Modern C++ Programming Cookbook

How it works...

std::conditional is a class template that defines a member called type as either one or the other of its two type template parameters. The selection is done based on a compile-time constant Boolean expression provided as an argument for a non-type template parameter. Its implementation looks like this:

    template<bool Test, class T1, class T2>    struct conditional    {      typedef T2 type;    };    template<class T1, class T2>    struct conditional<true, T1, T2>    {      typedef T1 type;    };

To help simplify the use of std::conditional, C++14 provides an alias template called std::conditional_t, that we have seen in the third example above, and that is defined as follows:

    template<bool Test, class T1, class T2> using conditional_t = typename conditional<Test,T1,T2>::type; ...