Re: forward declarations of gtkmm stuff

["Jonathon Jongsma" <jonathon quotidian org>]

On 1/2/08, Naveen Verma <ernaveenverma gmail com> wrote:
> Hi,
>
> I am sorry but somehow I got confused, I could not understand, does it make
> any difference in compilation if we include other headers(like windows.h
> etc) in a header file(e.g. foo.h) or in its source file(e.g. foo.c ) or in
> both, because in my understanding headers usually have #ifdef in it so it
> doesn't matter whether we include once or twice, it will be included only
> once at compile time, and at least we need to include once somewhere either
> in source or header.
>
> -Br
> Naveen

Yes, what you say is correct, but the point of forward-declaration is
somewhat different.  The point is not to prevent headers from being
included multiple times, but to prevent them from being included at
all if they're not absolutely necessary.  For example, say you have
the following setup:

A.h
A.cpp
B.h includes A.h
B.cpp
C.h includes B.h
C.cpp

This is a common (and valid) setup.  In this case, if you change A.h,
then B.cpp and C.cpp will need to be re-compiled (because they both
include B.h which then includes A.h).  But if B.h doesn't explicitly
need the full definition of A, then you could change it to have the
following setup:

A.h
A.cpp
B.h forward-declares A
B.cpp includes A.h
C.h includes B.h
C.cpp

Now, when you change A.h, you only need to re-compile B.cpp and not
C.cpp (since C.h includes B.h which only forward-declares A, and
doesn't include the full A.h header).  In addition, compiling C.cpp
should be a little bit faster, because the compiler doesn't need to
parse and compile all of A.h (and any headers that A.h includes).  For
a simple example, the difference is not significant, but for complex
projects, it can make a significant difference in compilation
depedencies and compilation times.

-- 
jonner