template<
      bool c
    , typename T1
    , typename T2
    >
struct if_c
{
    typedef unspecified type;
};
 
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.
 
| Parameter | Requirement | Description | 
|---|
| c | An integral constant | A selection condition. | 
| T1, T2 | Any type | Types to select from. | 
 
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; | 
|---|
 
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;
};