Thread: Understanding forward declarations of classes, pimpl

I read that sometimes it's good to have two sets of header files: one for declarations and one for definitions, but I'm having trouble understanding exactly when and where a forward declaration of a class can be used instead of the actual definition.

This is how I've always done things:

Code:

//myClass.h
#ifndef _MYCLASS_H
#define _MYCLASS_H

class myClass { /* Class stuff here */ };

#endif

Code:

//myClass.cxx
#include "myClass.h"

/* definitions of myClass member functions here */

Code:

// something like main.cxx or myOtherClass.h
#include "myClass.h"

/* Definitions of other classes and/or non-member functions that use or refer to myClass */

Where and how would a header file consisting only of forward declarations come into play? I've always just #included the definition header file in every other file that used the class. From what I understand, this isn't ideal because every change to the class definition would require the recompilation of everything that included it.

I'm trying to make something like myClassfwd.h with the forward declarations:

Code:

/* myClassfwd.h */
#ifndef _MYCLASSFWD_H
#define _MYCLASSFWD_H

	/* Class forward declarations */
	class myClass;

#endif

but I get compile-time errors like "error: invalid use of incomplete type" and "error: forward declaration of class blah blah blah" when I #include it instead of the definition header file. Including the myClass.h alongside the myClassfwd.h file gets rid of the errors, but obviously that's going against the whole purpose of it all.

And I mentioned pimpl in the title of the post because I know it's somehow related to all this, but I'm so confused right now I think I'll settle for understanding the basics.

Originally Posted by Boxknife

I'm having trouble understanding exactly when and where a forward declaration of a class can be used instead of the actual definition.

Typical cases include: when you are declaring a pointer or reference to an object of that class; when you are declaring (but not defining) a function with a parameter of that class type.

By the way, names that begin with an underscore followed by an uppercase letter, or that contain consecutive underscores, are reserved to the implementation for any use. Names that begin with an underscore are otherwise reserved to the implementation for use in the global and std namespaces. I suggest that you change your header inclusion guard naming convention.