c++ - 理解虚函数

Question

// multiple inheritance
#include <iostream>
using namespace std;

class Polygon {
  protected:
    int width, height;
  public:
    Polygon (int a, int b) : width(a), height(b) {}
    virtual int area()=0;
    virtual void print(){
        cout << "area = " << area() << endl;
    };
};



class Rectangle: public Polygon{
  public:
    Rectangle (int a, int b) : Polygon(a,b) {}
    int area () { return width*height; }
    void print(){
        cout << "area = " << area() << endl;
    };
};

class Square: public Rectangle{
  public:
    Square (int a, int b) : Rectangle(a,b) {}
    int area () { return width*height/2; }
};

int main () {
  Square sq (4,5);
  sq.print ();
  return 0;
}

在这个函数中，print调用了Square(不是Rectangle)的area()。为什么？由于 Rectangle 中的 area() 不是虚拟的，因此它应该从 Rectangle 中调用 area() 。最后的结果是10，按照我的说法应该是20。

Answer 1

Since area() in Rectangle is not virtual, it should call area() from Rectangle

它实际上是virtual，因为它在基类中被声明为virtual。此属性会自动转移到继承类的成员函数声明。

参见标准，虚函数 [class.virtual](强调我的):

If a virtual member function vf is declared in a class Base and in a class Derived, derived directly or indirectly from Base, a member function vf with the same name, parameter-type-list (8.3.5), cv-qualification, and refqualifier (or absence of same) as Base::vf is declared, then Derived::vf is also virtual (whether or not it is so declared) and it overrides Base::vf.

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旁注。从矩形导出正方形可以是problematic因为它违反了 Liskov 替换原则。