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Front Page / Metafunctions / Miscellaneous / identity

identity

Synopsis

template<
      typename X
    >
struct identity
{
    typedef X type;
};

Description

The identity metafunction. Returns X unchanged.

Header

#include <boost/mpl/identity.hpp>

Model of

Metafunction

Parameters

Parameter Requirement Description
X Any type An argument to be returned.

Expression semantics

For an arbitrary type x:

typedef identity<x>::type r;
Return type:

A type.

Semantics:

Equivalent to

typedef x r;
Postcondition:

is_same<r,x>::value == true.

Example

typedef apply< identity<_1>, char >::type t1;
typedef apply< identity<_2>, char,int >::type t2;

BOOST_MPL_ASSERT(( is_same< t1, char > ));
BOOST_MPL_ASSERT(( is_same< t2, int > ));

See also

Metafunctions, Placeholders, Trivial Metafunctions, always, apply