Question

Why declare the class that you included as a header file?

#include "TreeCallObj.h"
#include "TreeDevObj.h"
#include "TreeDevCallObj.h"

class TreeCallObj; //what is the purpose of this line ?
class TreeDevObj;  //what is the purpose of this line ?
class TreeDevCallObj;  //what is the purpose of this line ?


class Apple
{
public:
...
private:
...
}

Answer 1

Consider this:

//a.h
#ifndef A_H
#define A_H

#include "b.h"
class A
{
   B* b;
};
#endif

//b.h
#ifndef B_H
#define B_H

#include "a.h"
class B
{
   A* a;
};
#endif

Now you try to include one of the files in a different one, say you #include "a.h".

The compiler will parse it as:

#ifndef A_H
#define A_H

fine – A_H isn’t defined

#include "b.h"

try to paste the contents:

#ifndef B_H
#define B_H

ok, since B_H isn’t defined

#include "a.h"

this will not define A, because A_H is defined. So next, we have

class B
{
   A* a;
};

which will lead to an error, because A wasn’t defined or declared before the use.

The forward declaration fixes this.

Of course, the best solution to this is to not include at all (unless you absolutely have to).