Hi,

After quite a bit of procedural programming, we started inheritence today at school. Now, I understood what inheritence basically does, but I could not convince myself of its advantages. Moreover I seem to get confused asking myself "what should inherit what". We just had a discussion on the topic (all theoretical) today and haven't been given any problems to solve yet... but I have a question of my own (and I don't know even if inheritence is required or not):

Suppose I have a class student with all the necessary details. Now each student is issued a "school library card", which is a class on its own. Should, in this case, student inherit library-card of vice versa... or is this a simply absurd question that originated in my mind?

Please join this discussion... (and help me in this unknown territory)

Regards,
netnut.

The easiest way to think of inheritance is via the "IS-A" method. For example, you could have class Automobile which has characteristics of having 4 wheels and an engine. Subclasses could be Car, Truck, Minivan, etc. with each one inheriting class Automobile's wheels and engine but having their own characteristics (e.g. 4 doors vs. 2 doors, sliding passenger door, etc.).

This way you can say a Car "IS-A" Automobile, but you couldn't say Car "IS-A" Minivan. In your Student/Library card example, ask yourself if a Library Card "IS-A" type of Student. The answer should be "No".

A better way to use your example is Library Card "IS-A" Identification and Student "IS-A" School Member. Then class Identification could not only have Library Card as a subclass, but you could also have Driver's License, Student ID card, etc. and your School Member class could be subclassed into Student, Administrator, Teacher, Custodian, etc.

Hope that makes more sense now. It took me a while to figure out how it all works with objects, so don't feel bad or stupid.

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Thanks C. That "is-a" thing is a very good way to solve "what should inherit what" problem.
Now for the library card thing. This is what I understand of your code:

*There should be 2 base classes - SchoolMember & Identification
*Those 2 classes will have derived classes. Which would include student and library card respectively

So I believe, as of now, that student has no relation with a library card (It does not qualify the "is-a" relation). Now what if there is a such a scenario wherein every student is issued a school library card? Wouldn't I require an object of the "card class" inside the student class... isn't that inheritence?

netnut

As you drift into "has-a" relationships.

Here is some reading to get started at parashift.

Some stuff from Scott Meyers

Mark

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Thanks Mark, those links were excellent. I've bookmarked them for future reference.
The two articles seem to have generated in me a feeling that I have known this topic for long time. Let me try and solve a few problems, and I'll be back if I face any difficulty...

Thanks all..
netnut.

So after a few reads at the resources, here is what I gather (please correct me if you find anything wrong):

"Public inheritence" -> "Is-A" relationship
"Private inheritence" -> "Has-A" relationship

An employee is (should be) inherited publicly by a manager; "person" should be public for "student".
"Name" should be private for "person" which in turn should be public for a student.

What about protected inheritence and containership? In the real world how is a train related to a ticket? And what is the equivalent implementation in C++?

Thanks in advance.
netnut.