Front Page / Sequences / Intrinsic Metafunctions / key_type

# key_type

### Synopsis

    template<
          typename Sequence
        , typename X
        >
    struct key_type
    {
        typedef unspecified type;
    };

### Description

Returns the key that would be used to identify `X` in `Sequence`.

### Header

    #include <boost/mpl/key_type.hpp>

### Model of

Tag Dispatched Metafunction

### Parameters

Parameter | Requirement | Description
--- | --- | ---
`Sequence` | Associative Sequence | A sequence to query.
`X` | Any type | The type to get the key for.

### Expression semantics

For any Associative Sequence `s`, iterators `pos1` and `pos2` in `s`, and an artibrary type `x`:

    typedef key_type<s,x>::type k;

Return type: | A type.
Precondition: | `x` can be put in `s`.
Semantics: | `k` is the key that would be used to identify `x` in `s`.
Postcondition: | If `key_type< s,deref<pos1>::type >::type` is identical to `key_type< s,deref<pos2>::type >::type` then `pos1` is identical to `pos2`.

### Complexity

Amortized constant time.

### Example

    typedef key_type< map<>,pair<int,unsigned> >::type k1;
    typedef key_type< set<>,pair<int,unsigned> >::type k2;

    BOOST_MPL_ASSERT(( is_same< k1,int > ));
    BOOST_MPL_ASSERT(( is_same< k2,pair<int,unsigned> > ));

### See also

Associative Sequence, value_type, has_key, set, map