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.

Please add the class definition inheritance schema and use of virtual keyword may change the behavior of your exemple. — VGE, May 10 '11 at 11:53
It doesn't look like it's a very good book. Maybe you could find a different one with some more useful examples? — Stu Mackellar, May 10 '11 at 11:55

score 3 · Accepted Answer · edited May 23 '17 at 11:55

3

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.

edited May 23 '17 at 11:55

Community

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answered May 10 '11 at 11:52

Björn Pollex

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@Hassan: I added an explanation. A more detailed one can be found in the excellent answer I linked. – Björn Pollex May 10 '11 at 12:02
Thank you a lot. It helps me. The book mention the Slicing in two words, and I can't understand what it means. – Tom May 10 '11 at 12:06
Slicing has nothing to do with design it is a physical side effect of C++ semantics. It is also an ellusive concept to grasp especially when placed in the context of all the other features of C++ (multiple inheritance). A more sound answer should enhance understanding in as general a way as possible. – Hassan Syed May 10 '11 at 12:14

score 0 · Answer 2 · answered May 10 '11 at 11:54

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Window win

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

answered May 10 '11 at 11:54

onurozcelik

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score 0 · Answer 3 · answered May 10 '11 at 11:54

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

Window and Textwindow need to implement the "is-a" relationship. (so, class TextWindow : public Window {});
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.

c++ member function binding

3 Answers3