June 2013 - Boost-users - lists.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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[spirit] Compile error when using qi::locals<>
by Simon 07 Jun '13

07 Jun '13

Hi there, I'm pretty new to spirit and I need a semantic action to construct an object (that's fine) using a vector<double>, the vector is the problem. I'd really appreciate help with this error, or perhaps I'm not using the right technique, in which case, I'd need suggestions on how to proceed. Even if I don't use it anywhere, just adding the following to my grammar base class template parameter and adding the same to the rule declaration I need it for, it gives the monster error below. boost::spirit::qi::locals<std::vector<hs::Language::AX*> > I've also tried to make the vector a pointer, and allocate it on the first semantic action of a sequence. I've searched for an answer but found only one remote reference to boost needing to be recompiled to use c++11 features. So I did, but it didn't change anything (I use boost-1.49.0-r2). Also, I'm not sure if that is right, but my gcc command has this parameter: -std=c++0x I've put the full compile output at this address: http://next.n32.ca/locals.error.txt I will forget it there, so it should still be available in a few weeks/months... ;) And here are a few extracts of just the error lines: /usr/include/boost/spirit/home/qi/nonterminal/grammar.hpp:75:13: error: no matching function for call to 'assertion_failed(mpl_::failed************ (boost::spirit::qi::grammar<Iterator, T1, T2, T3, T4>::grammar(const boost::spirit::qi::rule<Iterator_, T1_, T2_, T3_, T4_>&, const string&) [with Iterator_ = __gnu_cxx::__normal_iterator<const char*, std::basic_string<char> >, T1_ = hs::Language::AX*(), T2_ = boost::spirit::unused_type, T3_ = boost::spirit::unused_type, T4_ = boost::spirit::unused_type, Iterator = __gnu_cxx::__normal_iterator<const char*, std::basic_string<char> >, T1 = hs::Language::AX*(), T2 = boost::spirit::locals<std::vector<hs::Language::AX*>*>, T3 = boost::spirit::unused_type, T4 = boost::spirit::unused_type, std::string = std::basic_string<char>]::incompatible_start_rule::************)(boost::spirit::qi::rule<__gnu_cxx::__normal_iterator<const char*, std::basic_string<char> >, hs::Language::AX*(), boost::spirit::unused_type, boost::spirit::unused_type, boost::spirit::unused_type>))' /usr/include/boost/proto/extends.hpp:537:29: error: use of deleted function 'boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::spirit::qi::reference<const boost::spirit::qi::rule<__gnu_cxx::__normal_iterator<const char*, std::basic_string<char> >, hs::Language::AX*(), boost::spirit::locals<std::vector<hs::Language::AX*>*>, boost::spirit::unused_type, boost::spirit::unused_type> > >, 0l>::expr()' /usr/include/boost/proto/detail/preprocessed/expr_variadic.hpp:37:12: error: 'boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::spirit::qi::reference<const boost::spirit::qi::rule<__gnu_cxx::__normal_iterator<const char*, std::basic_string<char> >, hs::Language::AX*(), boost::spirit::locals<std::vector<hs::Language::AX*>*>, boost::spirit::unused_type, boost::spirit::unused_type> > >, 0l>::expr()' is implicitly deleted because the default definition would be ill-formed: /usr/include/boost/proto/detail/preprocessed/expr_variadic.hpp:37:12: error: no matching function for call to 'boost::spirit::qi::reference<const boost::spirit::qi::rule<__gnu_cxx::__normal_iterator<const char*, std::basic_string<char> >, hs::Language::AX*(), boost::spirit::locals<std::vector<hs::Language::AX*>*>, boost::spirit::unused_type, boost::spirit::unused_type> >::reference()'

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Using boost with 'c' level interfaces ?
by avib369 . 07 Jun '13

07 Jun '13

I can not find any solution where a 'c' api can update/modify a std::vector Passing a std::vector<int>/std::vector<double> to a 'c' or fortran api function, that _reads_, works like a charm. However we are out of luck, if we need the 'c' api, to populate the array. When passing a std::vector will 'kind of' work. But you can never use any of the following // 1/ size() // 2/ empty() // 3/ begin()/ end()/ rbegin()/rend() // 4/ push_back() // 5/ pop_back() You have to separately carry the size of the vector, in order to use it. You then just end up using it like a 'c' array. I thought there may have been way of updating the vectors begin/finish after the api call. But there does not appear to any way to do this. One solution would be to use 'c' array when calling the interface, and then copy to a std::vector. However this would degrade performance, when dealing large data sets. Is there any boost libraries/containers that are applicable or that can handle this issue ? It must have similar properties to a vector, like contiguous memory ? I have included a sample program below, to demonstrate. Before resorting back to 'c' array I was wondering if any one else has come across this problem ? Ta, Avi ============================================================= #include <vector> #include <string> #include <string.h> // memcopy #include <stdlib.h> // malloc #include <iostream> #include <ostream> using namespace std; void c_function_that_reads(int* array, int size_of_array) { for(int i = 0; i < size_of_array; i++) cout << array[i] << ","; cout << "\n"; } void c_function_that_writes(int* array, int size_of_array) { for(int i = 0; i < size_of_array; i++) array[i] = i; } int main(){ { cout << "\ncalling c_function_that_reads\n"; vector<int> dataVec; dataVec.push_back(1); dataVec.push_back(22); c_function_that_reads(&dataVec[0],dataVec.size()); } { cout << "\ncalling c_function_that_writes\n"; int the_size = 10; int* int_array = (int*)malloc(the_size*sizeof(int)); c_function_that_writes(int_array,the_size); // Copy into vector. vector<int> dataVec; dataVec.insert(dataVec.end(), &int_array[0], &int_array[the_size]); for(size_t i = 0; i < dataVec.size(); i++) { cout << dataVec[i] << ","; } cout << "\n"; free(int_array); } { cout << "\ncalling c_function_that_writes\n"; int the_size = 10; vector<int> dataVec; dataVec.reserve(the_size); c_function_that_writes(&dataVec[0],the_size); cout << "dataVec.size() = " << dataVec.size() << ", dataVec.capacity() = " << dataVec.capacity() << " dataVec.empty() = " << dataVec.empty() << "\n"; cout << "for(size_t i = 0; i < dataVec.size(); i++): BROKEN\n"; for(size_t i = 0; i < dataVec.size(); i++) { cout << dataVec[i] << ","; } cout << "\n"; cout << "for(size_t i = 0; i < the_size; i++): WORKS\n"; for(int i = 0; i < the_size; i++) { cout << dataVec[i] << ","; } cout << "\n\n"; cout << "for(vector<int>::iterator i = dataVec.begin(); i != dataVec.end(); i++): BROKEN\n"; for(vector<int>::iterator i = dataVec.begin(); i != dataVec.end(); i++) { cout << (*i) << ","; } cout << "\n"; dataVec.push_back(11); the_size += 1; // dataVec.pop_back(); the_size -= 1; // also broken cout << "for(size_t i = 0; i < dataVec.size(); i++): BROKEN\n"; for(size_t i = 0; i < dataVec.size(); i++) { cout << dataVec[i] << ","; } cout << "\n"; cout << "for(size_t i = 0; i < the_size; i++): BROKEN\n"; for(int i = 0; i < the_size; i++) { cout << dataVec[i] << ","; } cout << "\n\n"; cout << "for(vector<int>::iterator i = dataVec.begin(); i != dataVec.end(); i++): BROKEN\n"; for(vector<int>::iterator i = dataVec.begin(); i != dataVec.end(); i++) { cout << (*i) << ","; } cout << "\n"; // Using std::vector is ok for read only access when calling a c api function // *however* Using a c/fortrans api to *populate* a std::vector leads to issues: // // The following will not work after the api call. // 1/ size() // 2/ empty() // 3/ begin()/ end()/ rbegin()/rend() // 4/ push_back() // 5/ pop_back() // 6/ will ~vector() work since that use begin()/end() ?? // Is it worth using std::vector, since many of invariants are broken ???? // We have to carry the length separately } }

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All talks for Meeting C++ 2013 are online!
by Jens Weller 06 Jun '13

06 Jun '13

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Re: [Boost-users] [1.54.0] Beta 1 Available
by Cromwell Enage 06 Jun '13

06 Jun '13

Hi, all. When calling the merge() member function of a boost::heap::d_ary_heap, is the heap passed in not supposed to be emptied? Here's my test program: ['' #include <boost/heap/d_ary_heap.hpp> #include <boost/heap/binomial_heap.hpp> #include <boost/heap/fibonacci_heap.hpp> #include <boost/heap/pairing_heap.hpp> #include <boost/heap/skew_heap.hpp> #include <boost/test/minimal.hpp> template <typename Heap> void test_heap_merge_memfun() { Heap heap_1, heap_2; heap_1.push(3); heap_1.push(5); heap_1.push(1); heap_2.push(2); heap_2.push(4); heap_2.push(0); heap_1.merge(heap_2); std::cout << " heap_2"; if (!heap_2.empty()) { BOOST_CHECK(4 == heap_2.top()); heap_2.pop(); BOOST_CHECK(2 == heap_2.top()); heap_2.pop(); BOOST_CHECK(0 == heap_2.top()); heap_2.pop(); std::cout << " not"; } std::cout << " emptied." << std::endl; BOOST_CHECK(heap_2.empty()); BOOST_CHECK(5 == heap_1.top()); heap_1.pop(); BOOST_CHECK(4 == heap_1.top()); heap_1.pop(); BOOST_CHECK(3 == heap_1.top()); heap_1.pop(); BOOST_CHECK(2 == heap_1.top()); heap_1.pop(); BOOST_CHECK(1 == heap_1.top()); heap_1.pop(); BOOST_CHECK(0 == heap_1.top()); heap_1.pop(); BOOST_CHECK(heap_1.empty()); } int test_main(int argc, char** argv) { test_heap_merge_memfun< boost::heap::d_ary_heap< int , boost::heap::arity<4> , boost::heap::mutable_<true> > >(); test_heap_merge_memfun<boost::heap::binomial_heap<int> >(); test_heap_merge_memfun<boost::heap::fibonacci_heap<int> >(); test_heap_merge_memfun<boost::heap::pairing_heap<int> >(); test_heap_merge_memfun< boost::heap::skew_heap<int,boost::heap::mutable_<true> > >(); return 0; } ''] When I run this program, I see that all the other heap types empty the heap passed in. I use gcc 4.2.1 with Apple clang version 4.0 on MacOSX 10.8 Cromwell D. Enage

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[Iostreams] Safely returning filtering_istream from function
by Szymon Gatner 06 Jun '13

06 Jun '13

Hi, I would like to return filtering_istream via std::unique_ptr<std::iostream> so that client code can just use iostream interface without knowing what is actual stream type. Inside the factory function filtering_istream is prepared with possible filters attached. Problem is filters are attached by reference. My question is: how to elegantly attach filters to a filtering_istream so that they live as long as filtering_istream itself? How to attach a filter via shared_ptr for example? Regards, Szymon Gatner

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Trouble with posix_time
by Davies, John 06 Jun '13

06 Jun '13

I'm trying to time part of my program and coming up with something I can't explain. I don't expect great accuracy with the sample code here. But I'd like to understand what is happening. static boost::posix_time::ptime MillisecondsFromNow(int milliseconds) { return(boost::posix_time::ptime(boost::posix_time::second_clock::universal_time()) + boost::posix_time::milliseconds(milliseconds)); } Code segment: nanosecond_type const twenty_seconds(20 * 1000000000LL); cpu_timer timer; long count = 0; while(true) { boost::thread::sleep(MillisecondsFromNow(1000)); cpu_times const elapsed_times(timer.elapsed()); nanosecond_type const elapsed(elapsed_times.wall); if (elapsed >= twenty_seconds) { break; } count++; } std::cout << count << '\n'; So I'm running for 20 seconds and delaying each time for a second. So I would expect to get output of 19, maybe 20 or 18. Instead I'm getting around 38-40 I've tried .system and .user and get different values. But never something close to 20. This is on Windows. ___________________________________ John Davies Contractor Home Respiratory Care Philips Home Healthcare Solutions 1740 Golden Mile Highway Monroeville, PA 15146 Email: john.davies(a)philips.com Fax: 724-387-4109 ________________________________ The information contained in this message may be confidential and legally protected under applicable law. The message is intended solely for the addressee(s). If you are not the intended recipient, you are hereby notified that any use, forwarding, dissemination, or reproduction of this message is strictly prohibited and may be unlawful. If you are not the intended recipient, please contact the sender by return e-mail and destroy all copies of the original message.

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[DNS] Heads up: svn.boost.org moving to a new server; IP address changing
by Beman Dawes 05 Jun '13

05 Jun '13

At 2:00 pm Eastern US time (i.e 30 minutes from now) we will switch svn.boost.org to a new server with a new IP address. It would be best to hold off on any SVN commits until after the change over, and after your DNS server has the new IP address. If command line "ping svn.boost.org" is going to [129.79.245.252], you are still getting the old IP address. Once svn.boost.org is resolving to [129.79.13.13] it should be OK to resume normal Subversion activity. Thanks, --Beman

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[iostreams] C++ 11 moving semantics
by Ovanes Markarian 05 Jun '13

05 Jun '13

Hello *, I am wondering if latest boost iostreams implementation (1.53) supports C++ 11 moving semantics. I would like to create a tee_device and related stream in a separate function and hide the complexity from the user and just return a stream object which moved out of the function. Any ideas how to do that, without using heap and returning smart pointers? Thanks for your help, Ovanes

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[program_options] Config file and/or response file
by Michael Powell 05 Jun '13

05 Jun '13

Hello, Couple of questions about program options config file (and/or response file). I've got an options set that is starting to get fairly verbose, it's rich, but is also impractical to expect users to type that many options. So I would like to capture some (or all) of those options in one or more different config files, depending on the options that need to be set for a given session. With config files, can the parse_config_file method build a vector of options? For instance if I define "value" as vector<int>, can I specify value=1, value=2, value=3 and expect {1,2,3}? Or does the convention go something like this, value=1 2 3, and so on? Next, how does the parse_config_file method treat boiler plate iterative sections? In other words, if I've got a pattern and would like to capture zero or more of that pattern: [pattern] name=test param=1 expected=2 actual=3 [pattern] name=result param=4 expected=5 actual=6 Those might inform a vector<pattern> type concern downstream of the parser. Possible? Third, in a response file, there is a statement about "limitations", which was left at that. Can we be more specific? What kind of limitations are we talking about? Obvious ones like the delimiters between tokens come to mind, but other than that, that's a nebulous statement at best. Thank you. Regards, Michael Powell

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