Class `list_inserter`

This class is responsible for inserting elements into containers and it is the key to extending the library to support your favourite class.

Synopsis

>     namespace boost
>     {
>     namespace assign
>     {
>         template< Function, Argument = void >
>         class list_inserter
>         {
>             Function fun;

>         public:
>             explicit list_inserter( Function fun );

>             // conversion constructor
>             template< class Function2, class Arg >
>             list_inserter( const list_inserter<Function2,Arg>& );

>         public:
>             template< class U >
>             list_inserter& operator,( U u );

>             template< class U >
>             list_inserter& operator=( U u );

>             // calls 'fun()' with default-constructed object
>             list_inserter& operator()();

>             template< class U >
>             list_inserter& operator()( U u );

>             template< class U, class U2 >
>             list_inserter& operator()( U u, U2 u2 )
>             {
>                //
>                // if 'Argument' is 'void'
>                //     fun( u, u2 );
>                // else
>                //     fun( Argument( u, u2 ) );
>                //
>                return *this;
>             }

>             //
>             // similarly up to 5 arguments
>             //
>         };

>         template< class C >
>         list_inserter< ... > push_back( C& );

>         template< class C >
>         list_inserter< ... > push_front( C& );

>         template< class C >
>         list_inserter< ... > insert( C& );

>         template< class C >
>         list_inserter< ... > push( C& );

>     } // namespace 'assign'
>     } // namespace 'boost'

Notice how the arguments to `operator,()` and `operator()()` are passed differently to `fun` depending of the type of `Argument`. So if we only pass one template argument to `list_inserter,` we can forward "arbitrary" argument lists of functions. If we pass two template arguments to `list_inserter` we can construct types with "arbitrary" constructors.

And because a reference to `list_inserter` is returned, we can chain argument list together in a very space-efficient manner.

### Function `make_list_inserter()`

A simple "constructor" function for `list_inserter`. A typical use of this function is to call it with the result of `boost::bind()` which in general returns some unreadable and weird class template.

Synopsis

>     namespace boost
>     {
>     namespace assign
>     {
>         template< class Function >
>         list_inserter<Function>  make_list_inserter( Function fun )
>         {
>             return list_inserter<Function>( fun );
>         }
>     }
>     }

### Customizing argument list sizes

This library uses the boost Preprocessor Library to implement overloaded versions of `operator()()` and `list_of()`. By default you can call these functions with five arguments, but you can also customize this number by defining a macros before including a header from this library:

>     #define BOOST_ASSIGN_MAX_PARAMS 10
>     #include <boost/assign.hpp>

## Exceptions and exception-safety

The exception guarantees by the library is the same as guarantee as the guarantee of the function that is forwarded to. For standard containers this means that the strong guarantee is given for a single insertions and that the basic guarantee is given for many insertions (provided that the object being copied gives the basic guarantee).

The functions may throw standard exceptions like `std::bad_alloc`. Note however that, unfortunately, the standard does not guarantee allocation-failures in standard containers to be reported by `std::bad_alloc` or exceptions derived from `std::exception`.

### Class `assignment_exception`

The exception is thrown by the conversion operator in the proxy object returned from `list_of()`.

>     namespace boost
>     {
>     namespace assign
>     {
>         class assignment_exception : public std::exception
>         {
>         public:
>             explicit assignment_exception( const char* what );
>             virtual const char* what() const throw();
>         };
>     }
>     }

## Extending the library

It is very simple to make the library work with new classes. This code shows how to use `operator+=()` with a container:

>     template< class V, class A, class V2 >
>     inline list_inserter< assign_detail::call_push_back< std::vector<V,A> >, V >
>     operator+=( std::vector<V,A>& c, V2 v )
>     {
>         return make_list_inserter( assign_detail::call_push_back< std::vector<V,A> >( c ) )( v );
>     }

where `call_push_back` is defined as

>     template< class C >
>     class call_push_back
>     {
>         C& c_;
>     public:

>         call_push_back( C& c ) : c_( c )
>         { }

>         template< class T >
>         void operator()( T r )
>         {
>             c_.push_back( r );
>         }
>     };

Note that we pass a second template argument to `list_inserter` so argument lists will be used to construct a `V` object. Otherwise we could end up trying to call `push_back()` with n arguments instead of one.

An alternative way would be to use `boost::function` and `boost::bind()` in combination. However, in this case one must remember that it is illegal to take the address of a function in the standard library.

Calling a function with more that one argument can be very useful too. This small example shows how we take advantage of this functionality:

>     //
>     // A class representing emails
>     //
>     class email
>     {
>     public:
>         enum address_option
>         {
>             check_addr_book,
>             dont_check_addr_book
>         };

>     private:

>         typedef std::map< std::string,address_option >  address_map;

>         //
>         // Store list of persons that must be cc'ed
>         //
>         mutable address_map cc_list;

>         //
>         // This extra function-object will take care of the
>         // insertion for us. It stores a reference to a
>         // map and 'operator()()' does the work.
>         //
>         struct add_to_map
>         {
>             address_map& m;

>             add_to_map( address_map& m ) : m(m)
>             {}

>             void operator()( const std::string& name, address_option ao )
>             {
>                 m[ name ] = ao;
>             }
>         };

>     public:

>         //
>         // This function constructs the appropriate 'list_inserter'.
>         // Again we could have use 'boost::function', but it is
>         // trivial to use a function object.
>         //
>         // Notice that we do not specify an extra template
>         // parameter to 'list_inserter'; this means we forward
>         // all parameters directly to the function without
>         // calling any constructor.
>         //
>         list_inserter< add_to_map >
>         add_cc( std::string name, address_option ao )
>         {
>             //
>             // Notice how we pass the arguments 'name' and 'ao' to
>             // the 'list_inserter'.
>             //
>             return make_list_inserter( add_to_map( cc_list ) )( name, ao );
>         }
>     };

>     //
>     // Now we can use the class like this:
>     //
>     email e;
>     e.add_cc( "Mr. Foo", email::dont_check_addr_book )
>             ( "Mr. Bar", email::check_addr_book )
>             ( "Mrs. FooBar", email::check_addr_book );

The full example can be seen in email_example.cpp

## Examples

Additional examples can be found in the test files:

- email_example.cpp
- my_vector_example.cpp
- multi_index_container.cpp
- array.cpp
- list_of.cpp
- std.cpp
- list_inserter.cpp
- list_of_work_around.cpp

## Supported libraries

Here is a list libraries has been tested with Boost.Assign:

1. `boost::array`
2. `boost::multi_index_container`
3. `Boost.Pointer Container`

## Portability

Library has been successfully compiled and tested with MVC++ 7.1, GCC 3.2 (under Cygwin) Comeau 4.3.3

There are known limitation on platforms not supporting templated conversion operators. The solution is to call certain member functions on the object returned by `list_of()`:

>     {
>         using namespace std;
>         using namespace boost;
>         using namespace boost::assign;

>         vector<int>         v = list_of(1)(2)(3)(4).to_container( v );
>         set<int>            s = list_of(1)(2)(3)(4).to_container( s );
>         map<int,int>        m = map_list_of(1,2)(2,3).to_container( m );
>         stack<int>         st = list_of(1)(2)(3)(4).to_adapter( st );
>         queue<int>          q = list_of(1)(2)(3)(4).to_adapter( q );
>         array<int,4>        a = list_of(1)(2)(3)(4).to_array( a );
>     }

Notice how one must supply the functions with an argument so the right return type can be deduced.

Some standard libraries are also broken. One problem is that `insert()` might not work:

>     map<int,int> next;
>     insert( next )(1,2)(2,3); // compile-time error

The solution is to use `map_list_of()` instead:

>     map<int,int> next = map_list_of(1,2)(2,3);

## History and Acknowledgment

The idea for an assignment/initialization library is not new. The functionality of this library resembles Leor Zolman's STL Container Initialization Library a great deal, but it does not rely on string parsing to achieve its goals.

The library is non-intrusive and puts only a minimum of requirements on its supported classes. Overloading operator comma is sometimes viewed as a bad practice [1]. However, it has been done with success in eg. the Generative Matrix Computation Library and Blitz to initialize matrices (see [2]) and [3]). The Initialization Library overloads the comma operator in a safe manner by letting free standing functions return an object that is responsible for the initialization. Therefore it takes explicit action from the programmer to begin using the overloaded `operator,()`.

There has recently been some discussion about enhancing the language to support better initialization (see [4]).

Special thanks goes to

- Leor Zolman for our many discussion that eventually led to this library.
- Tom Brinkman for being review manager.
- Joaquín Muñoz for vc6/vc7 portability.
- Pavel Vozenilek for his countless suggestions, improvements and portability fixes.
- Rene Rivera for Code Warrior portability.

## References

1. Scott. Meyers, "More Effective C++", Item 7, Addison Wesley, 1996
2. K. Czarnecki and U.W. Eisenecker, "Generative programming", Addison-Wesley, 2000
3. http://www.oonumerics.org/blitz/
4. Gabriel Dos Reis and Bjarne Stroustrup, "Generalized Initializer Lists", 2003

(C) Copyright Thorsten Ottosen 2003-2004