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Policy based threading question
by Aschwin Gopalan 24 May '03

24 May '03

Dear Boosters! I am designing a class template with policy controled threading behaviour, like template<ThreadingModel=SingleThreaded> class Something; where there are different ThreadingModels available (see Modern C++ Design by Alexandrescu for a discussion of policies). I would like to have Something to inherit ThreadingModel. The idea is that SingleThreaded does not impose any additional penalties. So my Idea was to have something like class SingleThreaded { public: void lock() {} void release() {} } class MultiThreaded { public: void lock() { lock_mutex(m_mutex) } void release() { release_mutex(m_mutex) } private: mutex m_mutex; } and have all functions call lock() and release() accordingly. But this is not good, since it does not use Resource Initialization is Aquisition and leads to exeption safety problems and so on. Furthermore, it is not (because of these reasons) possible to model using boost::mutex ´es since they do not expose a lock() release() interface but can only be locked using scoped_locks. So another idea is to have something like class SingleThreaded { class scoped_lock { // Empty class } } class MultiThreaded { public: class scoped_lock { scoped_lock() // constructor that locks m_mutex of // of MultiThreaded ~scoped_lock() // unlocks } private: boost::mutex m_mutex; } and have the member functions lock the mutex by instantiating an embeded scoped_lock object. Now there are some questions I have: 1.) Does this approach sound sensible, or am I overlooking a commonly used Idiom to achieve this functionality 2.) How can I access the m_mutex member of the right object in scoped_lock()? The only way I can imagin is to give the this pointer to the constructor (like scoped_lock lock(this);). It is somehow not nice, but possible 3.) The actual code I am using looks like //! Policy class. /*! Multithreading, object operations are synchronized via * a non-recursive lock, obeying strict Resource Acuisition is Initialization * semantics */ class LockDefault { public: LockDefault() : m_pmutex(new boost::mutex) { } class scoped_lock : public boost::mutex::scoped_lock { public: scoped_lock(LockDefault &obj) : boost::mutex::scoped_lock(obj->m_pmutex) { } ~scoped_lock() { } }; public: boost::mutex m_mutex; }; This works with gcc and I think it does what it should. It fails to compile with VC++6 giving error C2512: 'scoped_lock<class boost::recursive_mutex>' : no appropriate default constructor which I do not understand, since the default constructor is never called. Also, I do not like to have the mutex member public, but if I make it private and write friend class scoped_lock; this does not have the desired effect (MSVC still complains about private data member access) Somehow, I must be getting something completely wrong, so if somebody could point me in the right direction, I would be thankful. BTW: I am using boost_1_30_0 Thanks, Aki

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newbie: trouble composing with boost::bind
by Jacques A. Vidrine 23 May '03

23 May '03

Hello, I'm new to the Boost library and quite excited to see some of the tools available to assist writing functional-style code. Recently I wanted to perform an action on every object in a container of smart pointers (boost::shared_ptr). I couldn't seem to find a concise way to do so. Here is a small example (no container involved) demonstrating the problem I'm encountering. #include <iostream> #include <string> #include <boost/shared_ptr.hpp> #include <boost/bind.hpp> using namespace std; using namespace boost; void xprint(ostream *os, shared_ptr<string> p) { *os << p.get()->c_str() << endl; } void xprint2(ostream *os, string *p) { *os << p->c_str() << endl; } int main(int ac, char *av[]) { shared_ptr<string> p(new string("Test")); bind(xprint, &cerr, _1)(p); #ifdef THIS_DOES_NOT_WORK bind(xprint2, &cerr, _1)( bind(&shared_ptr<string>::get, _1)(p) ); #else string *q = bind(&shared_ptr<string>::get, _1)(p); bind(xprint2, &cerr, _1)(q); #endif return 0; } Using GCC 3.2, I encounter a `no match for call' error in the `THIS_DOES_NOT_WORK' section of code. (Full diagnostics below.) I have tried many variations using mem_fn, apply, and by specifying the return type of the functor explicitly, but with no success. I'm afraid I don't understand the problem. I hoped someone here might be able to hit me with a clue-stick :-) Cheers, -- Jacques Vidrine . NTT/Verio SME . FreeBSD UNIX . Heimdal nectar(a)celabo.org . jvidrine(a)verio.net . nectar(a)freebsd.org . nectar(a)kth.se g++ -DTHIS_DOES_NOT_WORK -I/usr/local/include example.cc example.cc: In function `int main(int, char**)': example.cc:31: no match for call to `(boost::_bi::bind_t<void, void (*)(std::ostream*, std::string*), boost::_bi::list2<boost::_bi::value<std::ostream*>, boost::arg<1> > >) ( std::string*)' /usr/local/include/boost/bind/bind_template.hpp:19: candidates are: boost::_bi::result_traits<R, F>::type boost::_bi::bind_t<R, F, L>::operator()() [with R = void, F = void (*)(std::ostream*, std::string*), L = boost::_bi::list2<boost::_bi::value<std::ostream*>, boost::arg<1> >] /usr/local/include/boost/bind/bind_template.hpp:25: boost::_bi::result_traits<R, F>::type boost::_bi::bind_t<R, F, L>::operator()() const [with R = void, F = void (*)(std::ostream*, std::string*), L = boost::_bi::list2<boost::_bi::value<std::ostream*>, boost::arg<1> >] /usr/local/include/boost/bind/bind_template.hpp:31: boost::_bi::result_traits<R, F>::type boost::_bi::bind_t<R, F, L>::operator()(A1&) [with A1 = std::string*, R = void, F = void (*)(std::ostream*, std::string*), L = boost::_bi::list2<boost::_bi::value<std::ostream*>, boost::arg<1> >] /usr/local/include/boost/bind/bind_template.hpp:37: boost::_bi::result_traits<R, F>::type boost::_bi::bind_t<R, F, L>::operator()(A1&) const [with A1 = std::string*, R = void, F = void (*)(std::ostream*, std::string*), L = boost::_bi::list2<boost::_bi::value<std::ostream*>, boost::arg<1> >]

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any_cast<double>(any_assigned_with_int) fails. workaround?
by Markus Werle 23 May '03

23 May '03

Hi! At present I learn what it means not to have strong typing: I play around with boost::any. A new and healing experience. Thank You, Bjarne, for insisting on strong typing for C++ ;-) Never made so much use of the debugger ... So now I found out that any_cast does not promote types that may be promoted like for int->double conversions. Any workaround in the lib I overlooked or is this the time for my_any_cast<T>(boost::any const &)? Markus -- Compile time analytic differentiation? Yes, at http://daixtrose.sourceforge.net/

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question about boost::bind
by Darin Buck 23 May '03

23 May '03

First, the working code: #include <boost/bind.hpp> #include <boost/function.hpp> #include <iostream> #include <string> #include <map> using namespace std; struct message { string mtablename; unsigned long long mpk; message(const string& tablename, const unsigned long long pk) : mtablename(tablename), mpk(pk) { } }; typedef boost::function < void, unsigned long long > MessageFunctor; typedef multimap < string, MessageFunctor > MessageFunctorMap; typedef pair < MessageFunctorMap::iterator, MessageFunctorMap::iterator > MessageFunctorTableMapIteratorPair; void callfunctor(pair < string, MessageFunctor > item, unsigned long long pk) { item.second(pk); } class MessageCenter { MessageFunctorMap mHandlerMap; public: MessageCenter() { } void sendMessage(message& amessage) { MessageFunctorTableMapIteratorPair iterPair = mHandlerMap.equal_range(amessage.mtablename); std::for_each(iterPair.first, iterPair.second, boost::bind (&callfunctor, _1, amessage.mpk)); // QUESTION LINE } void addHandler(string tableName, MessageFunctor handler) { mHandlerMap.insert(make_pair(tableName, handler)); } }; int gFooCounter = 0; void fooHandler(unsigned long long x) { cout << "fooHandler called on pk=" << x << endl; gFooCounter++; } void gooHandler(unsigned long long x){} int main(int argc, const char** argv) { MessageCenter messageCenter; messageCenter.addHandler("foo", fooHandler); messageCenter.addHandler("foo", fooHandler); messageCenter.addHandler("goo", gooHandler); // send some messages message fooMessage("foo", 123); messageCenter.sendMessage(fooMessage); if(gFooCounter != 2) cerr << "something didn't work"; else cout << "everything worked..." << endl; return 0; } now for the question: on the line labeled "QUESTION LINE", you can see that I'm binding to a functor called "callfunctor". All callfunctor does is call the functor embedded in the map's pair. Is there a way to simplify the line in question so that I don't need to create the functor just to get access to the pair.second member? -Darin

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smart pointer into classes this
by Erik Thiele 23 May '03

23 May '03

hi i use my own smartpointer class but hope to consolidate it into some other standard class some day (like the boost smart_ptr). an open point is the "this" pointer in classes. i have a problem with this situation: class myclass { void foo (); }; void myclass::foo() { // i do not know, if there is a smartpointer on me. // in some situations i'd like to have it! // that's why "this" exists, but "this" is not a // smartpointer! so either i have to get it in my // parameter list, or i have to be derived from // some "smartpointable" base class and call the // thissmartptr() member function. // but how should i implement the "thissmartptr" function? // in the class construction, nobody gets the smartpointer. } void main() { // sp is the smartpointer sp<myclass> m (new myclass()); // does this kind of initialisation still work // when the thissmartptr stuff shall be implemented? } i have no clue on how to implement the thissmartptr problem. my problems are class construction. do you have any plans, or is it even already implemented in the smart_ptr? cu & thx erik -- Erik Thiele

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building regex v3 with a "no-namespaces" compiler
by Nik Kolev 23 May '03

23 May '03

Hi all: Is it possible to build the regex v3 library with a compiler that does not recognize namespaces? Thanks, --Nik

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one last boost conversion question: notify_one
by Mark Sizer 22 May '03

22 May '03

I have a message queue class that has the following API: post( msg ); msg get(); where get( msg ) blocks until there is a message available and post( msg ) notifies when a message is posted. There are four threads waiting in get: MSG mqueue::get(); { MSG msgResult; { boost::mutex::scoped_lock lockQueue( _mutexQueue ); _conditionMessageReady.wait( lockQueue ); if ( _queueOut.size() > 0 ) { msgResult = _queueOut.front(); _queueOut.pop(); } } return msgResult; } The post method USED TO do this: void mqueue::post( MSG msg ) { { boost::mutex::scoped_lock lockQueue( _mutexQueue ); _queueOut.push(msg); } _conditionMessageReady.notify_one(); } When the queue was just a little bit busy (one thread processing, not all four), messages would go into the queue and never come out. I changed the .notify_one() to .notify_all() and I no longer have messages getting stuck in the queue. However, now I have a bunch of threads getting released when there is nothing for them to do (they just loop around and wait again, but YUCK!). Am I missing something or is this "expected" behavior? Thanks again, - Mark P.S. The lockQueue is locked when the condition is done waiting, right?

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newbie regex problem
by Mark Sizer 22 May '03

22 May '03

Everything builds and runs fine, I'm just not getting the result I expect. std::string sString = "Now is the time for all good men..."; std::string sRegEx1 = "^Now"; boost::regex regex1( sRegEx1 ); CPPUNIT_ASSERT( boost::regex_match(sString, regex1) ); is failing. I have to assume I'm doing something really stupid. Could someone tell me what? Thanks, - Mark

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boost::thread and detaching
by Aatu Koskensilta 22 May '03

22 May '03

Hello. I have a very simple server, in which there is a loop that looks like while( (c_sock = accept(...)) != -1) { try { boost::thread thr( boost::bind(&ServerClass::client_connection, this, c_sock)); }catch ( const std::exception & e) { std::cout << "Exception " << e << std::endl; close(c_sock); } } Now, it used to look more like while( (c_sock = accept(...)) != -1) { pthread_t thr; pthread_attr_t attrs; pthread_attr_init(&attrs); pthread_attr_setdetachstate(&attrs, PTHREAD_CREATE_DETACHED); thr_data * data = new thr_data(this, c_sock); pthread_create(&thr, &attrs, thread_proxy_function, thr_data); } (Actually, the pthread specific stuff and thread parameter wrapping and all that that was done with wrappers written by me. Also, there was proper error checking and all.) Now, the older version worked without any problems. However, the newer version using boost::thread ceased to work after approximately 256 threads had been created *all of which had exited*, pthread_create reporting an error Memory allocation failed which was reported as a resource allocation error by boost::thread. It seems that this problem was caused by the fact that the thread function usually got to execute and finish *before* the temporary thread object was destroyed (boost::thread destructor calls pthread_detach). Apparently LinuxThreads didn't consider it prudent to release the resources allocated to the thread upon call of pthread_detach if the thread had exited but no one had joined it yet. Is my analysis credible? Am I doing something completely idiotic here? I managed to get my server code functioning by poking at the boost::thread code and making it create threads detached by default, but that's hardly a very elegant solution. I'm testing this code in a RedHat 9 Linux box. -- Aatu Koskensilta (aatu.koskensilta(a)xortec.fi) "Wovon man nicht sprechen kann, daruber muss man schweigen" - Ludwig Wittgenstein, Tractatus Logico-Philosophicus

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feature request: date_time module: date_iterator--
by Erik Thiele 22 May '03

22 May '03

hi the date iterator only has ++ but no --. please add the -- cu & thx erik -- Erik Thiele

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