Are mutually recursive classes possible?

Question

I read how this can be made to work using forward declarations.

class A
{
    public:
    B *objB;

    void foo(){}
}

class B
{
    public:
    A *objA;

    void foo(){}
}

Just wanted to confirm if this design is ever possible ?

class A
{
    public:
    B objB;

    void foo(){}
}

class B
{
    public:
    A objA;

    void foo(){}
}

PS: If someone could also please explain why/why not this is possible logically in terms of classes, rather than just in terms of language, like by quoting some example. What exactly this signify in terms of classes ?

Answer 1

The second example is not possible. It says that the space allocated for an A contains room for a B, which in turn contains room for an A, etc. This would require an infinite amount of memory, and would take an infinite amount of time to construct.

Answer 2

No, it is not possible either in terms of language or in terms of classes.

In terms of classes: Every A instance contains a B instance which contains an A instance which... => infinite recursion. This is not a problem with the pointer version because the pointer may not point to a valid object, or all A pointers may point to the same object, etc.