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.