On Mon, 29 Jun 2009 12:55:45 +0200, Kristian Nielsen
<knielsen@stripped> wrote:
> Mats Kindahl <mats@stripped> writes:
>
>> Just to clarify, when we mention forward declarations here, we are
>> talking about
>> forward declarations of *classes* only, not variables, so the "extern
>> int foo"
>> is not an example of a forward declaration we are considering. For
>> functions and
>> variables, it is a requirement to use an include file.
>
> Mats, could you describe what the benefits are of using forward
> declarations
> of structs/classes in place of #including the definition?
>
> Clearly, self-references in structs (whether direct or recursive) require
> forward declarations, but otherwise?
>
> It seems to me to be a simpler if code referencing struct/class FOO would
> always do so by including file_defining_FOO.h? What am I missing? I don't
> remember seeing this style much in C/C++ code...
Please see
http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml#Header_File_Dependencies
for an explanation.
If a class 'Bar' is implemented in terms of another class 'Foo', or uses
it's interface in some way,
it will typically store a pointer to it (in its private section).
By forward declaring the class
class Foo;
rather than
#include "foo.h"
you can often drastically cut down on the number of header files included.
The savings in compile time can be quite substantial.
I remember doing this on a project sometime in the 90's cutting down
compile/build time from a day (yes 24 hours) to about one hour.
-- didrik
>
> (More generally, I think your proposal would benefit from explaining more
> concretely what problems you are trying to solve. Clearly there are some
> real
> issues that are being addressed, like the possibility to use parts of the
> server code in unit tests that has been hinted at. It seems to me that
> the
> proposal mixes some changes needed to solve concrete problems with other
> changes that are more a matter of style and preferences).
>
> - Kristian.
>
