isdefaultconstructible1.hpp

The following code example is taken from the book
C++ Templates - The Complete Guide, 2nd Edition
by David Vandevoorde, Nicolai M. Josuttis, and Douglas Gregor,
Addison-Wesley, 2017
© Copyright David Vandevoorde, Nicolai M. Josuttis, Douglas Gregor 2017


#include "issame.hpp"

template<typename T>
struct IsDefaultConstructibleT {
  private:
    // test() trying substitute call of a default constructor for T passed as U:
    template<typename U, typename = decltype(U())>
      static char test(void*);
    // test() fallback:
    template<typename>
      static long test(...);
  public:
    static constexpr bool value
      = IsSameT<decltype(test<T>(nullptr)), char>::value;
};