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> > ));