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### map

##### Description

map is an Associative Sequence of heteregenous typed data elements. Each element is a key/data pair (see `fusion::pair`) where the key has no data (type only). Type identity is used to impose an equivalence relation on keys. A map may contain at most one element for each key. Membership testing and element key lookup has constant runtime complexity (see Overloaded Functions).

##### Header

    #include <boost/fusion/container/map.hpp>
    #include <boost/fusion/include/map.hpp>
    #include <boost/fusion/container/map_fwd.hpp>
    #include <boost/fusion/include/map_fwd.hpp>

##### Synopsis

    template <
        typename T0 = unspecified
      , typename T1 = unspecified
      , typename T2 = unspecified
        ...
      , typename TN = unspecified
    >
    struct map;

The variadic class interface accepts `0` to `FUSION_MAX_MAP_SIZE` elements, where `FUSION_MAX_MAP_SIZE` is a user definable predefined maximum that defaults to `10`. Example:

    map<pair<int, char>, pair<char, char>, pair<double, char> >

You may define the preprocessor constant `FUSION_MAX_MAP_SIZE` before including any Fusion header to change the default. Example:

    #define FUSION_MAX_MAP_SIZE 20

##### Template parameters

Parameter | Description | Default
--- | --- | ---
`T0`...`TN` | Element types | unspecified-type

##### Model of

- Associative Sequence
- Forward Sequence

Notation

`M`

  A `map` type

`m`

  An instance of `map`

`e0`...`en`

  Heterogeneous key/value pairs (see `fusion::pair`)

`s`

  A Forward Sequence

##### Expression Semantics

Semantics of an expression is defined only where it differs from, or is not defined in Random Access Sequence and Associative Sequence.

Expression | Semantics
--- | ---
`M()` | Creates a map with default constructed elements.
`M(e0, e1,... en)` | Creates a map with element pairs `e0`...`en`.
`M(s)` | Copy constructs a map from a Forward Sequence `s`.
`m = s` | Assigns to a map, `m`, from a Forward Sequence `s`.

##### Example

    typedef map<
        pair<int, char>
      , pair<double, std::string> >
    map_type;

    map_type m(
        make_pair<int>('X')
      , make_pair<double>("Men"));

    std::cout << at_key<int>(m) << std::endl;
    std::cout << at_key<double>(m) << std::endl;