c++ - "Incomplete type" in class which has a member of the same type of the class itself

Question

I have a class that should have a private member of the same class, something like:

class A {
    private:
        A member;
}

But it tells me that member is an incomplete type. Why? It doesn't tell me incomplete type if I use a pointer, but I'd rather not use a pointer. Any help is appreciated

question from:https://stackoverflow.com/questions/6349822/incomplete-type-in-class-which-has-a-member-of-the-same-type-of-the-class-itse

Answer 1

At the time you declare your member, you are still defining the A class, so the type A is still undefined.

However, when you write A*, the compiler already knows that A stands for a class name, and so the type "pointer to A" is defined. That's why you can embed a pointer to the type your are defining.

The same logic applies also for other types, so if you just write:

class Foo;

You declare the class Foo, but you never define it. You can write:

Foo* foo;

But not:

Foo foo;

On another hand, what memory structure would you expect for your type A if the compiler allowed a recursive definition ?

However, its sometimes logically valid to have a type that somehow refer to another instance of the same type. People usually use pointers for that or even better: smart pointers (like boost::shared_ptr) to avoid having to deal with manual deletion.

Something like:

class A
{
  private:
    boost::shared_ptr<A> member;
};