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> ));

// allocates space for an object of class T on heap or "inplace"
// depending on its size
template< typename T > struct lightweight
{
    // ...
    typedef typename if_<
          less_equal< sizeof_<T>, sizeof_<T*> >
        , inplace_storage<T>
        , heap_storage<T>
        >::type impl_t;

    impl_t impl;
};