Forward declaration doesn't seem to help to resolve the Circular dependency

[fomas87]

Hi guys, I'm writing a framework for an sdl gui in c++, but got stuck on the circular dependency,
I've read a lot of articles about resolving dependencies and about forward declaration but none of
them seem to work for me.
Basically I've got the Component abstract class which is the base class for all elements like
Buttons, Labels, CheckBoxes, Containers and so on. And every Component has a parent of type
Container. But here is the thing, the Container extends the Component because it should have the
same properties as Component and it also has a list of Components that belong to it.
I've tried the forward declaration but it didn't help.
Here is an example of what I mean, I can't really give you the full source code, coz it's just to big.

/**
 * Container.h file
 */
#ifndef _CONTAINER_H_
#define _CONTAINER_H_
 
#include "Component.h" // If I comment this out it gives an error about the
                       // invalid use of undefined type 'struct Component'
                       // see the comment below
 
class Component; // Forward declaration.
 
class Container : public Component
{                      // The compiler complains here!
    public:
        Container();
        virtual ~Container();
 
        Component* getChild() const;
        void setChild(const Component* c);
 
        // Some other methods
 
    private:
        Component* _child;
 
        // Some other attributes
};
 
#endif // _CONTAINER_H_
 

/**
 * Component.h file
 */
#ifndef _COMPONENT_H_
#define _COMPONENT_H_
 
#include "Container.h"
 
class Container; // Forward declaration.
 
class Component
{
    public:
        Component();
        virtual ~Component();
 
        Container* getParent() const;
        void setParent(const Container* c);
 
        // Some other methods
 
    private:
        Container* _parent;
 
        // Some other attributes
};
 
#endif // _COMPONENT_H_
 

/**
 * Component.cpp file
 */
 
#include "Container.h"
#include "Component.h"
 
Component::Component()
  : // Some initialisations
{
}
 
Component::~Component()
{
    // If this Component gets destroyed I've to remove it
    // from the parent's list first.
    if (_parent != 0)
        _parent->remove(this); // This gives me an error:
                               // invalid use of undefined type 'struct Container'
}
 
// Here goes the rest of the methods
 
 

Some people say that the circular dependency is very bad, but this is the model
I've to implement, I mean each gui element has some common properties
that I keep in the Component class. The only thing is that each Component
must belong to some Container, which has the same properties as other Components.

Any ideas how to solve such problem?

Thanks.

[weaknessforcats]

Circular references aren't bad. You just need to use them correctly. In your case I added a class forward reference to one of your header files and everything compiled without error.

#ifndef _CONTAINER_H_
#define _CONTAINER_H_
 
 
//#include "Component.h" // If I comment this out it gives an error about the
                       // invalid use of undefined type 'struct Component'
                       // see the comment below
 
class Component; // Forward declaration by weaknessforcats.
 
class Container : public Component
{                      // The compiler complains here!
    public:
        Container();
        virtual ~Container();
 
        Component* getChild() const;
        void setChild(const Component* c);
        void remove(Component* c);
 
        // Some other methods
 
    private:
        Component* _child;
 
        // Some other attributes
};
 
#endif // _CONTAINER_H_
 

BTW I also added a Conbtainer::remove() for the compiler.

[fomas87]

That's strange the CodeBlocks still gives me an error at line 12 in your code. Did you compile the Component.cpp as well? Coz I also get an error when I call the remove method like this

_parent->remove(this);

What IDE do you use, by the way?

[fomas87]

Ok, it worked! Thanks a lot for your assistance weaknessforcats. I had to change it a bit to make it
work in CodeBlocks though, but it actually makes sense to me now. I suppose each compiler links those
files differently.

But here's what worked for me any way:

/**
 * Component.h file
 */
#ifndef _COMPONENT_H_
#define _COMPONENT_H_
 
class Container; // Forward declaration.
 
class Component
{
    public:
        Component();
        virtual ~Component();
 
        Container* getParent() const;
        void setParent(const Container* c);
 
    private:
        Container* _parent;
};
#endif // _COMPONENT_H_
 

/**
 * Container.h file
 */
#ifndef _CONTAINER_H_
#define _CONTAINER_H_
 
#include "Component.h"
 
class Container : public Component
{                // The compiler complains here!
    public:
        Container();
        virtual ~Container();
 
        Component* getChild() const;
        void setChild(const Component* c);
        void remove(Component* c);
 
    private:
        Component* _child;
};
#endif // _CONTAINER_H_
 

/**
 * Component.cpp file
 */
#include "Container.h"
#include "Component.h"
 
Component::Component()
{
}
 
Component::~Component()
{
    // If this Component gets destroyed I've to remove it
    // from the parent's list first.
    if (_parent != 0)
        _parent->remove(this); // This gives me an error:
                               // invalid use of undefined type 'struct Container'
}
 

/**
 * Container.cpp file
 */
#include "Container.h"
#include "Component.h"
 
void Container::remove(Component* c)
{
}
 

Could you please run this on your IDE and see if it works for your?

[weaknessforcats]

Yes, you are right.

What I did was paste your code into one file and compile it. In that file I had:

component.h
container.h
main()

So that is the equivalent of #include <component.h> in container.h.

I use Visual Studio.NET 2005.

[fomas87]

Ok, the main thing is that it makes sense to me now so I've learned something, I'd say if I come across any other circular references in the future, I'll be able to resolve it. Thanks again weaknessforcats!