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# map

### Description

`map` is a variadic, associative, extensible sequence of type pairs that supports constant-time insertion and removal of elements, and testing for membership. A `map` may contain at most one element for each key.

### Header

Sequence form | Header
--- | ---
Variadic | `#include <boost/mpl/map.hpp>`
Numbered | `#include <boost/mpl/map/map` n`.hpp>`

### Model of

- Variadic Sequence
- Associative Sequence
- Extensible Associative Sequence

### Expression semantics

In the following table and subsequent specifications, `m` is an instance of `map`, `pos` is an iterator into `m`, `x` and p1,p2,... pn are `pair`s, and `k` is an arbitrary type.

Expression | Semantics
--- | ---

    map<p1,p2,... pn>
    mapn<p1,p2,... pn>

`map` of elements p1,p2,... pn; see Variadic Sequence.

    map<p1,p2,... pn>::type
    mapn<p1,p2,... pn>::type

Identical to `map` n`<`p1,p2,... pn`>`; see Variadic Sequence.

`begin<m>::type` | An iterator pointing to the beginning of `m`; see Associative Sequence.
`end<m>::type` | An iterator pointing to the end of `m`; see Associative Sequence.
`size<m>::type` | The size of `m`; see Associative Sequence.
`empty<m>::type` | A boolean Integral Constant `c` such that `c::value == true` if and only if `m` is empty; see Associative Sequence.
`front<m>::type` | The first element in `m`; see Associative Sequence.
`has_key<m,k>::type` | Queries the presence of elements with the key `k` in `m`; see Associative Sequence.
`count<m,k>::type` | The number of elements with the key `k` in `m`; see Associative Sequence.
`order<m,k>::type` | A unique unsigned Integral Constant associated with the key `k` in `m`; see Associative Sequence.

    at<m,k>::type
    at<m,k,default>::type

The element associated with the key `k` in `m`; see Associative Sequence.

`key_type<m,x>::type` | Identical to `x::first`; see Associative Sequence.
`value_type<m,x>::type` | Identical to `x::second`; see Associative Sequence.

`insert<m,x>::type`

A new `map`, `t`, equivalent to `m` except that

    at< t, key_type<m,x>::type >::type

is identical to `value_type<m,x>::type`.

`insert<m,pos,x>::type` | Equivalent to `insert<m,x>::type`; `pos` is ignored.
`erase_key<m,k>::type` | A new `map`, `t`, equivalent to `m` except that `has_key<t, k>::value == false`.
`erase<m,pos>::type` | Equivalent to `erase<m, deref<pos>::type >::type`.
`clear<m>::type` | An empty `map`; see clear.

### Example

    typedef map<
          pair<int,unsigned>
        , pair<char,unsigned char>
        , pair<long_<5>,char[17]>
        , pair<int[42],bool>
        > m;

    BOOST_MPL_ASSERT_RELATION( size<m>::value, ==, 4 );
    BOOST_MPL_ASSERT_NOT(( empty<m> ));

    BOOST_MPL_ASSERT(( is_same< at<m,int>::type, unsigned > ));
    BOOST_MPL_ASSERT(( is_same< at<m,long_<5> >::type, char[17] > ));
    BOOST_MPL_ASSERT(( is_same< at<m,int[42]>::type, bool > ));
    BOOST_MPL_ASSERT(( is_same< at<m,long>::type, void_ > ));

### See also

Sequences, Variadic Sequence, Associative Sequence, Extensible Associative Sequence, set, vector