May 2013 - Boost-users - lists.preview.boost.org

[thread] synchronized_value: value and move semantics
by Vicente J. Botet Escriba 08 Jun '13

08 Jun '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]

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Using boost from XCODE 4.6.1 - what are the proper build settings?
by B Hart 01 Jun '13

01 Jun '13

I downloaded boost 1.53 and extracted to Users/user/Desktop/boost_1_53_0 I built boost as static libraries following instructions at: http://libcinder.org/docs/v0.8.5/_cinder_boost.html I changed XCODE's "Build Settings" "Header Search Paths" to /Users/user/Desktop/boost_1_53_0 as well as the "Library Search Paths" to Users/user/Desktop/boost_1_53_0/stage/lib Then I build and get errors: The first error I'm getting is "has_binary_operator.hpp" Parse issue Expected member name or ';' after declaration specifiers at the line: template < typename Lhs, typename Rhs > struct operator_exists { ... Does anyone have tips for what the build settings should be? The project contains some files that are cocoa/objective C (.m) and others that are C++(.mm) using boost. I'm completely new to trying to use boost under OSX mountain lion and XCODE 4.6.1.

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[mpl] Using max_element inside a fold while constructing a vector
by Brendon Costa 01 Jun '13

01 Jun '13

Hi, I have been using boost.mpl for the first time recently and have solved a number of tasks with boost.mpl now but have a problem with a bit of code I thought should be working. What is wrong with this bit of code using boost.mpl? typedef vector<int_<0>, int_<3>, int_<2> > v1; // I want to use max_element on a vector I am in the process of constructing using a fold typedef typename fold<v1, vector<int_<0> >, push_back<_1, deref<max_element<_1>::type >::type> >::type v2; // This code below compiles and runs fine to illustrate that max_element on a vector works, but is not what I am after. // I thought _1 in the max_element above would be a vector just as v1 below is, but the one below works and the one above does not. // typedef typename fold<v1, vector<int_<0> >, push_back<_1, deref<max_element<v1>::type >::type> >::type v2; I know this code is only going to create v2 like [0, 0, 0, 0] in this case but this is a smaller part of a larger expression that is failing and I cut it down to the minimum to show the problem at hand. In case it is useful I am using gcc 4.5.3 and boost 1.50

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[Thread] Can't compile package_task code with 1.54
by Szymon Gatner 01 Jun '13

01 Jun '13

Hi, I just tried to compile my 1.52- based code with 1.54 beta and run into package_task compilation issue. In 1.52 I was using BOOST_THREAD_VERSION=3 define but with 1.54 I complied with =4. Test program: #define BOOST_THREAD_VERSION 4 #include <boost/thread/future.hpp> #include <boost/function.hpp> struct MyFunc { void operator()()const {} }; int main() { boost::function<void()> f; MyFunc mf; boost::packaged_task<void()> t1(f); // error 1 boost::packaged_task<void()> t2(mf); // error 2 } error 1 being: d:\devel\boost_1_54_0_beta1\boost\thread\future.hpp(2848): error C2664: 'boost::detail::task_object<F,R>::task_object(boost::detail::task_object<F,R> &)' : cannot convert parameter 1 from 'boost::function<Signature>' to 'boost::detail::task_object<F,R> &' and error 2 similarly: d:\devel\boost_1_54_0_beta1\boost\thread\future.hpp(2848): error C2664: 'boost::detail::task_object<F,R>::task_object(boost::detail::task_object<F,R> &)' : cannot convert parameter 1 from 'MyFunc' to 'boost::detail::task_object<F,R> &' (I tried with custom functor because I thought maybe error was related to boost function) Compiling with Visual Studio 2012 Update 3 Regards, Szymon Gatner

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[1.54.0] Beta Release Candidates available
by Marshall Clow 01 Jun '13

01 Jun '13

Release candidate files for 1.54.0 beta 1 are available at http://boost.cowic.de/rc/ As always, the release managers would appreciate it if you download the candidate of your choice and give building it a try. Please report both success and failure, and anything else that is noteworthy. This helps ensure the candidates build OK before we push them out to SourceForge. The files (and associated md5s) are: 31266b7dcc6f557ad839e4e87317cd6e *boost_1_54_0_beta1_rc1.tar.gz fbf7be9cbaf9ea6fb21ae73d0c17100d *boost_1_54_0_beta1_rc1.tar.bz2 7de4299ab69c941e83fe3fc46dddc15f *boost_1_54_0_beta1_rc1.zip c73e5118ff585d9f1d68542f96a0758a *boost_1_54_0_beta1_rc1.7z Thanks, --The release managers

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[Xpressive] Nested patterns and sub-matches' semantic values
by Simon 30 May '13

30 May '13

Hi, For one part of a large project, I need to parse math and turn the "mathematical objects" into a C++ abstraction of them. Is it possible that a nested pattern hold semantic values within itself and can be queried for that value somehow? As if a match were a function of its sub-patterns' matches, if that makes sense... Code might speak better than my english: class math_number {...}; class math_addition {...}; class math_arglist {...}; class math_funcdef {...}; sregex integer = (s1= +_d) [ return math_number( as<int>(s1) ) ]; sregex addition = (s1= integer) >> '+' >> (s2= integer) [ return math_addition( as<math_number>(s1), as<math_number>(s2) ) ]; sregex arglist = (s1= identifier) >> *( ',' >> (s2= arglist)) [ std::vector<math_id> ids( as<math_arglist>(s2).begin(), as<math_arglist>(s2).end() ); ids.push_back(as<math_id>(s1)); return ids; ] sregex funcdef = ((s1= identifier) >> '(' >> (s2= arglist) >> ")=" >> (s3= addition)) [ return math_funcdef( as<math_id>(s1), as<math_arglist>(s2), as<math_addition>(s3) ) ]; I just want to avoid the funcdef above to be responsible for the construction of the various math_* objects the funcdef will hold (some of which will be complex to construct). And if I really must store the objects in an external stack, how should the parent-objects find their child-objects? Thanks, Simon

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Confused with BGL-- Six Degrees of Kevin Bacon example
by lizy10b 30 May '13

30 May '13

Hi there, I am new to BGL library. I am walking through the online tutorial and just finished the "10.2. Six Degrees of Kevin Bacon". As it is said at the end of the example: "Note that vertex descriptor objects can not always be used as indices into vectors or arrays such as bacon_number. This is valid with the adjacency_list class with VertexList=vecS, but not with other variations of adjacency_list. A more generic way to index based on vertices is to use the ID property map (vertex_index_t) in coordination with the iterator_property_map.". So I turn to have a look at the iterator_property_map page and its sample. First, this sample uses VertexList=vecS too. Second, I know this sample is using iterator_property_map to build a property map, but I think the property is still indexed by the edge descriptor objects. So what is the difference? Thanks. Zhiyu LI 2013-05-29 lizy10b

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[program options] parsing std::string
by Michael Powell 30 May '13

30 May '13

Hello, I want to parse std::string, perhaps into a vector<string>, but I do not want the command line argument to be positional in nature (i.e. like the examples demonstrate), because there may be a handful of parameters I need to capture into string or vector<string>. I've been able to parse integral types like int, even vector<int>, but haven't had much success with string. Possible or is this a program options "feature" and/or bug? Regards, Michael Powell

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Does the Proactor of boost uses edge-triggered epoll on linux?
by Journeyer J. Joh 30 May '13

30 May '13

Hello list, I have some questions about boost's async IO implementation for linux. 1. Does it use edge-triggered epoll to implement the Proactor (in terms of Reactor)? http://www.boost.org/doc/libs/1_53_0/doc/html/boost_asio/overview/core/asyn… The guide above says that boost's Proactor is implemented in terms of Reactor when it comes to Linux. If it is based on Reactor, I guess it should be level-triggered epoll. 2. Am I right? Then my question is 3. Does boost use edge-triggered epoll in any kind of async IO on linux? 4. Isn't edge-triggered epoll a good method to implement async IO on linux? Thank you very much. Sincerely Journeyer ---------------------------------------- Journeyer J. Joh o o s a p r o g r a m m e r a t g m a i l d o t c o m ----------------------------------------

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Install Boost.Build: gcc command not found (Windows)
by Tom C 29 May '13

29 May '13

I'm trying to build boost from sources following this guide: "http://www.boost.org/doc/libs/1_53_0/more/getting_started/windows.html#or-b…" I got the following outputs: here is bootstrap.log: here is my C:[...]\boost_1_53_0\tools\build\v2\user-config.jam: here is my PATH: It seems like the user-config.jam is not read by the bootstrap command. -- View this message in context: http://boost.2283326.n4.nabble.com/Install-Boost-Build-gcc-command-not-foun… Sent from the Boost - Users mailing list archive at Nabble.com.

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