June 2009 - Boost - lists.preview.boost.org

Proposal for #pragma once support
by Zachary Turner 11 Jun '09

11 Jun '09

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. If so, #pragma once is used. so, something like this: #if defined(_MSC_VER) /* || defined(OTHER_PRAGMA_ONCE_ENABLED_COMPILER) ... */ #pragma once #endif the rest of the header file, including the original #include guard, could be as normal. to demonstrate the motivation for this, consider the sample on this page: http://www.gamearchitect.net/Articles/ExperimentsWithIncludes.html. At the bottom there is a zip file with a visual studio solution that contains 3 different projects. Each project contains 200 header file, each of which includes all of the other 199 header files. There is a single cpp file with a main function which includes all 200 header files. All 3 projects use internal #include guards. In addition: Project 'RedundantGuard' - Uses external include guards (wrapping the invocation of #include itself inside the appropriate guard) as discussed in the book Large Scale C++ Software Design Project 'PragmaOnce' - Uses #pragma once Project 'Nothing' - Uses nothing additional other than the internal include guards. Here are the build timings on my machine (Visual Studio 2008 Service Pack 2) RedundantGuard - 1 second compile + link time PragmaOnce - 1 second compile + link time Nothing - 15 seconds compile + link time In addition I have tested this in the code base of a commercial product I am working on (~40,000 lines of code). It consists of about 6 different projects, some of which have dependencies between each other and some which don't, so that parallel compilation can happen in some instances RedundantGuard - Not Tested PragmaOnce - 1 minute, 25 seconds compile + link time Nothing - 2 minutes, 1 second compile + link time So, I believe that boost compile times can benefit significantly from this. Thoughts?

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Re: [boost] Proposal for #pragma once support
by Zachary Turner 11 Jun '09

11 Jun '09

On Tue, Jun 9, 2009 at 11:50 AM, <dherring(a)ll.mit.edu> wrote: > On Tue, 9 Jun 2009, Zachary Turner wrote: > > Here are the build timings on my machine (Visual Studio 2008 Service Pack >> 2) >> >> RedundantGuard - 1 second compile + link time >> PragmaOnce - 1 second compile + link time >> Nothing - 15 seconds compile + link time >> > ... > >> So, I believe that boost compile times can benefit significantly from >> this. >> >> Thoughts? >> > > Could you run benchmarks on gcc? Will my compile times suffer for your > improvement? > > - Daniel > It seems someone else in the thread has already run benchmarks on GCC, but regardless of how the benchmarks did or did not turn out, I can say with 100% certainty that nobody will suffer. That's the exact reason I mentioned that compilers should be whitelisted based on which ones benefit from #pragma once, and only use #pragma once for said compilers. #if defined(_MSC_VER) && (_MSC_VER >= 1020) #pragma once #endif That's all it takes. Everybody benefits. Of course I don't have licenses for every C++ compiler around, but I'm sure there are other compilres in which it benefits also. So I would need some assistance from other people who do have other compilers to help find out which compilers it does offer an improvement on. And of course, the default would be to NOT use pragma once, erring on the side of caution. If you see any downsides to this approach let me know, but it seems like a win/win for everyone. To make it even more explicit, there could even be a global macro, which defaults to undefined, called BOOST_USE_PRAGMA_ONCE. The programmer would be forced to explicitly declare this macro before including any boost headers, and if it is enabled, boost would use pragma once.

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possible problem with boost::iostreams::file_descriptor
by Zachary Turner 11 Jun '09

11 Jun '09

nested file_descriptor implementation class is defined as follows: struct impl { impl() : #ifdef BOOST_IOSTREAMS_WINDOWS handle_(reinterpret_cast<handle_type>(-1)), #else handle_(-1), #endif flags_(0) { } impl(handle_type fd, bool close_on_exit) : handle_(fd), flags_(0) { if (close_on_exit) flags_ |= impl::close_on_exit; } ~impl() { if (flags_ & close_on_exit) close_impl(*this); } enum flags { close_on_exit = 1, append = 4 }; handle_type handle_; int flags_; }; In the second constructor in particular we have this: impl(handle_type fd, bool close_on_exit) : handle_(fd), flags_(0) { if (close_on_exit) flags_ |= impl::close_on_exit; } I think this should be changed. Just because close_on_exit is specified to false does not mean a file is not open. It just means don't close it if it is. I think flags enumeration should be changed to: enum flags { close_on_exit = 1, good = 2, append = 4 }; and constructors / code should be changed accordingly. Otherwise following code (Windows) asserts, even though I don't think it should: bool is_open(HANDLE h) { return h && h!=INVALID_HANDLE_VALUE;} HANDLE h = ::CreateFile("C:\\autoexec.bat", GENERIC_READ, FILE_SHARE_READ|FILE_SHARE_WRITE,NULL,OPEN_EXISTING,FILE_ATTRIBUTE_NORMAL,NULL); boost::iostreams::file_descriptor fd(h, false); BOOST_ASSERT(fd.is_open() == is_open(h)); Thoughts?

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Re: [boost] Proposal: Monotonic Containers: Performance test
by Christian Schladetsch 11 Jun '09

11 Jun '09

void test_speed() { typedef monotonic::map<int, monotonic::list<int> > map_with_list; monotonic::inline_storage<1000000> storage; map_with_list m(storage); size_t count = 10000; boost::timer timer; for (size_t i = 0; i < count; ++i) { int random = rand(); map_with_list::iterator iter = m.find(random); if (iter == m.end()) m.insert(make_pair(random, monotonic::list<int>(storage))); else iter->second.push_back(i); } double elapsed = timer.elapsed(); cout << "monotonic: " << elapsed << endl; // do the same thing, with std::containers { typedef std::map<int, std::list<int> > map_with_list; map_with_list m; for (size_t i = 0; i < count; ++i) { int random = rand(); map_with_list::iterator iter = m.find(random); if (iter == m.end()) m[random] = std::list<int>(); else iter->second.push_back(i); } double elapsed = timer.elapsed(); cout << "std: " << elapsed << endl; } } monotonic: 0.005 std: 0.09

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bug-sprint #2114, 2 comments
by Alexander Arhipenko 11 Jun '09

11 Jun '09

I have 2 major comment on the updated patch. Consider following snippet from gcc.hpp: #if __GNUC__ >= 4 && !defined(__MINGW32__) && !defined(__CYGWIN__) # define BOOST_SYMBOL_EXPORT __attribute__((visibility("default"))) # define BOOST_SYMBOL_HIDE __attribute__((visibility("hidden"))) # define BOOST_SYMBOL_IMPORT BOOST_SYMBOL_EXPORT ...And assume following scenario: Someone is writing shared library (let's name it foo) with 'visilibility=hidden' option. Library foo is using e.g. boost_filesystem. In this case, all the symbols marked with BOOST_FILESYSTEM_DECL will be exported from foo library. This could easily break foo library's ABI (in case of migration to the newest boost version etc.), so BOOST_SYMBOL_IMPORT should be defined for gcc as empty macro: # define BOOST_SYMBOL_IMPORT. The second comment: BOOST_EXCEPTION_EXPORT/IMPORT are intended for re-exporting exceptions from shared library. So, they should be defined for gcc as: # define BOOST_EXCEPTION_EXPORT BOOST_SYMBOL_EXPORT # define BOOST_EXCEPTION_IMPORT BOOST_SYMBOL_EXPORT and not as: # define BOOST_EXCEPTION_EXPORT BOOST_SYMBOL_EXPORT # define BOOST_EXCEPTION_IMPORT BOOST_SYMBOL_IMPORT Note: for the current patch version this part works as expected, since BOOST_SYMBOL_EXPORT == BOOST_SYMBOL_IMPORT for gcc. See also this thread (search for exception): http://lists.boost.org/Archives/boost/2008/12/146068.php Regards

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[candidate bloom_filter] Default Sequence
by JOAQUIN M. LOPEZ MUÑOZ 11 Jun '09

11 Jun '09

Currently, the default Sequence template arg is class Sequence = fusion::vector< detail::default_hash<0>, detail::default_hash<1>, detail::default_hash<2> > which implies the default K is 3. I propose to generalize this a bit so that we have something like template<typename K> default_hash_sequence...; such that default_hash_sequence<K> is the fusion sequence default_hash<0>,...,default_hash<K-1> (I don't know Fusion but I'm sure this can be done) and then put class Sequence = default_hash_sequence<3> So, a user can easily set bloom_filter<foo,default_hash_sequence<K> > with a K other than 3. Joaquín M López Muñoz Telefónica, Investigación y Desarrollo

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Re: [boost] Proposal: Monotonic Containers
by joaquin＠tid.es 10 Jun '09

10 Jun '09

Stewart, Robert escribió: > joaquin(a)tid.es wrote: > On Tuesday, June 09, 2009 9:34 AM > >> Stewart, Robert escribió: >> >>> There's a problem with using an allocator for the node based >>> containers: they use the allocator to allocate the elements >>> but not the nodes. That implies free store (de)allocations >>> for the nodes which is contrary to the intended purpose of >>> this proposal. >>> >> Umm, I beg to differ. If we have a node-based container >> >> xxx<T,...,custom_allocator<T> > >> >> and node<T> is the internal node type, the container is required >> to do the allocation using an allocator of type >> >> custom_allocator<T>::rebind<node<T> >::other. >> >> The container is not allowed to do any kind of allocation using >> other means than the custom_allocator<X> family of allocators. >> > > If custom_allocator<node<T> > is not specialized to do the same as > custom_allocator<T>, or to use the same memory pool, then it > doesn't fit the proposal. Point conceded, but how would you do otherwise? Specifying a custom allocator is your only way to affect the container's allocation behavior. Seems like you're implying that the container wrapping approach is in some way more powerful than directly specifying the custom allocator, which is not. Your only chance to solve the problem you mention is to make custom_allocator<node_type<T> > use the same memory pool. There's a way to do this. If you construct the container with custom_allocator<T> a; xxx<T,...,custom_allocator<T> > x(a); then xxx will use an object of type custom_allocator<node<T> > for its allocation needs, which perforce will be an object differnt to the allocator a you've passed; but the container is required to construct this internal allocator as a copy of a: // inside xxx implementation custom_allocator<node<T> > internal_a(a); So you can use this knowledge to pass info around, for instance on the memory pool to be used. Joaquín M López Muñoz Telefónica, Investigación y Desarrollo

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first review submission of timer_queue.zip
by JohnQ 10 Jun '09

10 Jun '09

Hi, Thanks for pointing me to the submission requirements page... I just uploaded timer_queue.zip to boost vault for first review. Haven't got everything shipshape yet but I'm hoping I've satisfied enough of the requirements so I can start getting some feedback on functionality. If not, let me know so I can rework and resubmit. In essence, timer_queue is a library for creating timers that fire at some arbitrary time in the future and hand you back a void* that you provided when you created the timer. Timers can be deleted before they fire by providing an id (can be non-unique) when you create a timer that can be used to delete particular sets of timers later on. Thanks to any who take the time to review this and help me make it better. John Q.

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[mpl]iter_fold_if Forward Backward rationale?
by Larry Evans 10 Jun '09

10 Jun '09

Why is there both a ForwardOp and BackwardOp template argument to iter_fold_if? https://svn.boost.org/trac/boost/browser/trunk/boost/mpl/iter_fold_if.hpp#L… I would have thought that only 1 was needed. Since iter_fold_if is not documented here: http://www.boost.org/doc/libs/1_38_0/libs/mpl/doc/refmanual/refmanual_toc.h… I assume it's not meant to be used by the user, but is only an implementation detail; however, it, or a variation, might be useful to implement the and_seq mentioned here: http://article.gmane.org/gmane.comp.lib.boost.devel/186204 As shown in that post, the template, while_if_then was used, and it does not require anything like both a ForwardOp and BackwardOp, it just requires the If and Then unary metafunctions. One problem I encountered with while_if_then was moving to the end of the sequence, at which time the Then metafunction couldn't be called; hence, I had to make it lazy to avoid dereferencing the end iterator. Is that somehow related to the need for BackwardOp in iter_fold_if. IOW, because iter_fold_if might reach the end iterator, it has to backup? I'm really asking because in the variadic template version of mpl, I wondering if iter_fold_if could be replaced with something like while_if_then (which may be renamed fold_if to be more consistent with other names with similar function). Any insight about the rationale for the iter_fold_if would probably save me considerable time in deciding whether it can be replaced with the proposed fold_if. -regards, Larry

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[filesytem][1.39.0] basic_path::clear mia?
by Jeff Flinn 10 Jun '09

10 Jun '09

http://www.boost.org/doc/libs/1_39_0/libs/filesystem/doc/reference.html#bas… lists void clear(); as an available modifier on basic_path. Neither I nor MSVC8 seem to be able to find it. Am I doing something wrong? Thanks, Jeff

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