Avoid cyclic references caused by C++ includes headers [duplicate]

Question

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Resolve build errors due to circular dependency amongst classes (11 answers)

Closed 2 years ago.

I have 3 classes: A, B and C. C is #includeed by B, and B is #included by A. In class C i have defined a handler for a button, and when the button is pushed, C will PostMessage to object A. If i include A in C, i will have a cyclic reference, so what should i do to avoid this cyclic reference?

EDIT: All includes are made in implementation files.

Answer 1

You should use forward declarations. Since C is not the owner of A, I'll assume you have a pointer as a member. So you don't need to include:

class A; //forward declaration
class C
{
    A* a;
};

In the implementation file, you'll include A.h but that's OK. Also, if you can, use forward declarations in A.h and B.h where possible.

Answer 2

If class X uses class Y only by pointer, you should use a forward declaration of class Y before declaring class X.

It is a good idea to keep your forward declarations in a separate file. iosfwd is a standard example of this approach

If class X uses class Y in other way, for instance has a member of type Y, then you need a whole definition of class Y and forward declaration won't do.

Answer 3

If you're referring to a cyclic reference of the same header file, a common convention is to wrap the header in a define so the definitions are only included once, like this MyHeader.h file:

#ifndef MyHeader_h
#define MyHeader_h

// my definitions here

#endif  // MyHeader_h

Also widely supported is the #pragma once directive.