§7.4派生类的构造和析构函数 显式调用 class poin七 I public Point (float =0.of, float =0.0f)i }; 派生类的声明 class Circle public Point Circle (float r=0.of, float x=0.Of, float y=0. of)i Circle:: Circle (float r, float a, float b) Point (a b) 派生类的实现§7.4 派生类的构造和析构函数  显式调用： class Point { public: Point (float = 0.0f, float = 0.0f); … … }; class Circle : public Point { Circle (float r=0.0f, float x=0.0f, float y=0.0f); … … }; Circle::Circle (float r, float a, float b) : Point (a, b) { … … } 派生类的声明 派生类的实现