I.cc

#include <iostream>

class I {
  private:
    int value;
  public:
    I(int i): value(i) {}
    I operator+(int i) const {
	return I(value - i);
    }
    operator int() const {
	return value;
    }
};

int main(int argc, char *argv[]) {
    I i(2);
    std::cout << i << "+" << 3 << "=" << i + 3 << std::endl;
    return 0;
}