2.10. Class Declarations and Definitions

2.10. Class Declarations and Definitions
	Chapter 2. Top of the `class`

[ fromfile: classdeclarations.xml id: classdeclarations ]

Figure 2.4. Bidirectional Relationship

Bidirectional relationships appear quite often in classes.
the compiler needs to have some knowledge of each class before defining the other.
You might try to have each header file #include the other, as shown in Example 2.12 and Example 2.13.

[ . . . . ]
#include "chicken.h"
class Egg {
 public:
    Chicken* getParent(); 
};
[ . . . . ]

[ . . . . ]
#include "egg.h"

class Chicken {
 public:
    Egg* layEgg();
};
[ . . . . ]

The preprocessor does not permit circular dependencies such as these.
In this example, neither header file needed to include the other.
In each case, doing so created an unnecessarily strong dependency between header files.

Under the right conditions, C++ permits you to use a forward class declaration instead of #including a particular header.

[ . . . . ]
class Chicken;              
class Egg {
 public:
    Chicken* getParent();    
};
[ . . . . ]

	Forward class declaration.
	Okay in declarations if they are pointers.

Classes that are declared but not defined can only be used as types for pointers or references, as long as they are not dereferenced in the file.

We define getParent() in the source code module, egg.cpp, shown in Example 2.15.
Notice that the .cpp file can #include both header files without causing a circular dependency between them.
The .cpp file has a strong dependency on both headers, while the header files have no dependency on one another.

#include "chicken.h"
#include "egg.h"

Chicken* Egg::getParent() {
    return new Chicken();    
}

Requires definition of Chicken.

For further details, see Section C.2


2.9. The Keyword `static`		2.11. Copy Constructors and Assignment Operators