I have to write a program that uses 3 C++ classes. The user knows only about one of them (let's say class C). The C constructor, depending on the input received should decide with what type of object to work with. Any ideas? Thanks.

not sure exactly what you mean here...

so you have 3 classes, A B and C

since you say C is exposed, and 'uses' the other classes, does that mean you want C to contain an instance of either A or B, and it will choose which one based on the C constructor?

Yes John, exactly.

i'm not sure if this is the most elegant way to do it, but could you have a member variable in C called useA, and initialize it accordingly in the C constructor.

and then in every C member that would use either class A or B, you'd go

if (useA) {A related code} else {B related code}

Ok just to make sure what you are asking, as you said "exactly" to something that isn't polymorphism. Are A and B types of C?
If they are and the user does not know about A and B then the correct way would be to use the design pattern factory.

You're right John it's not very elegant.

Please elaborate dmail. I thought I'd use polymorphism. A and B don't have to be types of C. The design is up to me. Thank you both for for ideas.

I'm in a good mood so GPL factory pattern
Although you have me confused maybe you could explain what the class are.
[edit]removed link[/edit]

class A should be a string and class B a bool array - I thought about templates, but it won't work.

That's all there is to the classes?
If so then just use an std::vector<T> where T is string(ie chars) or bool

No, I have to use 3 classes: one to call one of the other 2; it's a school assignment.

Why didn't you say this in the first place
You will have to think about it yourself then or consult your tutor. You will not learn by somebody supplying the answer for you.

I didn't ask for the code only the idea. What's the point of this forum if not to ask questions and it's not like you've told me much. Thanks anyway for your time.

did you happen to review the Factory pattern that dmail mentioned. that may shed some light on it for you, and then you could ask questions that make a little more sense.

I agree that you should ask your instructor... after you have thoroughly reviewed the material to see if you can discover the answer for yourself.

Instructors need feedback! (Believe me, I know...) They can be at the front of that classroom thinking that they are absolutely in a vacuum. When one student comes back with a question, they know that many other students must have the same problem; without it, they don't.

Perhaps you do not yet understand the problem; to me, your description of it was pretty vague. If you don't understand what is being asked of you, that might make it impossible to arrive at the answer. And if that be the case, I can flatly say to you that you are not the only one. "It happens. A lot." Believe me, it's okay to (timely...) ask the instructor to clarify. I assure you that there have been many times when I have sent a "broadcast message to all students" in response to a single inquiry from any one.

That instructor is, after all, in a way working for you. You're paying good money to learn this stuff. It's a two way street. The point, after all, is not that the instructor knows the material! The point is that, by the end of the course, you do, and (of course) you've got the good grades to prove it. If information is flowing in one direction only, it ain't gonna happen and you're wasting your time.