std::conditional(3)

NAME
std::conditional - std::conditional

Synopsis
   Defined in header <type_traits>
   template< bool B, class T, class F > (since C++11)
   struct conditional;
   Provides member typedef type, which is defined as T if B is true at compile time, or
   as F if B is false.
   The behavior of a program that adds specializations for conditional is undefined.

Member types
   Member type Definition
   type T if B == true, F if B == false

Helper types
   template< bool B, class T, class F > (since C++14)
   using conditional_t = typename conditional<B,T,F>::type;

Possible implementation
   template<bool B, class T, class F>
   struct conditional { using type = T; };
   template<class T, class F>
   struct conditional<false, T, F> { using type = F; };

Example
// Run this code
 #include <iostream>
 #include <type_traits>
 #include <typeinfo>
 int main()
 {
     typedef std::conditional<true, int, double>::type Type1;
     typedef std::conditional<false, int, double>::type Type2;
     typedef std::conditional<sizeof(int) >= sizeof(double), int, double>::type Type3;
     std::cout << typeid(Type1).name() << '\n';
     std::cout << typeid(Type2).name() << '\n';
     std::cout << typeid(Type3).name() << '\n';
 }

Possible output:
 int
 double
 double

See also
   enable_if conditionally removes a function overload or template specialization from
   (C++11) overload resolution
             (class template)