class figure { protected : double x,y; public: void set_dim(double i, double j=0) { x = i ; y = j ; } //声明纯虚函数 show_area }; class triangle : public figure { public : void show_area() { cout< }; class square : public figure { public: void show_area() { cout< }; class circle : public figure { public: void show_area() { cout< cout< } };