Question

In Effective C++ (3rd edition), Scott Meyers, in Item 31, suggests that classes should have, on top of their classic Declaration (.h) and Definition (.cpp) files, a Forward Declaration Include File (fwd.h), which class that do not need the full definition can use, instead of forward declaring themselves.

I somewhat see the case for it, but I really don’t see this as a viable option… It seems very hard to maintain, rather overkill and hardly necessary.

I can, however, see its use for template forward declarations, which are rather heavy. But for simple classes? It seems to be that it’s a pain to maintain and will create a whole lot of almost empty include files that serve a very small purpose… is it worth the hassle?

Here’s a example:

// Class.h
class Class
{
    Class();
    ~Class();
};

// ClassFwd.h
class Class;

// Class.cpp
Class::Class()
{
}

Class::~Class()
{
}

My question:

What do you guys think? If this a good practice?

NOTE I am mostly interested in the arguments FOR this practice, to see if I missed something that would make me agree with Scott Meyers.

Answer 1

I used forward declaration header files for all my libraries. A library would typically have this structure:

lib/
  include/
    class headers + Fwd.h
src/
  source files + internal headers

The lib/include directory would contain all public classes headers along with one forward declarations header. This made the library light-weight on the include side. Any header outside of this library only includes the forward header (Fwd.h), while sources outside of this library includes the necessary complete headers. One can also provide a convenience header (Lib.h) that includes all the other headers, for use in source files.

Another thing to place in the forward declaration header is typedefs for shared_ptr, especially in the case of an inheritance hierarchy with factory classes that return pointers to implementations.

The above is useful for larger applications with lots of internal libraries. A refinement of the above for this case would be to place the public headers in lib/include/lib. This way clients of your library would have to include lib/.... Think of this as a namespace for your headers.

Good luck!