Inheritance

[Harris1995]

What is the difference between HAS-A and IS-A relationship ?

[razzle]

What is the difference between HAS-A and IS-A relationship ?

It's like the difference between having a question and being the question.

Now code some C++ and ask about that.

[2kaud]

See http://en.wikipedia.org/wiki/Is-a

A car is-a vehicle and has-a steering wheel.

[Harris1995]

Thanks.

[2kaud]

Incidentially, how are you learning c++ - from a course, on-line material, books?

You may be interested in these
http://www.learncpp.com/
http://www.cplusplus.com/doc/tutorial/
http://www.tutorialspoint.com/cplusplus/index.htm
http://www.programminghelp.org/lib/c++/
http://www.parashift.com/c++-faq/

[Harris1995]

Thank you very much.

[OReubens]

and the unfortunate reality is that sometimes you think something IS-A something else (because it's assumed so in the real world), when it really isn't in the context of programming.

A circle is NOT a special case of an ellipse in C++ class design, even though every math book will tell you it is. Equally a square is not a rectangle.

In the case of C++ and all other programming languages:
"x IS-A y" means that x shares all of the properties of y.
this is why a circle isn't an ellipse, because an ellipse can have a different width and height where a circle can't.
Trying to make a class design where a circle is derived from an ellipse will have several failure points because of this.

HAS-A is all about ownership "it's the stuff you have in your pockets".

[razzle]

Trying to make a class design where a circle is derived from an ellipse will have several failure points because of this.

It's quite easily fixed actually. One way is to make the classes immutable. Then the Liskov Substitution (LSP) principle isn't broken anymore and the design is fine.

I think you're being too categorical. The ellipse/circle is a good example because it clearly demonstrates some design pitfalls. But that doesn't mean it's an impossible design. It just shows you need to be careful and explicit with your definitions and assumptions.

IS-A vs. HAS-A is a first approximation only. The next step is always to align the design with language oriented principles,

http://en.wikipedia.org/wiki/SOLID_(...iented_design)