October 2014 - Boost - lists.preview.boost.org

Re: [boost] Is there interest in a library for string-convertible enums?
by Felix Uhl 29 Oct '14

29 Oct '14

Artyom Tokmakov wrote: >In addition to various useful suggestions on the list, I would ask for >implementation to support (almost) any large number of enum elements >(enumerators) within enum declaration.: >I have ad-hoc implementation of my own which uses Boost.Proprocessor's >sequence to define enum elements. Then I can generate whatever >parser/printer/other code I want (I find that very useful). >There's a disadvantage, however: obvious limitation of max 256 >elements (at least, by default) coming from Boost.Preprocessor. And that’s where the problem lies. To enable large numbers of enum values, I’d have to find a solution without using Boost.PP. Without C++ introducing argument number based macro overloading, that is not possible, short of defining my own macro sequences for more than 256 elements, which I would like to avoid. I am still thankful that you asked for this, because it uncovered a huge implementation problem which induces enormous compilation overhead (1 minute for 48 enum values on my laptop). I will do my best to make the library comprehend large enums, but I can’t guarantee anything larger than 256. --- Felix Uhl

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Is there interest in a library for string-convertible enums?
by Felix Uhl 29 Oct '14

29 Oct '14

So far, the only way to convert an enumeration value to a string is a lot of boilerplate code consisting of switch case statements for every single possible value of the enum. I’ve written a small header-only library that uses Boost.Preprocessor and templates to ease the definition of named enumerations that are convertible to and from strings. Those can also be used with the std::cin and std::cout streams naturally, while maintaining all syntactic and semantic properties, regarding initialisation, assignment, type safety and conversion to underlying types. Currently, I am documenting the first version of the library to make it available on the Boost Library Incubator website, but I wanted to get some initial feedback from the developers mailing list, too. I would be glad to extend the library to a general extension of enumeration behaviour, like changing the generation of underlying values from a continuous increment to a continuous shift left by one, effectively making the underlying values binary flags. What features would you want from such a library? Is there even a chance that such a library would get included in boost (given that it satisfied the quality requirements), or is there too little interest in this functionality?

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Re: [boost] Is there interest in a library for string-convertible enums?
by Felix Uhl 29 Oct '14

29 Oct '14

Pieter wrote: > Are you aware of the earlier effort by Frank Laub? Frankly (no pun intended), I am not. It would be interesting to see how he has implemented that and what caveats his implementation has. --- Felix

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Re: [boost] Is there interest in a library for string-convertible enums?
by Felix Uhl 29 Oct '14

29 Oct '14

Jeffrey Bush wrote: > I have also offered a class for doing this as well, although it didn't draw > the interest that this one did. > Here was my attempt at it with a test/example class: > https://gist.github.com/coderforlife/6d8bac451d49dd1a0c81 Looks good to me, and seems to be quite close to what I aim at. But the definition seems very user unfriendly. Is it correct that you always have to declare enums in the public namespace? --- Felix

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Re: [boost] Is there interest in a library for string-convertible enums?
by Felix Uhl 29 Oct '14

29 Oct '14

Rob Stewart wrote: > You should not add features because someone on this list asked for them. > If you're not convinced of the utility of something, push back > to understand the request better. This sounds like advice I should follow. I tend to forget that I’m not writing for a specific client. Also, I noticed that the library has too much compile time overhead already, which should be fixed before introducing any additional ground-breaking features. So I’ve gone through the features suggested and ordered them by priority, perhaps the requesters of the following ones could answer my questions about them and specify why they think these features would be a good addition to the library: By Damien Buhl: - Adaptation of enums after declaration with something like BOOST_ADAPT_ENUM Is this really worth the effort? If somebody knows they want to string-convert an enum (or enum class), why would they not define it as one in the first place? Or is this about enumerations that are already defined by other libraries? Would it be ok to instead define a new enum that has the “to adapted” one as its underlying type and can thus be converted to and from it? There are implementational problems with offering both namespace independent definition of those enums and adaptability. By Gavin Lambert: - Specify custom and multiple strings In which use cases would this actually be beneficial for anyone? Could those use cases be solved by using custom char_traits or even custom string classes? - Specify different conversion tables for use in different contexts What do you exactly mean by that? Conversion to different string values? Conversion by different algorithms? Again, if you could answer those questions, it would really help evaluation. --- Felix

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[gsoc] Any interest in Google Code In?
by Niall Douglas 29 Oct '14

29 Oct '14

The application period for Google Code In 2015 has opened. This is a "tiny GSoC" programme for high school students where the tasks set might take the student three to five hours each. Stuff like writing documentation, improving unit testing, duplicating bug reports, making YouTube video tutorials, blogging about or writing an essay about getting a Boost development setup ready, even refactoring small segments of code if the student is up to it. The important part is the shortness of work item, and of course that you review it and give constructive feedback. More detail is at https://developers.google.com/open-source/gci/. Some example tasks from last year are at https://developers.google.com/open-source/gci/resources/example-tasks. Is there any interest from the Boost community in mentoring such high school student work? If so, now is the time to say. Niall -- ned Productions Limited Consulting http://www.nedproductions.biz/ http://ie.linkedin.com/in/nialldouglas/

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Re: [boost] Is there interest in a library for string-convertible enums?
by Artyom Tokmakov 29 Oct '14

29 Oct '14

> What features would you want from such a library? Hello, thanks for the proposal, it's very useful! In addition to various useful suggestions on the list, I would ask for implementation to support (almost) any large number of enum elements (enumerators) within enum declaration.: I have ad-hoc implementation of my own which uses Boost.Proprocessor's sequence to define enum elements. Then I can generate whatever parser/printer/other code I want (I find that very useful). There's a disadvantage, however: obvious limitation of max 256 elements (at least, by default) coming from Boost.Preprocessor. In fact, I hit some other limitations even before that. One of them was max 'if else' statements (I used them in parser implementation. Don't judge me, it was good enough!). When I fixed that, my VC compiler started complaining about some macro processor limitations around 220 enum elements (don't have details handy). Speed of parsing/printing implementation doesn't matter for me. PS: This is my first post in this DL, sorry if I broke some rules. -- Best regards, Artem Tokmakov

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Re: [boost] Is there interest in a library for string-convertible enums?
by Jeffrey Bush 29 Oct '14

29 Oct '14

> > So far, the only way to convert an enumeration value to a string is a lot > of boilerplate code consisting of switch case statements for every single > possible value of the enum. > > > I?ve written a small header-only library that uses Boost.Preprocessor and > templates to ease the definition of named enumerations that are convertible > to and from strings. Those can also be used with the std::cin and std::cout > streams naturally, while maintaining all syntactic and semantic properties, > regarding initialisation, assignment, type safety and conversion to > underlying types. > > > Currently, I am documenting the first version of the library to make it > available on the Boost Library Incubator website, but I wanted to get some > initial feedback from the developers mailing list, too. I would be glad to > extend the library to a general extension of enumeration behaviour, like > changing the generation of underlying values from a continuous increment to > a continuous shift left by one, effectively making the underlying values > binary flags. > > > What features would you want from such a library? Is there even a chance > that such a library would get included in boost (given that it satisfied > the quality requirements), or is there too little interest in this > functionality? > I have also offered a class for doing this as well, although it didn't draw the interest that this one did. Here was my attempt at it with a test/example class: https://gist.github.com/coderforlife/6d8bac451d49dd1a0c81 Supports value to string and string to value, including multiple values with | between them, meaning all combinations can be round-tripped. Supports default power-of-two values and custom values, listing all valid values (as enum types, but each is easily convertible to integral and string values), Although you cannot specify custom string names for an enum value that is not the "STRINGIZE" name, however you can choose to have multiple enum entries with the same value and different names. Jeff

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Re: [boost] Is there interest in a library for string-convertible enums?
by Felix Uhl 28 Oct '14

28 Oct '14

Marcel Raad wrote: >being able to specify the underlying >type, not being implicitly convertible >to any other type, and being able to >forward-declare them. Apart from forward-declaration, that’s what I’ve implemented. I want to stay as close to the stock enum class as possible. Forward declaration is added to the feature list :) --- Felix Uhl

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Re: [boost] Is there interest in a library for string-convertible enums?
by Felix Uhl 28 Oct '14

28 Oct '14

On 28/10/2014, Rob Stewart wrote: >That sounds interesting. Ease of use and compilation overhead are concerns, of course. I’ll answer your ease of use concern with example code for definition of the enum class Zott: >BOOST_ADVANCED_ENUM_DEFINE( Zott, > zero, > one, > two, > three >); And now for the mentioned “flag” values: >BOOST_ADVANCED_ENUM_DEFINE_W_SUPPLY( > TwoToThePowerOf, > ::boost::advanced_enum::supplies:shiftL1::values, > zero, > one, > two, > three >); Seems easy enough. The second one could be shortened to another macro, of course. Arbitrary values have a bit strange syntax, though: >BOOST_ADVANCED_ENUM_DEFINE_W_SUPPLY( > BOOST_ADCANCED_ENUM_ARBITRARY(4, 8, 6, 20), > four, > eight, > six, > twenty >); I’ll see what I can do about that. Conversions to and from string are done via static_cast (not sure if that’s the ideal solution), in- and output work naturally like this: >Zott myZott = Zott:one; >std::cin >> myZott; //if input is not convertible, no changes are made >std::cout << myZott; I think that's as easy as it can get :) Compilation overhead indeed needs to be considered. >What about enum classes? My mistake, with “named enums” I meant “enum classes”, but now that you say it, I should try to implement string convertible regular enums as well. Thank you for your feedback! --- Felix Uhl Von: Rob Stewart Gesendet: ‎Dienstag‎, ‎28‎. ‎Oktober‎ ‎2014 ‎22‎:‎23 An: boost(a)lists.boost.org On October 28, 2014 7:09:35 AM EDT, Felix Uhl <felix.uhl(a)outlook.de> wrote: >So far, the only way to convert an enumeration value to a string is a >lot of boilerplate code consisting of switch case statements for every >single possible value of the enum. As Niall noted, most of us have done that before. >I’ve written a small header-only library that uses Boost.Preprocessor >and templates to ease the definition of named enumerations that are >convertible to and from strings. That sounds interesting. Ease of use and compilation overhead are concerns, of course. >Currently, I am documenting the first version of the library to make it >available on the Boost Library Incubator website, I can hear Robert yelling, "Yes!" > but I wanted to get >some initial feedback from the developers mailing list, too. I would be >glad to extend the library to a general extension of enumeration >behaviour, like changing the generation of underlying values from a >continuous increment to a continuous shift left by one, effectively >making the underlying values binary flags. That would certainly be useful. What about enum classes? ___ Rob (Sent from my portable computation engine) _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost

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Re: [boost] Is there interest in a library for string-convertible enums?
by Felix Uhl 28 Oct '14

28 Oct '14

On 08/10/2014, Damien Buhl wrote: >What I would like to is to have a complete adapted enum for >Boost.Fusion, this was implemented at some point in some old version of >Boost.Fusion but I cannot find it anymore. Yes, interoperability with other boost libraries is a huge point to consider, and I like the concepts of Boost.Fusion a lot. I also thought about MPL, getting the names as compile time strings could be really useful. >And before all it's important I believe that it doesn't ask the user to >write the enums with macros, but that an additional call later to >something like BOOST_ADAPT_ENUM adds the boilerplate code. >It's naturally possible to offer both choice to the user like >BOOST_FUSION_ADAPT_STRUCT / BOOST_FUSION_DEFINE_STRUCT. I don’t think it is possible to offer an ADAPT_STRUCT macro without forcing the client to either rewrite every single value of the enumeration or define the enum with another macro in the first place. The former would could become problematic when clients change what values the enum actually contains but forget to change that in the macro. I would really want to enable the most natural syntax possible, but even a flexible language like C++ has boundaries where you can’t bend it anymore. So yes, I will offer both to the user. Thanks for your feedback! Felix Uhl Von: Damien Buhl Gesendet: ‎Dienstag‎, ‎28‎. ‎Oktober‎ ‎2014 ‎22‎:‎47 An: boost(a)lists.boost.org Hi Felix, Personally I'm interested, because so far I've been always using a silly macro for this, which doesn't make much : > #ifndef SWISSARMYKNIFE_ENUMS_ADAPT_ENUM_HPP > #define SWISSARMYKNIFE_ENUMS_ADAPT_ENUM_HPP > > #include <string> > #include <ostream> > #include <boost/preprocessor/cat.hpp> > #include <boost/preprocessor/stringize.hpp> > #include <boost/preprocessor/seq/for_each.hpp> > > > #define SWISSARMYKNIFE_ADAPT_ENUM_EACH_ENUMERATION_ENTRY_C( \ > R, unused, ENUMERATION_ENTRY) \ > case ENUMERATION_ENTRY: \ > return BOOST_PP_STRINGIZE(ENUMERATION_ENTRY); \ > break; > > #define SWISSARMYKNIFE_ADAPT_ENUM(ENUM_TYPE, ENUMERATION_SEQ) \ > inline std::string to_string(const ENUM_TYPE& enum_value) { \ > switch (enum_value) { \ > BOOST_PP_SEQ_FOR_EACH( \ > SWISSARMYKNIFE_ADAPT_ENUM_EACH_ENUMERATION_ENTRY_C, \ > unused, ENUMERATION_SEQ) \ > default: \ > return BOOST_PP_STRINGIZE(ENUM_TYPE); \ > } \ > } \ > \ > inline std::ostream& operator<<(std::ostream& os, const ENUM_TYPE& value) { \ > os << to_string(value); \ > return os; \ > } > > #endif What I would like to is to have a complete adapted enum for Boost.Fusion, this was implemented at some point in some old version of Boost.Fusion but I cannot find it anymore. Because if this had the extension struct_member_name or some other better named extension, then I could print out all the possible states with some generic code like I print struct member name with "boost::fusion::for_each_member(enumValue, *this);" > #ifndef BOOST_FUSION_FOR_EACH_MEMBER_HPP > #define BOOST_FUSION_FOR_EACH_MEMBER_HPP > > #include <functional> > > #include <boost/fusion/include/adapt_struct.hpp> > > #include <boost/fusion/sequence/intrinsic/begin.hpp> > #include <boost/fusion/sequence/intrinsic/end.hpp> > #include <boost/fusion/sequence/intrinsic/front.hpp> > #include <boost/fusion/iterator/equal_to.hpp> > #include <boost/fusion/iterator/next.hpp> > #include <boost/fusion/iterator/deref.hpp> > #include <boost/fusion/iterator/distance.hpp> > #include <boost/fusion/support/category_of.hpp> > #include <boost/mpl/bool.hpp> > > namespace boost { namespace fusion { > > namespace detail { > > template <typename First, typename Last, typename F> > inline void > for_each_member_linear(First const& first, > Last const& last, > F const& f, > boost::mpl::true_) {} > > template <typename First, typename Last, typename F> > inline void > for_each_member_linear(First const& first, > Last const& last, > F const& f, > boost::mpl::false_) { > > f( > extension::struct_member_name< > typename First::seq_type, First::index::value > >::call(), > *first > ); > > for_each_member_linear( > next(first), > last, > f, > result_of::equal_to< typename result_of::next<First>::type, Last>() > ); > } > > template <typename Sequence, typename F> > inline void > for_each_member(Sequence& seq, F const& f) { > > detail::for_each_member_linear( > fusion::begin(seq), > fusion::end(seq), > f, > result_of::equal_to< > typename result_of::begin<Sequence>::type, > typename result_of::end<Sequence>::type>() > ); > } > > } > > template <typename Sequence, typename F> > inline void > for_each_member(Sequence& seq, F f) { > detail::for_each_member(seq, f); > } > > }} > > #endif And before all it's important I believe that it doesn't ask the user to write the enums with macros, but that an additional call later to something like BOOST_ADAPT_ENUM adds the boilerplate code. It's naturally possible to offer both choice to the user like BOOST_FUSION_ADAPT_STRUCT / BOOST_FUSION_DEFINE_STRUCT. Cheers, -- Damien Buhl On 10/28/2014 12:09 PM, Felix Uhl wrote: > So far, the only way to convert an enumeration value to a string is a lot of boilerplate code consisting of switch case statements for every single possible value of the enum. > > > Iâ€™ve written a small header-only library that uses Boost.Preprocessor and templates to ease the definition of named enumerations that are convertible to and from strings. Those can also be used with the std::cin and std::cout streams naturally, while maintaining all syntactic and semantic properties, regarding initialisation, assignment, type safety and conversion to underlying types. > > > Currently, I am documenting the first version of the library to make it available on the Boost Library Incubator website, but I wanted to get some initial feedback from the developers mailing list, too. I would be glad to extend the library to a general extension of enumeration behaviour, like changing the generation of underlying values from a continuous increment to a continuous shift left by one, effectively making the underlying values binary flags. > > > What features would you want from such a library? Is there even a chance that such a library would get included in boost (given that it satisfied the quality requirements), or is there too little interest in this functionality? > > _______________________________________________ > Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost > _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost

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[Lockfree] has_trivial_destructor and has_trivial_assign restrictions
by Dan Dillon 28 Oct '14

28 Oct '14

I've been doing some thinking about this, and to my untrained eye these restrictions do not seem to be a requirement of the lock-free structure. That is to say, they need not be atomic anyhow, so why does it matter if they are trivial or not? The question goes to making the data structures more globally useful. The more structures they can be used with, the better they are (at least in my opinion). Thanks, Dan

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Custom logging backend difficulty
by Kenneth Adam Miller 28 Oct '14

28 Oct '14

So, I need malleable logging facilities, and so far I've identified boost's logging facilities as useful. I've read the docs on logging, and right now I'm having difficulty achieving what I want-basically, I have a custom backend, in order that when logging is done, it gets routed to an alternative destination - over the network namely. I extended from text_ostream_backend, and basically I'm having the following trouble: I don't understand the record_view visitation API. I'd like the ability to set formatters and what not to remain untouched, but have the final formatted result sent off else where. I can change this by sending the string received in the consume function that has to be overrode, but the values are lost in the record_view. void consume (const record_view & rec, std::string s); where I do { send(s); } for the body. What I think is necessary is just to retain the default behavior of accepting whatever formatter was specified... What is the best way to go about achieving what I want?

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1.57.0 beta 1 is in a new location
by Marshall Clow 27 Oct '14

27 Oct '14

Tom Kent pointed out that I put the beta 1 files in a folder that we traditionally use for Windows binaries, so I have moved the beta files to https://sourceforge.net/projects/boost/files/boost/1.57.0.beta.1/ These are the same files as last Monday - merely in a new location. If you have already downloaded and tested the beta, thanks! - no need to do it again. — Marshall

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Re: [boost] Non-allocating constexpr functional continuations-based future-promise
by Bjorn Reese 26 Oct '14

26 Oct '14

On 10/23/2014 11:00 PM, Niall Douglas wrote: > Given Hartmut's feedback from my previous attempt at a non-allocating > future-promise implementation, I now have a second, much more generic > demonstration prototype which is based on heterogenous sequences of > constexpr functional calls. I think this design may have huge potential. This is a very nice evolution of your previous promise-future framework. > template<class T> struct then_continuation > { > std::vector<std::function<void(T)>> _conts; > template<class U> auto then(U &&c) > { > typedef decltype(c(std::declval<T>())) result_type; > promise<result_type> p; > future<result_type> ret(p.get_future()); > // Unfortunately libstdc++'s std::function currently always tries to > copy lambda types > //_conts.emplace_back([p=std::move(p), c=std::forward(c)](T > v){p.set_value(c(v));}); > _conts.push_back([p = > std::make_shared<promise<result_type>>(std::move(p)), > c=std::forward(c)](T v){p->set_value(c(v));}); > return ret; > } > BOOST_CONSTEXPR T operator()(T v) const > { > for(auto &i : _conts) > i(v); > return v; > } > }; > > All this continuation does is provide a .then(callable) which stores those > callables into a std::vector and its operator() executes the previously > stored callables. Previously you defined callables as composed functions, but here you execute them sequentially. That is, then(f1).then(f2) gets executed as f1(v),f2(v) instead of f2(f1(v)). Or am I missing something?

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[boost.lockfree] spsc_queue preallocated memory block
by Philip Bennefall 25 Oct '14

25 Oct '14

Hi all, I have a feature request for Boost.lockfree (spsc_queue in particular). When using the runtime sized version, could there possibly be a constructor that takes max_size as well as a T* that is then used as the memory buffer? This would remove the need for a specialized allocator if I want to quickly initialize the buffer with memory that does not come from the default std allocator. Kind regards, Philip Bennefall

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Re: [boost] Applying Boost Phoenix to infinite sequence
by Aaron Levy 25 Oct '14

25 Oct '14

25.10.2014, 00:54, "Aaron Levy" <aaron.levy(a)yandex.com>: > Is there a way to use Boost Phoenix to define an infinite sequence and evaluate specific terms of it, compute partial sums, etc? I am looking for pointers to specific libraries or examples. > Indeed, what would are some examples of common programming problems that can be succinctly solved using Phoenix? Aaron.

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Why is iterator_traits<unordered_map<K, M>>::pointer a detail::ptr_node type rather than value_type *?
by Alex Mohr 24 Oct '14

24 Oct '14

The iterator type looks like: template <typename NodePointer, typename Value> struct iterator : public boost::iterator< std::forward_iterator_tag, Value, std::ptrdiff_t, NodePointer, Value&> The NodePointer argument here (which is boost::unordered::detail::ptr_node<...>) becomes the iterator_traits<>::pointer type. But iterator's operator->() returns Value *: Value* operator->() const { return &node_->value(); } Why shouldn't iterator_traits<>::pointer be Value* here? This detail::ptr_node<> type seems intended to be completely internal. Thanks, Alex

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Non-allocating constexpr functional continuations-based future-promise
by Niall Douglas 23 Oct '14

23 Oct '14

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Non-allocating constexpr functional continuations-based future-promise
by Niall Douglas 23 Oct '14

23 Oct '14

Dear Boost, Given Hartmut's feedback from my previous attempt at a non-allocating future-promise implementation, I now have a second, much more generic demonstration prototype which is based on heterogenous sequences of constexpr functional calls. I think this design may have huge potential. Consider some heterogenous sequence of callables C { f1, f2, f3, f4, ... } such that fN(... f4(f3(f2(f1(Args...))))) -> R i.e. if you call the heterogenous sequence with arguments Args..., f1(args...) will be called and fed to f2() which then is fed to f3() up to fN() whose return type is R. If I understand proposed Boost.Hana correctly, these potentially constexpr sequences of heterogeneous callables are the main bridge between pure compile-time heterogeneous type sequences and doing something with them in real code as they can decay on demand into actual code when needed. And for proposed Boost.Expected, its monadic programming sequences bind(), map() and then() all functionally operate on some expected<T, E>, consuming an instance and returning new instances, so these monadic sequences are also potentially constexpr sequences of heterogenous callables. Now, what new basic_future-basic_promise will do is to allow you to insert a suspend-resume of execution into some functional sequence C, so essentially: split<idx>(C) => [ basic_promise<X>, basic_future<Y> ] ... where X is some split front part of C and Y is some back part of C. You can now execute the first part of C via the call operator: basic_promise<X>(Args...) Execution of the C0...Cn making up X occurs, but instead of continuing into Y it stores the result of Cn and makes that available to the corresponding basic_future. At some later point you can resume the execution of C: basic_future<Y>() -> R ... and thus complete the execution of the functional sequence C, returning the same value R as invoking the original functional sequence C with the same input parameters. For those who prefer to understand theory via code, here is some example C++ 14 code: // Let this be an arbitrarily long sequence of functional callables auto ct=make_continuations([](auto x){return 5+x;}, [](auto x){ return x+6;}); { // Make a future promise pair for if we called c(int) auto fp=ct.suspend_resume<1, int>(); // Execute promise fp.first(1); // Execute future b=std::move(fp.second)(); } { // Make a future promise pair for if we called c(double) auto fp=ct.suspend_resume<1, double>(); // Execute promise fp.first(1.0); // Execute future d=std::move(fp.second)(); } // b=12, d=12.0 There are a multitude of uses for this very fundamental pattern. Let us try implementing something closely resembling the Concurrency TS's dynamic continuations support .then(launch, callable). Let us define this functional continuation: template<class T> struct then_continuation { std::vector<std::function<void(T)>> _conts; template<class U> auto then(U &&c) { typedef decltype(c(std::declval<T>())) result_type; promise<result_type> p; future<result_type> ret(p.get_future()); // Unfortunately libstdc++'s std::function currently always tries to copy lambda types //_conts.emplace_back([p=std::move(p), c=std::forward(c)](T v){p.set_value(c(v));}); _conts.push_back([p = std::make_shared<promise<result_type>>(std::move(p)), c=std::forward(c)](T v){p->set_value(c(v));}); return ret; } BOOST_CONSTEXPR T operator()(T v) const { for(auto &i : _conts) i(v); return v; } }; All this continuation does is provide a .then(callable) which stores those callables into a std::vector and its operator() executes the previously stored callables. Now let us define a std::promise lookalike, with its main deficiencies being a total lack of exception_ptr support (one would use expected<T, E> as the transport type for that) and of course any thread safety: template<class T> class promise : public basic_promise<detail::then_continuation<T>, T, detail::then_continuation<T>> ... basic_promise takes three template parameters, the type of the front split of the continuations, the transported type between the two halves of continuations, and the type of the back split of the continuations. Here, we are defining promise as transporting a type T, and two then_continuation continuations exist before and after the transport of the type T. To our future<T> lookalike we now add this: template<class U> auto then(launch policy, U &&c) { if(policy==launch::deferred) return Base::_after.then(std::forward(c)); else if(Base::_other) return Base::_other->_before.then(std::forward(c)); } So, if the .then() dynamic continuation is deferred, push it into the latter then_continuation, else push it into the former. And voila, you can now do this very similar to Concurrency TS code: promise<int> p; future<int> f(p.get_future()); future<double> f2(f.then(launch(), [](int a){return a+1.0;})); p.set_value(5); return f2.get(); And you get back 6.0, as expected. However here is where things get neato. How many instructions do you think GCC 4.9 generates for this sequence? promise<int> p; future<int> f(p.get_future()); p.set_value(5); return f.get(); In traditional future-promise, that is an operation perhaps consuming thousands of CPU cycles and at least one memory allocation and deallocation. Well, this is the assembler produced by the above by GCC 4.9: _Z19test_futurepromise4v: movq $0, -88(%rsp) movq $0, -80(%rsp) movl $5, %eax movq $0, -72(%rsp) movq $0, -48(%rsp) movq $0, -40(%rsp) movq $0, -32(%rsp) movl $5, -56(%rsp) movq $0, -64(%rsp) movq $0, -24(%rsp) ret Believe it or not, this is actually bad output - there is a bug in GCC's optimiser and it should in fact be outputting a single instruction rather than writing zeros into the stack for no purpose - it seems to get confused by the double declation of a std::vector which is never used, because having just one std::vector produces the ideal output but having two produces the above. However clang 3.4's optimiser does far worse, and a bug there with how non-immediately defined types are treated causes complete expansion of code in its entirety, even if you remove all std::vector's (supplying any type not immediately declared in the same function breaks the optimiser for some unknown reason. I haven't tried clang 3.5 yet, it may be fixed). Other very interesting policy continuations which can be statically mixed into custom future promise types include calling some application specific callback without needing to indirect via a pointer to a function. This lets you combine future promise with invoking callbacks, useful for legacy code. The idea behind this suite of future-promise fundamental parts is that they can be mixed at will into bespoke future-ish promise-ish application specific future promise types. Because they all share the same foundations, a when_any() or when_all() can accept heterogenous future types - and I don't mean future<int> with future<double>, rather I mean afio::future<> with qt::future<int>, that sort of heterogenity. Another very interesting idea is combining these with signals and slots, so via custom continuations you can keep slots and fire signals in sync with future promises. This design I think shows a lot of potential. Anyway, I feel very happy with this design, and something along these lines is where I shall proceed next. The eventual intention is for these future promises to become the default Boost.Thread future promises when C++11 is available, though this will be up to a year from now. I won't be able to support future-promise custom allocators, but I certainly should be able to completely avoid memory allocation and sometimes the generation of any code at all with this design. The demonstration prototype code is at https://github.com/ned14/boost.spinlock/blob/expected_future/test_constexpr_ future_promise2.cpp for anyone interested. It needs a C++ 14 compiler, so a minimum of libstdc++ 4.9 and either GCC 4.9 or clang 3.4 or better. Comments are welcome. Niall

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DTLS support in boost
by Indtiny S 22 Oct '14

22 Oct '14

Hi, Currently I am using boost for one of my cpp application ... now my application needs DTLS security support ... I currently have written using openssl APIs .. I want nowb...does boost asio has any APIs for DTLS handshake ? Rgds Indhra

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[thread] linux.hpp, BOOST_HAS_PTHREAD_YIELD, BOOST_HAS_SCHED_YIELD
by Rob Conde 22 Oct '14

22 Oct '14

Reposting this here since I got no response on "users" but this list seems more appropriate: Using boost 1.55, Ubuntu 12.04, gcc 4.6 I'm building boost on linux using the lsb sdk (4.1.8) . I was getting the failure (paraphrased): pthread_yield not declared in libs/thread/src/thread.cpp 513 The documentation for pthread_yield (http://man7.org/linux/man-pages/man3/pthread_yield.3.html) says: This call is nonstandard, but present on several other systems. Use the standardized sched_yield(2)<http://man7.org/linux/man-pages/man2/sched_yield.2.html> instead. If I change boost/config/platform/linux.hpp to define BOOST_HAS_SCHED_YIELD instead of BOOST_HAS_PTHREAD_YIELD Then sched_yield is used instead and the build finishes properly. I'm posting this for two reasons: 1. To help others running into the same issue 2. To question whether linux.hpp should be updated officially as I did above 3. If not, why Thanks, Rob Conde

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Help with boost spirit
by Theodore Papadopoulo 22 Oct '14

22 Oct '14

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Sorry, this is probably a beginner question, but I'm stuck on it for quite some time. I'm trying to write a small string like parser in which the size of the string is first given. Some thing like: int_ >> repeat(_1)[ char_ ] But I cannot figure out the right way of doing this. The natural: const bool result = boost::spirit::qi::parse(it,str.end(),int_ >> repeat(_1)[ char_ ]); fails to compile (and I tried many variations of val, etc) with no success. If I replace _1 by 10, everything works fine. Another question is that of the associated attribute. From the documentation, I would have expected it to be: typedef std::pair<int,std::vector<unsigned char>> Attribute; but that does not work. I have managed to work around this using a second rule, using an attribute std::vector<char>(int) but then again I do not know how to pass the parameter. 10 works but _1 does not. Any help will be appreciated. Some test code is attached, compile with: g++ -g -std=c++11 -dboost_spirit_use_phoenix_v3 Test.C. Variants are in comment. Thank's in advance for any help. Theo. -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iEYEARECAAYFAlQ9GqgACgkQEr8WrU8nPV0b2gCeNFzQXwm1Uux4Eua2+jq8LcZk DW0AoKBKY9KN5kGxYoPbnxwncwFSyH3M =L0bR -----END PGP SIGNATURE-----

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[1.57.0] Release Notes
by Daniel James 22 Oct '14

22 Oct '14

Hi, Sorry this is late, but if you haven't written your release notes, you should do so soon. You can create a pull request on github by clicking on the pencil icon on the toolbar on this page while logged in: https://github.com/boostorg/website/blob/master/feed/history/boost_1_57_0.q… Or if you prefer you can email me, and I'll add it for you. thanks, Daniel

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Non-allocating future-promise
by Niall Douglas 21 Oct '14

21 Oct '14

Dear Boost, I have been working on non-allocating future-promise which when combined with proposed expected<T, E> (i.e. basic_promise<expected<T, E>>) will provide a close substitute to std::future/promise, except that it doesn't use an allocator and can reduce to pure constexpr when possible e.g. this code: __attribute__((noinline)) int test1() { promise<int> p; auto f(p.get_future()); p.set_value(5); return f.get(); } ... reduces to exactly: _Z5test1v: # @_Z5test1v .cfi_startproc # BB#0: # %_ZN7promiseIiJEED2Ev.exit2 movl $5, %eax ret The non-allocating future-promise is intended to be compatible with the Concurrency TS extensions (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4123.html) where futures can have continuations scheduled against them via a future.then(callable). However, this involves an unavoidable memory allocation, which while fine for dynamically assigned continuations is overkill for statically assigned continuations. There is no reason why one could not also have a method of statically adding continuations which uses type extension to store the additions. Let us say we have a template<class T, class... Continuations> class promise and that this promise also provides a .then(callable) like this: template<class F> promise<T, Continuations..., F> then(F &&c) { return promise<T, Continuations..., F>(std::move(*this), std::forward<F>(c)); } ... and therefore to statically add continuations to the future promise, one simply iterates .then(callable) on the promise like this: auto p=promise<int>().then([]).then([]) ...; auto f=p.get_future(); p.set_value(5); // executes all the continuations My first question to the Boost community is this: is there some less ugly way? I'm thinking maybe the promise's constructor could take a set of continuations, and then a make_promise(callable, callable, ...) could construct a statically continued future-promise? My second question to the Boost community is this: should static continuations be able to see the value being set? Or indeed, to modify it before it gets sent to future.get()? My thanks in advance. Niall -- ned Productions Limited Consulting http://www.nedproductions.biz/ http://ie.linkedin.com/in/nialldouglas/

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