July 2013 - Boost - lists.boost.org

[thread] synchronized_value: value and move semantics
by Vicente J. Botet Escriba 24 Sep '13

24 Sep '13

Hi, boost::synchronized_value (not released yet [1][2][3]) is based on [4]. See below part of this paper adapted to the Boost.Thread interface. Currently boost::synchronized_value is in addition Copyable and Swappable. I was wondering if it is worth adding value and move semantics to synchronized_value. Making it EqualityComparable, LessThanComparable and Movable if the underlying type satisfy these requirements will allow to store them on a Boost.Container/C++11 container. Do you see something completely wrong with this addition? Has some of you had a need for this? Could you share the context? Best, Vicente [1] https://svn.boost.org/svn/boost/trunk/boost/thread/synchronized_value.hpp [2] https://svn.boost.org/svn/boost/trunk/libs/thread/example/synchronized_valu… [3] https://svn.boost.org/svn/boost/trunk/libs/thread/example/synchronized_pers… [4] "Enforcing Correct Mutex Usage with Synchronized Values" by Anthony Williams http://www.drdobbs.com/cpp/enforcing-correct-mutex-usage-with-synch/2252002… ------------- [section The Problem with Mutexes] The key problem with protecting shared data with a mutex is that there is no easy way to associate the mutex with the data. It is thus relatively easy to accidentally write code that fails to lock the right mutex - or even locks the wrong mutex - and the compiler will not help you. std::mutex m1; int value1; std::mutex m2; int value2; int readValue1() { boost::lock_guard<boost::mutex> lk(m1); return value1; } int readValue2() { boost::lock_guard<boost::mutex> lk(m1); // oops: wrong mutex return value2; } Moreover, managing the mutex lock also clutters the source code, making it harder to see what is really going on. The use of synchronized_value solves both these problems - the mutex is intimately tied to the value, so you cannot access it without a lock, and yet access semantics are still straightforward. For simple accesses, synchronized_value behaves like a pointer-to-T; for example: boost::synchronized_value<std::string> value3; std::string readValue3() { return *value3; } void setValue3(std::string const& newVal) { *value3=newVal; } void appendToValue3(std::string const& extra) { value3->append(extra); } Both forms of pointer dereference return a proxy object rather than a real reference, to ensure that the lock on the mutex is held across the assignment or method call, but this is transparent to the user. [endsect] [/The Problem with Mutexes] [section Beyond Simple Accesses] The pointer-like semantics work very well for simple accesses such as assignment and calls to member functions. However, sometimes you need to perform an operation that requires multiple accesses under protection of the same lock, and that's what the synchronize() method provides. By calling synchronize() you obtain an strict_lock_ptr object that holds a lock on the mutex protecting the data, and which can be used to access the protected data. The lock is held until the strict_lock_ptr object is destroyed, so you can safely perform multi-part operations. The strict_lock_ptr object also acts as a pointer-to-T, just like synchronized_value does, but this time the lock is already held. For example, the following function adds a trailing slash to a path held in a synchronized_value. The use of the strict_lock_ptr object ensures that the string hasn't changed in between the query and the update. void addTrailingSlashIfMissing(boost::synchronized_value<std::string> & path) { boost::strict_lock_ptr<std::string> u=path.synchronize(); if(u->empty() || (*u->rbegin()!='/')) { *u+='/'; } } [endsect] [/Beyond Simple Accesses] [section Operations Across Multiple Objects] Though synchronized_value works very well for protecting a single object of type T, nothing that we've seen so far solves the problem of operations that require atomic access to multiple objects unless those objects can be combined within a single structure protected by a single mutex. One way to protect access to two synchronized_value objects is to construct a strict_lock_ptr for each object and use those to access the respective protected values; for instance: synchronized_value<std::queue<MessageType> > q1,q2; void transferMessage() { strict_lock_ptr<std::queue<MessageType> > u1 = q1.synchronize(); strict_lock_ptr<std::queue<MessageType> > u2 = q2.synchronize(); if(!u1->empty()) { u2->push_back(u1->front()); u1->pop_front(); } } This works well in some scenarios, but not all - if the same two objects are updated together in different sections of code then you need to take care to ensure that the strict_lock_ptr objects are constructed in the same sequence in all cases, otherwise you have the potential for deadlock. This is just the same as when acquiring any two mutexes. In order to be able to use the dead-lock free lock algorithms we need to use instead unique_lock_ptr, which is Lockable. synchronized_value<std::queue<MessageType> > q1,q2; void transferMessage() { unique_lock_ptr<std::queue<MessageType> > u1 = q1.unique_synchronize(boost::defer_lock); unique_lock_ptr<std::queue<MessageType> > u2 = q2.unique_synchronize(boost::defer_lock); boost::lock(u1,u2); // dead-lock free algorithm if(!u1->empty()) { u2->push_back(u1->front()); u1->pop_front(); } } [endsect] [/Operations Across Multiple Objects]

6 19

[Operators] empty_base should be untemplated?
by Andrew Ho 21 Sep '13

21 Sep '13

I've noticed about the empty_base class is that it's templated with type T, and I don't think it should be. Fyi, the empty_base class is there to turn multiple inheritance into a single inheritance chain (due to compiler deficiencies/object size bloat on at least Visual Studio). I don't see why this empty_base should be templated, and it would be better suited if it wasn't since all I can think it does is make the compiler work harder. Does anyone know why it was templated to begin with? Additionally, are we safe to change empty_base to be not use templates?

4 4

[asio] boost::asio::async_connect reports success on failure
by Ben Pope 13 Sep '13

13 Sep '13

Hi all, I've found a regression between 1.53 to 1.54; if you compile boost/libs/asio/example/cpp11/chat/chat_client.cpp and point it at a host and port that doesn't exist or refuses the connection, instead of coming back with an error, such as host not found or connection refused, it reports success (on the conversion to bool). (you might want to embellish the example with a std::cout << error_code.message() << std::endl;) This appears to be the case across Linux, Darwin, iOS and Android :( synchronous connect seems to work correctly (i.e., echo/blocking_tcp_echo_client.cpp) Ben

2 4

Removing old config macro and increasing compiler requirements.
by Stephen Kelly 12 Sep '13

12 Sep '13

Hi there, I've been working on the modularization effort from a CMake point of view, and from an 'actual modularization' point of view, looking at interdependencies: http://thread.gmane.org/gmane.comp.programming.tools.ryppl.devel/9/focus=26 I'd like to start on reducing interdependencies, and I'd like to start reasonably small, as no-one in the boost community knows me and see how it goes from there. I'm very active in CMake, Qt and KDE. Currently, in the modularized boost repos, boost::config and boost::core depend on each other. I listed 3 ways of fixing this on the ryppl list: http://thread.gmane.org/gmane.comp.programming.tools.ryppl.devel/201 My preference is the removal of BOOST_NO_EXPLICIT_FUNCTION_TEMPLATE_ARGUMENTS, and requiring compilers to have the feature. That way boost/config/suffix.hpp will no longer need to include boost/{non_,}type.hpp. My preference is increasing the compiler requirement, because that may open up more similar opportunities for reducing dependencies and interdependencies throughout boost. Grepping indicates that that means increasing the compiler requirements to something like __DMC__ > 0x840, GCC > 3.2, BOOST_INTEL_CXX_VERSION > 500, VC++ > 7.0. Comments? Thanks, Steve.

15 82

[thread] Can boost::this_thread::sleep_for() totally stop the thread?
by Klaim - Joël Lamotte 03 Sep '13

03 Sep '13

I'm having, sometime, trouble with a rare heisenbug (rare but appearing several times a week). I described it there: http://stackoverflow.com/questions/17918548/can-windows-totally-stop-a-thre… There I ask if it's possible for windows to totally stop a thread that is sleeping too often. As I'm using boost::thread for sleeping this specific thread that, for unknown reason, sometime totally stop working, I'm also wondering if the boost::this_thread() implementation could not be doing something that the OS might not like? ----- I'll report the content of the page here: Can Windows totally stop a thread if it's sleeping too often? I have a rare heisenbug in a multi-threaded application where the main thread, and only this thread, will just do nothing. As it's an heisenbug it's really hard to understand why this is happening. The main thread is basically just looping. In the loop, it check several concurrent priority queues which contain tasks ordered by time to be executed. It pop a task, see if it's time to execute it. If it's time, it will just schedule it into TBB's task scheduler (using a root task which is the parent of all other tasks). If it's not time, the task is pushed again in the priority queue. That's for one cycle. At the end of the cycle, the main thread is put to sleep for a very short time that I expect will be longer in practice but it's not really a problem, I just don't want it to use too much resources when not necessary. Litterally: static const auto TIME_SCHEDULED_TASKS_SPAWN_FREQUENCY = microseconds(250); while( !m_task_scheduler.is_exiting() ) // check if the application should exit { m_clock_scheduler.spawn_realtime_tasks(); // here we spawn tasks if it's time this_thread::sleep_for( TIME_SCHEDULED_TASKS_SPAWN_FREQUENCY ); } m_clock_scheduler.clear_tasks(); m_root_task.wait_for_all(); I have a special task that just log a "TICK" message each second. It is automatically rescheduling until the end of the program. However, when the heisenbug appear, I can see the "TICK" disappearing and the application not doing anything else than the work that occurs in non-main threads. So it appear that only the main thread is touched. The problem can come from different places, maybe in the scheduling logic, but then it's also the only thread that have a sleep call. That sleep is a boost::this_thread::sleep_for(). My question is: **Is it possible that Windows (7 64bit) consider the main thread to be sleeping often and decide that it should sleep for a longer period of time than asked or be definitely ended?** I expect that it is not possible but I would like to be sure. I didn't find any precision on this in online documentation so far. ----- I really suspect that the problem can't come from the sleeping function but I need to eliminate each potential source of problem to isolate it. Thanks for your time. Joel Lamotte

2 9

[multimethods] Proposal: Open Multi-Methods
by Jean-Louis Leroy 01 Sep '13

01 Sep '13

Hi, I am working on library that pretty much implements what Pirkelbauer, Solodkyy and Stroustrup describe in their paper "Open Multi-Methods for C++" - although there are some divergences. I have posted it on GitHub: https://github.com/jll63/multimethods. Please examine the files in the "examples" directory to get an idea of how to use it. Salient features are: * syntax: is relatively uncluttered. There are no limitations on the number of virtual arguments. Virtual and non-virtual arguments can be arbitrarily mixed. Multiple inheritance is supported, with some limitations - see below. * speed: close to a virtual function call when the hierarchies involved in the virtual arguments collaborate with the library. Calling a method that does nothing, with a single virtual argument in a single inheritance hierarchy is 33% slower than the equivalent virtual function call. The difference becomes unnoticeable if the functions perform a few simple maths operations. See tests/benchmarks.cpp. * size: the dispatch table is constructed in terms of class groups. This results, most of the time, in a table devoid of redundancies. The size is bounded by the product of the number of specializers - not classes - in each dimension. * support for "foreign" class hierarchies: the library can be used without modifications to existing classes, at the cost of lower performance. Collaborating and foreign arguments can be freely mixed. Performance is still quite good, see the benchmarks. * next: a pointer to the next most specialized method is available inside method specializations - see examples/next.cpp. Alternatively, it is possible to call a specialization directly. Current limitations are: * restrictions on hierarchy: the classes involved in a method call must have a single root. Repeated inheritance is not supported. Multiple inheritance is supported, provided that the two conditions above are respected. * no overloading: multi-methods are implemented as constexpr function objects. This does not allow for overloading. This restriction can be easily side-stepped by wrapping multi-methods with different names in a set of overloaded functions. I know how to make multi-methods functions instead of objects, at the cost of a slightly less nice syntax for specializers. This is open to discussion. Besides, the library differs from Stroustrup et al's paper on the following points: * a specializer specializes only one multi-method: what would "next" mean otherwise? What would its static type be? * pure multi-methods are allowed: if called, an exception is thrown. This seems a better choice given that most of the time the declared virtual arguments will be abstract classes anyway. The library is essentially feature complete (if anything ever is). I am now going to work on the documentation, a build system, some clean up and compatibility (I tested it only with g++ 4.7.2 only so far). Is there interest in the Boost community for this stuff? If yes, I will adapt it to the guidelines and submit it for formal review. Of course, suggestions are welcome. Thank you for your time, Jean-Louis Leroy

5 10

[type_triats] [move] is_copy_constructible<T> trait
by Antony Polukhin 26 Aug '13

26 Aug '13

Hi, I have an implementation of is_copy_constructible<T> trait. It works great with C++11, in C++03 it detects constructors assuming that if type is derived from boost::noncopyable or uses BOOST_MOVABLE_BUT_NOT_COPYABLE macro - it has no copy constructor. Questions are: * May I add that trait to trunk version of Boost.TypeTraits (with documentation and tests, as usual)? * May I or Boost.Move maintainer apply this patch to BOOST_MOVE_IMPL_NO_COPY_CTOR_OR_ASSIGN in boost/move/core.hpp : #ifdef BOOST_NO_CXX11_DELETED_FUNCTIONS #define BOOST_MOVE_IMPL_NO_COPY_CTOR_OR_ASSIGN(TYPE) \ private:\ TYPE(TYPE &);\ TYPE& operator=(TYPE &);\ + public: \ + typedef int boost_move_no_copy_constructor_or_assign; \ + private: \ // #else Why do I need this trait? Well, it allows to implement move_if_noexcept() that is required for circular_buffer and possibly for other containers. -- Best regards, Antony Polukhin

2 4

Pre-build MSVC Boost binaries
by Fredrik Orderud 13 Aug '13

13 Aug '13

Over the last year, I've set up a small SourceForge "WinBin" project [1] for distributing Microsoft Windows binaries for Boost (and some other projects). The repository is sorted by Boost version, compiler version and shared vs. static linking. Each file archive contains pre-build boost in debug/release for both 32/64-bit, following a include/lib[64]/bin[64] folder structure. There also seems to be some other sources for pre-built boost around (e.g. https://sourceforge.net/projects/boost/files/boost-binaries/ and http://boost.teeks99.com/bin/) However, these do not seem to provide boost built for both static and shared linking. Also, they do not appear to adhere to the "quasi standard" include/lib[64]/bin[64] folder naming convention used by many libraries. Finally, they seem to lack information on which version of python (and possibly other 3rd party SW) they are compiled against. Therefore, I decided to set up my own boost-binaries webpage. Please let me know if you might be interested in either improving the "WinBin" project, or somehow joining forces to merge the various sources of Boost binaries into a single, more "authoritative" repository. I would be more than happy to shut WinBin down if "similar enough" binaries can be downloaded elsewhere. Thanks in advance, Fredrik Orderud [1] https://sourceforge.net/projects/winbin/files/boost/

5 10

Line endings fixes
by Vitaly Budovski 13 Aug '13

13 Aug '13

Hi, Please see the attached files. boost_endings1.patch just sets some of the cpp and other text files to have file endings consistent with the rest of the repository. boost_endings2.patch may be controversial as it does the same but for the svg files. Some of them are marked as text, others as binary. It's a real mess. Most of the svg files are generated from other sources so should probably be omitted from the repository and simply regenerated when docs are built. Came across these issues when checking out the modularised boost git repository. Regards, Vitaly

7 32

[mixin] New library reminder
by Borislav Stanimirov 11 Aug '13

11 Aug '13

Hello, Some of you might remember the previous discussion on the subject (archived here: http://lists.boost.org/Archives/boost/2013/01/200598.php) Anyway, in short, I'm developing a library with the intention to formally submit it to Boost by the end of the year. You can find it here: https://github.com/iboB/boost.mixin The latest news about it is that I think that the library's features for the formal submission are done and all that's left are minor tweaks, bug fixes, tests, examples, and, most importantly, a complete documentation. In the meantime, I again invite you all to check it out. Any comments, suggestions, and questions will be greatly appreciated. Thanks, -- Borislav

3 17