Question

I’m looking to have a hierarchical class structure in which a level of controller ‘parent’ classes are responsible for creating/directing a number of ‘child’ classes. The parent class should be able to reference each child it creates directly, and each child should be able to reference its parent (and, assuming this child is not also a parent of more classes, only its parent) directly. This allows for the referencing of siblings through the parent. I’ve found this paradigm to be useful in JIT compilation languages like Java and C#, but C++ presents a unique problem…

My first attempt to implement this paradigm was as follows:

Parent Class TreeRoot.h

#ifndef __CCPP_SCENE_H__
#define __CCPP_SCENE_H__
#include "ChildA.h"
#include "ChildB.h"

class TreeRoot : 
{
private:


ChildA* a;
ChildB* b;

public:

//member getters
ChildA* getA();
ChildB* getB();
};

#endif // __CCPP_SCENE_H__

Child class ChildA.h

#ifndef CHILDA_H_
#define CHILDA_H_


#include "TreeRoot.h"


class ChildA
{

private:
TreeRoot* rootScene;

public:
ChildA(TreeRoot*);
~ChildA(void);

TreeRoot* getRootScene();
void setRootScene(TreeRoot*);

};

#endif /*CHILDA_H_*/

Child class ChildB.h

#ifndef CHILDB_H_
#define CHILDB_H_


#include "TreeRoot.h"


class ChildB
{

private:
TreeRoot* rootScene;

public:
ChildB(TreeRoot*);
~ChildB(void);

TreeRoot* getRootScene();
void setRootScene(TreeRoot*);

};

#endif /*CHILDB_H_*/

Now of course that wouldn’t compile because of the circular include (TreeRoot.h includes ChildA.h and ChildB.h, which both include TreeRoot.h etc) So I tried using forward declaration instead:

Parent Class TreeRoot.h

#ifndef __CCPP_SCENE_H__
#define __CCPP_SCENE_H__
#include "ChildA.h"
#include "ChildB.h"

class TreeRoot : 
{
private:


ChildA* a;
ChildB* b;

public:

//member getters
ChildA* getA();
ChildB* getB();
};

#endif // __CCPP_SCENE_H__

Child class ChildA.h

#ifndef CHILDA_H_
#define CHILDA_H_


//#include "TreeRoot.h" //can't use; circular include!
class TreeRoot;

class ChildA
{

private:
TreeRoot* rootScene;

public:
ChildA(TreeRoot*);
~ChildA(void);

TreeRoot* getRootScene();
void setRootScene(TreeRoot*);

};

#endif /*CHILDA_H_*/

Child class ChildB.h

#ifndef CHILDB_H_
#define CHILDB_H_


//#include "TreeRoot.h" //can't use; circular include!
class TreeRoot;



class ChildB
{

private:
TreeRoot* rootScene;

public:
ChildB(TreeRoot*);
~ChildB(void);

TreeRoot* getRootScene();
void setRootScene(TreeRoot*);

};

#endif /*CHILDB_H_*/

that implementation almost works in that I can successfully broadcast messages to the child objects and perform callbacks from the child objects to the parent class as follows:

TreeRoot.cpp

...
a->someChildMethod();
a->getRootScene()->someParentMethod();

However when I try the following:

ChildA.cpp

...
rootScene->someParentMethod(); //ERROR C2027: use of undefined type TreeRoot

I get an undefined type error. This makes sense since using forward declaration as above doesn’t inform the compiler of what TreeRoot actually is. The question then is how can I enable calls from the child objects like the rootScene->someParentMethod() call above? Perhaps some use of generic types via templates would make the compiler happy and provide the functionality I’m looking for?

thanks,
CCJ

Answer 1

Use a forward declaration in all your .h files. You can do this since you’re only storing pointers as class members so you don’t need the full class declaration.

Then, in all your corresponding .cpp files, #include the header files for the classes you need.

So, in TreeRoot.h you forward declare ChildA and ChildB. In TreeRoot.cpp, you #include ChildA.h and ChildB.h.

Rinse and repeat for your 2 other classes.

Take note that this will solve your current problem, but this design seems flaky at best.