has_key

Synopsis

template<
      typename Sequence
    , typename Key
    >
struct has_key
{
    typedef unspecified type;
};

Description

Returns a true-valued Integral Constant if Sequence contains an element with key Key.

Header

#include <boost/mpl/has_key.hpp>

Model of

Tag Dispatched Metafunction

Parameters

Parameter	Requirement	Description
`Sequence`	Associative Sequence	A sequence to query.
`Key`	Any type	The queried key.

Expression semantics

For any Associative Sequence s, and arbitrary type key:

typedef has_key<s,key>::type c;

Return type:	Boolean Integral Constant.
Semantics:	`c::value == true` if `key` is in `s`'s set of keys; otherwise `c::value == false`.

Complexity

Amortized constant time.

Example

typedef map< pair<int,unsigned>, pair<char,long> > m;
BOOST_MPL_ASSERT_NOT(( has_key<m,long> ));

typedef insert< m, pair<long,unsigned long> > m1;
BOOST_MPL_ASSERT(( has_key<m1,long> ));