C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 6:
6.1 Is C++ a practical language?
6.2 Is C++ a perfect language?
6.3 What's the big deal with OO? Updated!
6.4 What's the big deal with generic programming?
6.5 Is C++ better than Ada? (or Visual Basic, C, FORTRAN, Pascal, Smalltalk, or any other language?)
6.6 Who uses C++?
6.7 How long does it take to learn OO/C++?
6.8 What are some features of C++ from a business perspective?
6.9 Are virtual functions (dynamic binding) central to OO/C++?
6.10 Why do virtual functions (dynamic binding) make a big difference?
6.11 Is C++ backward compatible with ANSI/ISO C?
6.12 Is C++ standardized? Updated!
6.13 Where can I get a copy of the ANSI/ISO C++ standard?
6.14 What are some "interview questions" I could ask that would let me know if candidates really know their stuff?
6.15 What does the FAQ mean by "such and such is evil"?
6.16 Will I sometimes use any so-called "evil" constructs?
6.17 Is it important to know the technical definition of "good OO"? Of "good class design"?
6.18 What should I tell people who complain that the word "FAQ" is misleading?
6.19 How can I help make the C++ FAQ even better??!??
[6.10] I'm from Missouri. Can you give me a simple reason why virtual functions (dynamic binding) make a big difference?

Dynamic binding can improve reuse by letting old code call new code.

Before OO came along, reuse was accomplished by having new code call old code. For example, a programmer might write some code that called some reusable code such as printf().

With OO, reuse can also be accomplished by having old code call new code. For example, a programmer might write some code that is called by a framework that was written by their great, great grandfather. There's no need to change great-great-grandpa's code. In fact, it doesn't even need to be recompiled. Even if all you have left is the object file and the source code that great-great-grandpa wrote was lost 25 years ago, that ancient object file will call the new extension without anything falling apart.

That is extensibility, and that is OO.