c++ member function binding

Question

I read one of the book which deals with the issues of member function binding in c++.

and it's giving the next example:

void Window::oops()  { printf("Window oops\n"); } 
void TextWindow::oops() { 
printf("TextWindow oops %d\n", cursorLocation);

Window      win; 
Window     *winPtr;
TextWindow *txtWinPrt = new TextWindow; 
win    = *txtWinPrt; 
winPtr =  txtWinPtr;
win.oops();     // executes Window version 
winPtr->oops(); // executes TextWindow or Window version;

I didn't understand why win.oops would executes window version? win is defined as Textwindow.

Thank you for your help.

Answer 1

This is caused by slicing . If you assign to an object of the super-class, the information from the subclass is lost. The problem is this statement:

win    = *txtWinPrt;

Since you assign an object of a subclass ( TextWindow ) to an object of the super-class ( Window ), all the information of TextWindow that is not in Window is sliced away.

Answer 2

Window win 

is an object of Window class. It should be pointer or reference to call derived class method with base class instance.

Answer 3

Two things are needed for dynamic polymorphism using object orientation (which is what you are asking for).

1. Window and Textwindow need to implement the "is-a" relationship. (so, class TextWindow : public Window {} );
2. A virtual function is needed in a base-clase in order to get runtime polymorphism, generally a destructor if you cannot find a virtual function naturally. A virtual function causes the compiler to put down a v-table.

Without these two things, the compiler will not put a v-table down at callsites. The v-tables enables runtime polymorphism as function calls are indirected through it.

Alternatively you could resort to c-style function pointers, or something like boost::bind. But this defeats OO programming. I personally use a v-table very rarely.