Often times it\'s a good idea to have an abstract base class to isolate the inte

Question

Often times it's a good idea to have an abstract base class to isolate the interface of the object.

The problem is that copy construction, IMHO, is pretty much broken by default in C++, with copy constructors being generated by default.

So, what are the gotchas when you have an abstract base class and raw pointers in derived classes?

class IAbstract
{
    ~IAbstract() = 0;
}

class Derived : public IAbstract
{
    char *theProblem;
    ...
}

IAbstract *a1 = new Derived();
IAbstract a2 = *a1;//???
And now do you cleanly disable copy construction for the whole hierarchy? Declare copy construction as private in IAbstract?

Are there any rules of three with abstract base classes?

Explanation / Answer

Copy construction on an abstract class should be made private in most cases, as well as assignment operator.

Abstract classes are, by definition, made to be a polymorphic type. So you don't know how many memory does your instance use, and so cannot copy or assign it safely. In practice, you risk slicing.

Polymorphic type, in C++, mustn't be manipulated by value. You manipulate them by reference or by pointer (or any smart pointer).

This is the reason why Java has made object manipulable by reference only, and why C# and D has the separation between classes and structs (the first one being polymorphic and reference type, the second one being non polymorphic and value type).

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