Re: [boost] Boost.Math: Ignored trac patches

From: John Maddock (john_at_[hidden])
Date: 2007-12-15 04:49:05

Jens Seidel wrote:
>> Why?  It makes no difference, except to improve performance for
>> compilers that don't implement the include-guard detection trick
>> that gcc uses.
>
> Heh? What trick?

GCC has an optimisation: it spots when a file has already been included and 
auto-magically prevents it from being included twice, if the effect would be 
to include an empty file.  It's similar to Microsoft's #pragma once but 
automatic.

> To ensure that an include file is not included twice one normally
> defines a unique preprocessor macro at the beginning of the header
> file
> and skips processing if it was previously defined.
>
> Doesn't every Boost header does this? Do you consider using #define a
> trick?

That's not what I'm talking about, and those files do use the std include 
guard mechanism, it's just that's there's a second catch to prevent multiple 
inclusion, so the file doesn't get opened twice.  But in all honesty it's a 
triviality ;-)

> I just want to mention that
>  #include <boost/math/complex/details.hpp>
> is easier to read than
>  #ifndef BOOST_MATH_COMPLEX_DETAILS_INCLUDED
>  #  include <boost/math/complex/details.hpp>
>  #endif
>
> It is also more secure because you increase the chance to make a typo
> in
> the macro which could be hard to track down.
> You're right that it could be a little bit faster but I have never
> seen
> it somewhere (including Boost).

As noted by Edward, various vendors use this in their own headers.

Is it a good idea?  I don't know, I think the jury is still out on that one. 
If more compilers would follow GCC's lead and automatically recognise 
include guards, the the discussion would be moot anyway...

John.