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Tip of the Day
Language: C++
Expertise: Beginner
Dec 1, 1999

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Circular Pointers

Question:
I'm trying to make two classes point to one another. I know the syntax is weird, but it's just to illustrate:
 
class x
{ y* ptry; }

class y
{ x* ptrx; }
It won't compile. I get the error "missing storage class." How do I get around this?

Answer:
You need to forward-declare a class before you can create a pointer to it:

 
class y; //fwd declaration

class x
{ y* ptry; };

class y
{ x* ptrx; };
Without the forward declaration, the compiler doesn't recognize y as a class name because the declaration of y hasn't been seen. A forward declaration instructs the compiler to accept the pointer definition even if it hasn't seen yet the declaration of y. Note that you don't need a forward declaration for ptrx because by the time the compiler sees its definition, class x has already been seen.
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