// An illustration of function objects. Very close to the program on page 787 of "The C++ Programming Language" 4ed.

#include <iostream>
#include <type_traits>

// Conditional is a syntactial embellishment of the application of underlying conditional template:
template<bool B, typename T, typename F>
using Conditional = typename std::conditional<B, T, F>::type;    

struct X{
  void operator()(int x){
    std::cout << "X: " << x << std::endl;
  }
  // ...
};

struct Y{
  void operator()(int y){
    std::cout << "Y: " << y << std::endl;
  }
  // ...
};

void f(){
  Conditional<(sizeof(int)>4), X, Y>{}(7);       // Select either X or Y depending on the value of sizeof(int)>4.
                                                 // Y is selected, instantiated, and called on 7.
  Y{}(7);                                        // Equivalent.  Y: 7

  using Z = Conditional<std::is_polymorphic<X>::value, X, Y>;  // X is not polymorphic, therefore Z becomes an alias of Y.
  Z zz{};  // makes an X or a Y
  zz(8);   // calls an X og a Y.     // Y: 8
}

int main(){
  std::cout << "size(int): " << sizeof(int) << std::endl;  // 4 on my laptop.
  f();
}