April 2012 - Boost - lists.preview.boost.org

Metaprogram benchmarks
by Dave Abrahams 09 Apr '12

09 Apr '12

In order to measure the compile-time efficiency of various C++11 metaprogramming techniques, I'd like to put together a set of benchmarks, and I wanted to discuss here what kinds of tests might be appropriate. Aleksey and I came up with some benchmarks for an appendix to http://boostpro.com/mplbook/, but some of those are lost and they're all getting a bit crusty. Certainly, I have no confidence that they are realistic or useful. If anyone has ideas about this, I'd be very glad to hear them. Thanks in advance, -- Dave Abrahams BoostPro Computing http://www.boostpro.com

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[string_algo] ilexicographical_compare()/is_iless() bug?
by Dmitry Vinogradov 08 Apr '12

08 Apr '12

I want to share a problem using ilexicographical_compare(). I think that ilexicographical_compare() must compare two strings in "alphabetical" order, the order letters appear in alphabet. But ilexicographical_compare() uses is_iless() to compare letters. And such comparison looks like: std::toupper<T1>(Arg1,m_Loc)<std::toupper<T2>(Arg2,m_Loc); So, in fact, letters are compared depending on their position in a charset. It's not always match the alphabet order. Examples are Cyrillic letters "Io" and "i" (Unicode 0451 and 0456). I think the right solution is to compare like this: T1 Ch1 = std::toupper<T1>(Arg1,m_Loc); T2 Ch2 = std::toupper<T2>(Arg2,m_Loc); return std::use_facet< std::collate<typename CharType> > (m_Loc).compare(&Ch1, &Ch1 + 1, &Ch2, &Ch2 + 1);

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[SVN] Spurious changes to lots of files....
by John Maddock 08 Apr '12

08 Apr '12

I'm not picking on anyone in particular here, just noticing that some stuff that really shouldn't have changed is quite often getting "touched" due to changes to svn:mergeinfo. For example, if I look at the revision log for boost/config.hpp in the release branch I see: Revision: 77591 Author: eric_niebler Date: 27 March 2012 20:28:30 Message: merge [77416] from trunk, fixes #6131, fixes #6455 Revision: 77589 Author: eric_niebler Date: 27 March 2012 19:53:50 Message: merge [77417],[77437],[77422],[77587] from trunk ---- Revision: 77447 Author: eric_niebler Date: 21 March 2012 04:22:38 Message: merge [77446] from trunk ---- Revision: 77420 Author: eric_niebler Date: 19 March 2012 21:33:14 Message: merge [77419] from trunk ---- Revision: 77413 Author: eric_niebler Date: 19 March 2012 20:26:44 Message: merge [77412] from trunk, fixes #6702 ---- Revision: 77411 Author: eric_niebler Date: 19 March 2012 20:07:11 Message: merge [77320] from trunk ---- Revision: 76784 Author: eric_niebler Date: 29 January 2012 21:58:13 Message: merge [76637] from trunk ---- Revision: 76536 Author: jewillco Date: 15 January 2012 23:52:45 Message: Merged r75066, r75067, r75124, r75140, r75165, r75861, r75862, r75863, r75878, r75906, r75970, r75973, r75974, and r76394 from trunk ---- Revision: 76535 Author: jewillco Date: 15 January 2012 23:32:09 Message: Merged r76050, r75547, r75891, r76049, r76083, and r76439 from trunk (reverse_graph bug fixes and fix for #6293); refs #6293 ---- Revision: 76480 Author: eric_niebler Date: 14 January 2012 05:21:11 Message: merge [76479] from trunk ---- Before finally we get to a "real" change: Revision: 76339 Author: johnmaddock Date: 07 January 2012 17:29:04 Message: Merge config changes from Trunk. ---- If we pick out the latest of the changes above we see that a huge number of files have been "touched" in this way: Revision: 77591 Author: eric_niebler Date: 27 March 2012 20:28:30 Message: merge [77416] from trunk, fixes #6131, fixes #6455 ---- Modified : /branches/release Modified : /branches/release/INSTALL Modified : /branches/release/Jamroot Modified : /branches/release/LICENSE_1_0.txt Modified : /branches/release/boost 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/branches/release/status/Jamfile.v2 Modified : /branches/release/status/expected_results.xml Modified : /branches/release/status/explicit-failures-markup.xml Modified : /branches/release/status/explicit-failures.xsd Modified : /branches/release/tools Modified : /branches/release/tools/bcp Modified : /branches/release/tools/boostbook Modified : /branches/release/tools/build/v2 Modified : /branches/release/tools/build/v2/tools/mpi.jam Modified : /branches/release/tools/inspect Modified : /branches/release/tools/quickbook Modified : /branches/release/tools/regression Modified : /branches/release/tools/regression/src/library_status.cpp Modified : /branches/release/tools/release Modified : /branches/release/tools/wave As well as cluttering up the revision logs, this often leads to property and/or tree conflicts when merging your own stuff, and I can't tell you how annoying that is to figure out! :-( BTW, my sincere apologies to Eric here - he just happened to be the latest culprit - can some SVN guru suggest how to prevent this sort of spurious changing going on? BTW2, lest this turn into "my extra advanced version control system doesn't do messy stuff like this" conversation - I'm sure you're correct - some other version control system probably doesn't have this trap - but I sure bet it has other traps just as annoying for the unwary. Just saying.... Thanks, John.

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C++ Now! Early Bird Registration Ending Soon
by Jon Kalb 08 Apr '12

08 Apr '12

The fifteenth of this month is the end of Early Bird registration. Register by the end of next week to reserve your place and save on registration. Featuring 3 keynote speakers headlining 3 tracks of 50+ session brought to by 35+ presenters, this year¹s conference is our largest and most ambitious. What isn¹t bigger is the registration fee. If you beat the April 15th deadline you can enjoy the conference offering the most complete C++11 tutorial coverage of the year and save money too. See you in Aspen! Registration: http://cppnow.eventbrite.com/?ref=ecount Jon

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Nightly testing broken due to missing file
by Belcourt, K. Noel 07 Apr '12

07 Apr '12

error: Unable to find file or target named error: './sync/mutual_exclusion/locks/upgrade_lock/cons/ move_ctor_shared_lock_until_pass.cpp' error: referred from project at error: '../libs/thread/test'

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C++11 Metaprogramming
by Dave Abrahams 07 Apr '12

07 Apr '12

Hi All, I am on the C++Now 2012 schedule giving a talk on metaprogramming in C++11, which is really just supposed to be an overview of the state of the art. I am just at the beginnings of my research for this presentation, having learned a few things and done a few experiments, and it seemed to me foolish not to ask the Boost community for its insights. I'm sure y'all have come up with many neat tricks and techniques. If you'd care to share them here, that would be much appreciated. TIA, -- Dave Abrahams BoostPro Computing http://www.boostpro.com

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Re: [boost] C++11 Metaprogramming
by Andrzej Krzemienski 07 Apr '12

07 Apr '12

> On 06/04/12 09:04, Andrzej Krzemienski wrote: >>> And this is the worst part: I forgot the >> "typename". Having to type this "typename" is really terrible and just >> scares off many people. If you do not mind it, it only means that you got >> used to it; but people should not be forced to get used to too many >> gotchas. > > The language requires to specify whenever dependent names in template > contexts are anything else than values (i.e. types or templates). > > There is no substitute for learning the basics of the language. This is > not a gotcha, just normal usage of templates. Clearly, if you do not find requiring of the programmers to type this "typename" in these places even slightly wrong, you will not appreciate alias templates as a useful addition to meta-programming. Also, if you did a lot of advanced meta-programming, you have probably mastered the rules of template syntax, and find it difficult to sympathize with those that did not master the subject. But looking at it from the fresh programmer's perspective, the fact that the language requires "to specify whenever dependent names in template contexts are anything else than values" means that the language is not friendly to the fresh programmers (I understand that there are good reasons for requiring this, but still...). Based on the Wkipedia's definition of "gotcha" I interpret this typename as one. "In programming, a gotcha is a feature of a system, a program or a programming language that works in the way it is documented but is counter-intuitive and almost invites mistakes because it is both enticingly easy to invoke and completely unexpected and/or unreasonable in its outcome." It (the usage of typename) only appears as "normal usage" after you are past the stage of surprise. The context of your reply seems to imply that whoever finds this behavior surprising is guilty of not having acquired the basics of the language. Unless I misunderstood you, I think this position is a bit unfair. Regards, &rzej

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boost.asio and boost.bind question
by Philip Bennefall 07 Apr '12

07 Apr '12

Hi all, I have been using Asio and Bind for a server with great success, however I just thought of something that I wanted to clarify. If I have a class method and construct a string within this method, and then call post on the IO service with a bind generated function object that takes this local std::string as one of its arguments, is the string then copied? Naturally it will be destroyed after the asynchronous invokation when the method returns, so is it just by chance that this has worked so far? Is it undefined behavior? Example: void dummy() { string final_message="some_data"; the_client->partner_sync.post( boost::bind( &deliver_message, the_client, the_client->partner, final_message) ); } deliver_message is a global function. the_client and the_client->partner are both smart pointers to instances of the client class, and partner_sync is a strand that belongs to the client class. Is this legal usage, or am I messing things up? Do I need to keep the string around until the completion handler is invoked? Or does bind take care of this? I keep the buffer around whenever I use the asynchronous sending methods in Boost Asio as this is explicitly stated in the documentation, but is this case the same? Thanks in advance for any clarifications. Kind regards, Philip Bennefall

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[config] BOOST_STL_SUPPORTS_NEW_UNICODE_LOCALES macro?
by Antony Polukhin 07 Apr '12

07 Apr '12

Hi, I'm interested in preprocessor macro, that provides info about std::locale() usability with char16_t and char32_t character types. Currently, some compilers provide new Unicode character types, but calls to std::locale() generate runtime error. I'd like to use that macro to: * provide compile time errors instead of runtime errors * enable/disable some Boost.Lexical_cast tests. Another useful macro would be BOOST_UNICODE_FULL_SUPPORT. It can be defined like this: #if !defined(BOOST_NO_CHAR32_T) \ && !defined(BOOST_NO_CHAR16_T) \ && !defined(BOOST_NO_UNICODE_LITERALS) \ && defined(BOOST_STL_SUPPORTS_NEW_UNICODE_LOCALES) #define BOOST_UNICODE_FULL_SUPPORT #endif -- Best regards, Antony Polukhin

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Re: [boost] C++11 Metaprogramming
by Andrzej Krzemienski 06 Apr '12

06 Apr '12

> on Thu Apr 05 2012, Andrzej Krzemienski <akrzemi1-AT-gmail.com> wrote: > >> For the end users of meta-programming library (at least those interested in >> type transformations), I believe that alias templates offer a significant >> improvement to the interface: >> http://akrzemi1.wordpress.com/2012/03/19/meta-functions-in-c11/ > > Yeah... I started doing a bunch of that, but I am not yet convinced it's > an overall improvement. One thing you emphatically *don't* get with > that formulation is lazy evaluation. remove_pointer<int> is valid until > you instantiate it but RemovePointer<int> is not, and the former is > often quite useful. Also, you can't use metafunction forwarding—which > also eliminates vast amounts of typing—with RemovePointer. Also, you > end up needing to define more names than you otherwise would (e.g. both > remove_pointer and RemovePointer). My standard approach when doing this > has been to name them remove_pointer_ and remove_pointer, so at least > they have an obvious and mechanical relationship. I guess my understanding of the name "meta-programming" might be a bit different, and term "end user" might have been too much a mental shortcut, but let me share my view of someone that doesn't want to go to deep into the meta-programming but still needs to use the simple forms thereof (like type traits). This is a somewhat simplified version of what I was facing recently. I need a function that perfect-forwards the argument but returns by value: template <class T> T fun(T&& v); The above is incorrect, because T may be deduced to be an lvalue reference. But be sympathetic, perfect-forwarding is already hard enough to acquire. I need type trait std::decay: template <class T> std::decay<T> fun(T&& v); But this does not work! I forgot "::type". Let's try again: template <class T> std::decay<T>::type fun(T&& v); No. It still doesn't work. And this is the worst part: I forgot the "typename". Having to type this "typename" is really terrible and just scares off many people. If you do not mind it, it only means that you got used to it; but people should not be forced to get used to too many gotchas. Alias templates offer a simplification that can improve the "teachability" of type traits. You may not consider type traits a proper meta-programming, and all I say might be a bit off topic. But if we had these alias templates for type traits, it looks to me, it would make a soft introduction to meta-programming for the newbies. Regards, &rzej

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