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if_

Synopsis

template< 
      typename C
    , typename T1
    , typename T2
    >
struct if_
{
    typedef unspecified type;
};

Description

Returns one of its two arguments, T1 or T2, depending on the value C.

Header

#include <boost/mpl/if.hpp>

Parameters

Parameter Requirement Description
C Integral Constant A selection condition.
T1, T2 Any type Types to select from.

Expression semantics

For any Integral Constant c and arbitrary types t1, t2:

typedef if_<c,t1,t2>::type t;
Return type:Any type.
Semantics:If c::value == true, t is identical to t1; otherwise t is identical to t2.

Example

typedef if_<true_,char,long>::type t1;
typedef if_<false_,char,long>::type t2;

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

See also

Metafunctions, Integral Constant, if_c, eval_if