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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...)

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