... the #pragma which can do anything at all... Emil Dotchevski
Really? Describe two things that #pragma-once can do? I can think of only one. ------------------------------------------------------------------------ ---------------- Look, large builds take major advantage of the capability of the #pragma-once statement. I added the #pragma-once to my company's builds and shaved off %26 from the build time. That's pretty significant within large builds. Along with precompiled headers, the #pragma- once feature is a very cool thing to have on your side. In fact, this feature is so cool that its capability was added to the #ifdef header guards in some compilers. But that to me is the wrong way to go because it makes the #ifdef non- standard. Remember that #define can be un-defined, and then what exactly does the header guard do in that case? #ifndef __BOOST_HEADER_GUARD__ #define __BOOST_HEADER_GUARD__ // this macro can be undefined #pragma once // the pragma-once cannot be turned off I'm not going to split hairs over what compilers do with their #define header-guards, but the point is that this cool feature is being used by loads of major C++ compilers. In every case the compiler treats the #pragma-once in exactly the same way. This is how standards come to be in future versions of products. shared_ptr<> was a boost thing and now it's a C++0x thing. The same thing appears to be happening with #pragma-once in an undocumented way because it's way too cool. I mean take a look at this ugly code below: #ifndef __BOOST_HEADER_GUARD__ #define __BOOST_HEADER_GUARD__ #if defined( _MSC_VER ) && ( _MSC_VER >= 1020 ) # pragma once #endif First off, it's just plain ugly. It's also error prone since it's got to be typed in exactly, but most important, it implies that no compiler other than Microsoft can take advantage of the #pragma-once capabilities of our modern day compilers. Boost libraries are constantly being cleaned up by deprecating stuff no longer needed, and this is one those things that needs to be deprecated, even though it's a very minor thing. I would be more than happy to perform the deprecation work. Regards, -Sid Sacek