From: Kristian Nielsen
Date: June 30 2009 9:00am
Subject: Re: MySql coding style: Request for deprecation of UPPERCASE typedefs
List-Archive: http://lists.mysql.com/internals/37045
Message-Id: <87tz1yw0xs.fsf@knielsen-hq.org>
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Tor Didriksen <Tor.Didriksen@stripped> writes:

> On Mon, 29 Jun 2009 12:55:45 +0200, Kristian Nielsen
> <knielsen@stripped> wrote:

>> Mats, could you describe what the benefits are of using forward
>> declarations
>> of structs/classes in place of #including the definition?

> 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.

Ok, so to reduce compile time.

> 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.

If you expect this kind of improvements on today's computers you may be
disappointed. CPUs are _much_ faster, C++ compilation can be parallelised
across multiple cores (as opposed to eg. linking), and I suspect compilers
these days spend proportionally less time on parsing #include compared to code
generation.

Just for fun, I did some quick benchmarking on building the latest mysql 5.1
bzr tree on a cheap 2-year old quad-core. First, compiling one file in sql/
vs. compiling all:

`touch sql/mysqld.cc ; make -j5` -> 11.35 seconds, 121% CPU.

`touch sql/*.cc ; make -j5` -> 120.87 seconds, 352% CPU.

So that is a factor of only 10 between compiling one and all C++ sources. Of
course that is still something if we can get down to only one (or a few) files
#including something vs. all doing so.

Second, I tried adding #include <elf.h> (random 100k bytes, 2.5k lines file)
at the start of every .cc file in sql/, and timing `make -j5` after
`make clean`:

Without #include <elf.h>: 5 min 14 sec

With #include <elf.h>: 5 min 17 sec

So not much difference there (may even be less than the statistical noise).

Anyway, thanks for the explanation,

 - Kristian.