Directly invoke a function object as a regular function without having to explicitly construct it

direct-function-object-call.cpp

//Author: Ugo Varetto
//
//function objects do not allow a call to the object's operator()(...)
//without an explicit creation:
// struct neg {
//   int operator()(int i) const { return -i; }
// };
// neg n;
// std::cout << n(4) << std::endl
// or, if the () operator is const
// std::cout << neg()(4) << std::endl;
// prints -4
// the following code allows to call the neg function
// object as 'neg(4)'
// The trick is to replace(or add to) the () operator with:
// - a constructor taking the same arguments as the () operator
//   and storing their references internally
// - a conversion operator returning the same type as the () operator
//   where the actual operations are performed

#include <iostream>

struct neg {
  neg() : n_(0) {} // useful if default-constructible required
  neg(const int& n) : n_(&n) {}
  operator int() const { return -(*n_);}
  const int* n_;
};

int main(int, char**) {
  std::cout << neg(4) << std::endl;
	return 0;
}