The MPL Reference Manual: has_key

has_key

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

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

#include <boost/mpl/has_key.hpp>

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

For any Associative Sequence s, and arbitrary type key:

typedef has_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`.

Amortized constant time.

typedef map< pair, pair > m;
BOOST_MPL_ASSERT_NOT(( has_key ));

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

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