Avoid cyclic references caused by C++ includes headers

Question

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.

You haven't specified if you're including all these files in the implementation (.cc, .cpp) or the header (.h). Looks like most answerers are assuming you meant the header file. — i_am_jorf, Sep 24 '12 at 15:25

score 7 · Accepted Answer · answered Sep 24 '12 at 15:23

7

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.

answered Sep 24 '12 at 15:23

Luchian Grigore

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Well, my class/object structure looks like this: `A` is in fact a window with a `tab control`, each of it's tabs is a `B` object (which is another `tabcontrol`), and each `B` has a `C` obj for each of his tabs. `C` class has a few buttons, that when i press, sends a message to `A`( each button send a different message). And all of those, are part of a another window, the main one. A `C` object does not have a pointer to `A`, and `A` has an indirect pointer to `C` through `B`. – MRM Sep 25 '12 at 15:54
@MRM did you understand what a forward declaration is and tried using it? – Luchian Grigore Sep 25 '12 at 15:56
Nice. It's exactly what i needed. Thank you! – MRM Oct 01 '12 at 22:04

score 1 · Answer 2 · answered Sep 24 '12 at 15:28

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.

score 0 · Answer 3 · answered Sep 24 '12 at 15:38

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.

Avoid cyclic references caused by C++ includes headers

3 Answers3