CppCoreGuidelines C.126 抽象类一般不需要编写构造函数

class Shape {
public:
    // no user-written constructor needed in abstract base class
    virtual Point center() const = 0;    // pure virtual
    virtual void move(Point to) = 0;
    // ... more pure virtual functions...
    virtual ~Shape() {}                 // destructor
};

class Circle : public Shape {
public:
    Circle(Point p, int rad);           // constructor in derived class
    Point center() const override { return x; }
};