Including child class into basic C++

Question

I have three classes:
BaseClass, that contains ElementClass as field
ElementClass, that contains BaseClass as field and ChildClass as return type (pointer)
ChildClass, that's inherits from BaseClass.
When I'm trying to compile this, I am getting
expected class-name before '{' token
for ChildClass, that was included in ElementClass, that was included in BaseClass.
I understand, why is this happens. Because I'm trying to inherit child class from nonexistent, for compiler, class BaseClass.
I can't understand hot to fix this. Thank you.

Here is code example with this problem
BaseClass.h

#ifndef INHERITTEST_BASECLASS_H
#define INHERITTEST_BASECLASS_H

#include "ElementClass.h"

class ElementClass;

class BaseClass
{

private:
    ElementClass *m_someField;
};


#endif

ElementClass.h

#ifndef INHERITTEST_ELEMENTCLASS_H
#define INHERITTEST_ELEMENTCLASS_H

#include "ChildClass.h"

class ChildClass;

class ElementClass
{
private:
    ChildClass *m_class;
};


#endif

ChildClass.h

#ifndef INHERITTEST_CHILDCLASS_H
#define INHERITTEST_CHILDCLASS_H

#include "BaseClass.h"

class ChildClass : public BaseClass
{

};


#endif

related/dupe: http://stackoverflow.com/questions/625799/resolve-circular-dependencies-in-c — NathanOliver, Mar 16 '16 at 17:36
1. sounds like your class design could use more work, 2. forward declaration. — crashmstr, Mar 16 '16 at 17:37
3. show actual code instead of describing how the code looks like. — bolov, Mar 16 '16 at 17:37
If `BaseClass` contains one `ElementClass` field and if `ElementClass` contains one `BaseClass` field, the program is ill-formed and will not compile. C++ will only accept a fully defined class for a member. — Serge Ballesta, Mar 16 '16 at 17:40
@SergeBallesta I meant, that fields are pointers to objects. — Megaxela, Mar 16 '16 at 18:20
@NathanOliver No, I have another problem. I have problem with inherit, not with cycle pointer fields. — Megaxela, Mar 16 '16 at 18:21
Hm. It's pretty big, so I'll try to make small alternative. If I'll can't I will post all my code. — Megaxela, Mar 16 '16 at 18:38
@lispHK01 Ok. Edited question with sample project, introducing this "problem" — Megaxela, Mar 16 '16 at 18:47
@bolov Added example code with this "problem" that is not compiling. — Megaxela, Mar 16 '16 at 18:49
@Megaxela No you didn't, your BaseClass.h and ChildClass.h are identical — Mooing Duck, Mar 16 '16 at 18:51
Your problem is probably, that you are indirectly including `ChildClass.h` in `BaseClass.h`, resulting in you trying to define `ChildClass` before `BaseClass`. Just remove the extra `#include`s and only use forward declarations. Only include you need here is `BaseClass.h` in `ChildClass.h` (since that's not a pointer). — hyde, Mar 16 '16 at 19:55

score 0 · Answer 1 · edited Jun 20 '20 at 09:12

BaseClass, that contains ElementClass as field

ElementClass, that contains BaseClass as field

So, a BaseClass instance contains an ElementClass instance which contains a BaseClass instance which contains an ElementClass instance which contains... instances all the way down. One instance is going to use all of your memory and then some. That cannot work.

It also cannot work because when defining the first class, you need to have a complete definition of the second class, which needs a complete definition of the first class, which wasn't defined yet.

You must use indirection. At least one of the classes must not contain the other instance inside, but instead a pointer or a reference to an instance.

I tried to answer by quoting my own answer inside itself. You beat me to the punch. I'm stil copy-pasting... — bolov, Mar 16 '16 at 17:42

Including child class into basic C++

1 Answers1