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if_c

Synopsis

template<
      bool c
    , typename T1
    , typename T2
    >
struct if_c
{
    typedef unspecified type;
};

Description

Returns one of its two arguments, T1 or T2, depending on the value of integral constant c. if_c<c,t1,t2>::type is a shorcut notation for if_< bool_<c>,t1,t2 >::type.

Header

#include <boost/mpl/if.hpp>

Parameters

Parameter Requirement Description
c An 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<c,t1,t2>::type t;
Return type:Any type.
Semantics:Equivalent to typedef if_< bool_<c>,t1,t2 >::type t;

Example

typedef if_c<true,char,long>::type t1;
typedef if_c<false,char,long>::type t2;

BOOST_MPL_ASSERT(( is_same<t1, char> ));
BOOST_MPL_ASSERT(( is_same<t2, long> ));
// allocates space for an object of class T on heap or "inplace"
// depending on its size
template< typename T > struct lightweight
{
    // ...
    typedef typename if_c<
          sizeof(T) <= sizeof(T*)
        , inplace_storage<T>
        , heap_storage<T>
        >::type impl_t;

    impl_t impl;
};

See also

Metafunctions, Integral Constant, if_, eval_if, bool_