[prog] [C/C++] what to put in header files.

[Hemingway, Neil]

Hello,

	I've read this thread with interest because I've been bitten by this
in the past, and I wanted to pass on my experience...

For small scale projects what has been said is accurate, but as projects
get larger, what comes to dominate the build time is header inclusion.
To mitigate this you should only include header files when you absolutely
must, and most people are surprised when they find out the cases when you
do _not_ have to include the header file.

For C++, it boils down to whether the compiler needs to see the class
definition, or whether it simply needs to know that a class with such
a name exists somewhere.  For example, if a .cpp file refers to a attribute
of a class, then the compiler needs to know not only the address of the
class,
but also the offset into the class for the attribute.  In order to do this,
it
must have the class definition, and therefore you need to include the
header file.

On the other hand, if there is a class named Foo, and another class (Bar)
has a member attribute of type pointer to Foo, then the compiler does _not_
need to see the definition of class Bar to compile Foo.  Consequently, you
do
not need to include the header, you can simply say within the file defining
Foo:

	class Bar;

and this will satisfy the compiler.

The book I refer to below has all the details, and lists all of the
situations
where you do and where you do not need to include the header file.  I only
don't
include them here because I can't remember them all.

Suffice it to say that by taking this approach, you can dramatically cut the
compilation time for a large project.

Reference: "Large Scale C++ Design" by John Lakos published by Addison
           Wesley (ISBN: 0201633620)

Regards, Neil

-----Original Message-----
From: Kathryn Hogg [mailto:kjh at flyballdogs.com]
Sent: 19 September 2002 15:26
To: programming
Subject: Re: [prog] [C/C++] what to put in header files.



> One rule that really helps with compiling huge projects is that each
> include file should include as few other files as possible.  If class
> Foo uses class Bar, it's much better to include Bar.h in Foo.cpp rather
> than in Foo.h.

May I add something to this advice?  If Foo.h uses anything from class Bar,
then Foo.h should include Bar.h.  Not only does this make it easier users of
class Foo since they only need to include Foo.h but it may be required for
C++ template instantiation on some systems.  Of course, I always guard
multiple inclusion of dependent header files within files.

 > Those things you often see at the beginning and end of an include file:
>
> #ifndef FOO_H
> #define FOO_H
> [ ... body of file ... ]
> #endif /* FOO_H */

I always use some sort of organization tag like CES_FOO_H since I may use
code developed elsewhere that uses FOO_H.

-- 
Kathryn


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