value_type

Synopsis

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

Description

Returns the value that would be used for element X in Sequence.

Header

#include <boost/mpl/value_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 value for.

Expression semantics

For any Associative Sequence s, and an artibrary type x:

typedef value_type<s,x>::type v;

Return type:	A type.
Precondition:	`x` can be put in `s`.
Semantics:	`v` is the value that would be used for `x` in `s`.
Postcondition:	If .. parsed-literal: has_key< s,key_type<s,x>::type >::type then .. parsed-literal: at< s,key_type<s,x>::type >::type is identical to `value_type<s,x>::type`.

Complexity

Amortized constant time.

Example

typedef value_type< map<>,pair<int,unsigned> >::type v1;
typedef value_type< set<>,pair<int,unsigned> >::type v2;

BOOST_MPL_ASSERT(( is_same< v1,unsigned > ));
BOOST_MPL_ASSERT(( is_same< v2,pair<int,unsigned> > ));