On 11 Jun 2009, at 19:40, Nevin :-] Liber wrote:
2009/6/9 Zachary Turner <divisortheory@gmail.com>
I would like to propose conditional support for #pragma once be added to all header files in boost. This would involve, at the beginning of every header file, a preprocessor check to determine whether or not the particular compiler is on a whitelist of known compilers to optimized #pragma once.
1. To make a case for using the non-portable #pragma once vs. portable include guards, you'll have to come up with metrics comparing #pragma once with include guards and show that #pragma once is significantly faster for the compilers listed at < http://www.boost.org/development/tests/release/developer/ summary.html>.
2. When compilers get #pragma once wrong (as in < http://gcc.gnu.org/ml/gcc-bugs/2009-01/msg03268.html> earlier this year), who exactly is going to revert all of Boost back?
3. The semantics of #pragma once are different than the semantics of include guards (I believer that #pragma once tends to use the OS notion of identical files, and can fail under certain pathological cases). We would still need include guards everywhere for correctness.
While I agree (1) needs doing, both (2) and (3) fall under the same category. I don't believe anyone is suggesting header guards be removed, and all bugs in #pragma once I've ever seen have involved multiple inclusions of a file, not skipping a file. Chris