Re: C-style unit -> C++ class, implementation function / structure issue

"Alf P. Steinbach" <>
Thu, 12 Jul 2007 21:14:04 +0200
* James Kanze:

On Jul 12, 7:15 pm, Jason Doucette <> wrote:

I'm not sure I understand. How did you get by in C without
defining the struct in the header? You can wrap exactly the
same technique in a class.

(I was using a C++ compiler, so my original code wasn't technically
plain C code... I was using C++ without classes. I was using C-style
units of functions, where the implementation existed all in the .cpp
file, and the interface was visible in the .h file)

I defined the struct in the .cpp file, since only the implementation
functions in the .cpp file used it. The struct was not required for
any interface functions, so it didn't have to exist in the .h file.

In other words, the compilation firewall (pimpl) idiom. You can
do exactly the same in C++.

Yes, I also suggested PIMPL, plus a special case (singleton) where no
dynamic allocation is required.

However, the OP is very very vague about his code, and PIMPL isn't
necessarily required.

For example,

   //------------------- the.h

   #ifdef THE_H
   #error On the usenet, we strive to minimize typing.
   #define THE_H

   class SillyClass
       struct Foo;
       void get( Foo& );
       void doThings();

   //------------------- the.cpp

   #include "the.h"

   struct SillyClass::Foo
       char data[100000];

   void SillyClass::get( Foo& fooData ) { ... }

   void SillyClass::doThings()
       Foo fooData;
       get( fooData );

A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

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