eval_if

Synopsis

template<
      typename C
    , typename F1
    , typename F2
    >
struct eval_if
{
    typedef unspecified type;
};

Description

Evaluates one of its two nullary-metafunction arguments, F1 or F2, depending on the value C.

Header

#include <boost/mpl/eval_if.hpp>

Parameters

Parameter	Requirement	Description
`C`	Integral Constant	An evaluation condition.
`F1`, `F2`	Nullary Metafunction	Metafunctions to select for evaluation from.

Expression semantics

For any Integral Constant c and nullary Metafunctions f1, f2:

typedef eval_if<c,f1,f2>::type t;

Return type:	Any type.
Semantics:	If `c::value == true`, `t` is identical to `f1::type`; otherwise `t` is identical to `f2::type`.

Example

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

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