How does the C++ compiler know which implementation of a virtual function to call?

Question

Here is an example of polymorphism from http://www.cplusplus.com/doc/tutorial/polymorphism.html (edited for readability):

// abstract base class
#include <iostream>
using namespace std;

class Polygon {
    protected:
        int width;
        int height;
    public:
        void set_values(int a, int b) { width = a; height = b; }
        virtual int area(void) =0;
};

class Rectangle: public Polygon {
    public:
        int area(void) { return width * height; }
};

class Triangle: public Polygon {
    public:
        int area(void) { return width * height / 2; }
};

int main () {
    Rectangle rect;
    Triangle trgl;
    Polygon * ppoly1 = &rect;
    Polygon * ppoly2 = &trgl;
    ppoly1->set_values (4,5);
    ppoly2->set_values (4,5);
    cout << ppoly1->area() << endl; // outputs 20
    cout << ppoly2->area() << endl; // outputs 10
    return 0;
}

My question is how does the compiler know that ppoly1 is a Rectangle and that ppoly2 is a Triangle, so that it can call the correct area() function? It could find that out by looking at the "Polygon * ppoly1 = ▭" line and knowing that rect is a Rectangle, but that wouldn't work in all cases, would it? What if you did something like this?

cout << ((Polygon *)0x12345678)->area() << endl;

Assuming that you're allowed to access that random area of memory.

I would test this out but I can't on the computer I'm on at the moment.

(I hope I'm not missing something obvious...)

Answer 1

Each object (that belongs to a class with at least one virtual function) has a pointer, called a vptr. It points to the vtbl of its actual class (which each class with virtual functions has at least one of; possibly more than one for some multiple-inheritance scenarios).

The vtbl contains a bunch of pointers, one for each virtual function. So at runtime, the code just uses the object's vptr to locate the vtbl, and from there the address of the actual overridden function.

In your specific case, Polygon, Rectangle, and Triangle each has a vtbl, each with one entry pointing to its relevant area method. Your ppoly1 will have a vptr pointing to Rectangle's vtbl, and ppoly2 similarly with Triangle's vtbl. Hope this helps!

Answer 2

Chris Jester-Young gives the basic answer to this question.

Wikipedia has a more in depth treatment.

If you want to know the full details for how this type of thing works (and for all type of inheritance, including multiple and virtual inheritance), one of the best resources is Stan Lippman's "Inside the C++ Object Model".