Abstract base class for derived classes with functions that have a return type of the derived class

Question

I would like to force a certain API for all classes derived from the base class. Normally, you do that using an abstract base class that has purely virtual functions. However, how do you handle functions that return the derived type? How do I go about forcing that type of function?

struct base
{
    virtual base func() = 0;
};

struct deriv1 : base
{
    deriv1 func();
};

struct deriv2 : base
{
    deriv2 func();
};

This example will give an error like "invalid abstract return type for member function". I've seen some answers that suggest returning pointers, but I don't particularly want to dip into dynamic memory for that and keeping track of all the allocated pointers would be a special kind of hell. Any ideas?

Answer 1

When a virtual function returns a pointer or reference to a class, a class which inherits from the base class and overrides the function is allowed to change the return type to a pointer or reference to a class which is derived from the original return type.

You can't return base by value as it is abstract so you can't actually create one by itself.

http://en.wikipedia.org/wiki/Covariant_return_type

When using virtual functions and base classes, you usually have to use dynamic allocation to create your objects. I suggest you look into smart pointers to help manage the memory.