...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Header: #include
<boost/type_traits/conditional.hpp>
or #include <boost/type_traits.hpp>
namespace boost { template <bool B, class T, class U> struct conditional; template <bool B, class T, class U> using conditional_t = typename conditional<B, T, U>::type; // C++11 and above }
If B is true, the member typedef type shall equal T. If B is false, the member typedef type shall equal U.