Understanding virtual function

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

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

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

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

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


In this function, print the call area () of a square (not a rectangle). What for? Since area () in Rectangle is not virtual, it must call area () from Rectangle. The end result is 10. For me, it should be 20.


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1 answer

Since area()

in is Rectangle

not virtual, it must call area()


In fact, thisvirtual

is since it was declared virtual

in the base class. This attribute is automatically wrapped in function declarations of inherited classes.

See the standard, Virtual Functions [class.virtual] (emphasis mine):

If a virtual member function is vf

declared in class Base and in a class Derived obtained directly or indirectly from Base, a member function of vf

the same name, parameter list (8.3.5), cv-qualification and refqualifier (or no same) as and Base::vf

, then Derived :: vf is also virtual ( no matter declared ) and it overrides Base::vf


Side note. Derivald square from rectangle can be problematic because it violates Liskov's substitution principle.



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