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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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Re: [boost] C++11 Metaprogramming
by Andrzej Krzemienski 05 Apr '12

05 Apr '12

> 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. 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/ Regards, &rzej

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Belgium C++ Users Group
by Francisco Almeida 05 Apr '12

05 Apr '12

Hello to all, I just subscribed to this mailing list to ask the Boost developer community for anyone actively working on new libraries/library refactoring if they would be interested in presenting to us a lecture about the major changes Boost will undergo, now that the new C++ standard has been published and that support for it is quickly being implemented by the most popular compilers. Of course, assuming any generous Boost developer who happens to live/work in Benelux would be willing to visit us and share his/her experiences/ambitions with us. Best regards, Francisco Almeida

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[type_traits] is_scalar - fit into register?
by Oliver Kowalke 05 Apr '12

05 Apr '12

Hi, is it a correct assumtion that all types which evaluate 'is_scalar<T>::value' to 'true' will fit into a processor register? Oliver

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[math] Explicit message for configure checks
by Steven Watanabe 05 Apr '12

05 Apr '12

AMDG The attached patch just makes the output a little nicer. Okay to commit? In Christ, Steven Watanabe

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