#include <iostream> // overloading "operator << " outside class // << 应该定义在类之外。 ////////////////////////////////////////////////////////// class Rectangle { public: Rectangle(int w, int h) : width(w), height(h) {}; ~Rectangle() {}; public: int width; int height; }; ////////////////////////////////////////////////////////// std::ostream& operator<< (std::ostream& os, const Rectangle& rec) // 这里的 const 是必要的,rec应该是一个常量,请注意 { os << rec.width << ", " << rec.height; return os; } ////////////////////////////////////////////////////////// int main() { Rectangle a(40, 10); Rectangle b(41, 11); Rectangle c(42, 12); std::cout << "a = " << a << std::endl << "b = " << b << std::endl << "c = " << c << std::endl; return 0; }