Resolve circular include of 3 classes in C++

Question

I have 3 classes which I am going to refer to as A, B and C (names in this example are arbitrary). All 3 of these classes consist of a header-file and a cpp-file each.

I have the following dependencies:

A needs to know B as A has a member of type B
B needs to know A and C as B has members of both types
C needs to know A as C has a member of type A

I have tried out several options already (include this header here, forward-declaration of class there), but compilation was never succesful. I also couldn't extract any helpful information from answers to problems which solved circular inclusion problems between only 2 classes.

EDIT: Here a minimal (and hopefully sufficient) example of my current setup:
A.h

class A {
private:
    B myB;
};

B.h

class B {
private:
    A* myA;
    C myC;
};

C.h

class C {
private:
    A* myA;
}

I should perhaps also note that B::myA and C::myA are always going to point to the same A-instance.

`I have tried out several options already (include this header here, forward-declaration of class there), but compilation was never succesful.` Forward declarations are the way to go, we need an [MCVE]. — tkausl, Dec 01 '19 at 17:55
Show us the (possibly abbreviated) declarations and definitions for your classes, and I'm 100% sure somebody here will give you a solution to your problem (or maybe more than one solution). — Adrian Mole, Dec 01 '19 at 17:56
We really need to see a clear example of what you're trying to do, especially in regards to which members are pointers/references vs whole objects. At the moment I could read you explanation as wanting for each A to contain a B and then for each B to contain an A which isn't going to work since that leads to infinity. — TheUndeadFish, Dec 01 '19 at 18:30
[https://stackoverflow.com/questions/625799/resolve-build-errors-due-to-circular-dependency-amongst-classes](https://stackoverflow.com/questions/625799/resolve-build-errors-due-to-circular-dependency-amongst-classes) — drescherjm, Dec 01 '19 at 18:37
The headers for class `B` and `C` don't need or want to include the header for `A`. The header for class `A` needs to include the header for class `B` — drescherjm, Dec 01 '19 at 18:42
Per drescherjm: possible duplicate of [Resolve build errors due to circular dependency amongst classes](https://stackoverflow.com/questions/625799/resolve-build-errors-due-to-circular-dependency-amongst-classes) — Davis Herring, Dec 01 '19 at 18:56

Adrian Mole · Accepted Answer · 2019-12-01T19:00:15.473

As class C only includes a pointer to class A, it doesn't need the full definition for either A or B (which it doesn't use); so, just a statement declaring A as a class will suffice:

C.h

class A; // Declare that A is a class - then we can have a pointer to it...
class C {
private:
    A* myA;
}

Class B needs the definition of C, because it includes an instance of C; and, like class C, simply declaring A as a class (as is done already in C.h) will do:

B.h

#include "C.h" // Note: This header already declares "A" as a class!
class B {
private:
    A* myA;
    C myC;
};

Class A needs the definition of B, as it contains an instance of B. But note that B.h already includes C.h:

A.h

#include "B.h"
class A {
private:
    B myB;
};

Note that any other files that use one or more of A, B and C need only include the A.h header, as this will itself include the others.

Thank you very much, after applying your solution to my project and additionally including A.h in C.cpp it was compiled without issues. — Lux, Dec 01 '19 at 19:04

Resolve circular include of 3 classes in C++

1 Answers1