July 2011 - Boost-users - lists.preview.boost.org

HasPlus
by Robert Jones 26 Jul '11

26 Jul '11

Hi All I'm clutching at straws here, so this may be a complete fiction, but I seem to recall mention of purely syntactic concept definitions as part of the now-abandoned C++0x concept proposal. Things like hasPlus. Maybe I've just made this up! Does any such thing exist in boost anywhere? Maybe as a concept check or type trait? Thx - Rob.

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[Thread] Assertion on mutex when using multiple threads and mutexes
by Tim van den Kerkhof 26 Jul '11

26 Jul '11

Hi all, As a part of a project I'm writing a logger function. This logger function sends an e-mail when the program wants to log something. Since it has happened that the SMTP server was non responsive, I've decided to do the sending of the mails in a separate thread. This thread reads messages from an std::deque which is filled by the logging function. The thread is setup as follows: while (!boost::this_thread::interruption_requested()) { EmailItem emailItem; { boost::unique_lock<boost::mutex> lock(mMutex); while (mEmailBuffer.empty()) mCond.wait(lock); bufferOverflow = mBufferOverflow; mBufferOverflow = false; nrOfItems = mEmailBuffer.size(); if (nrOfItems> 0) { emailItem = mEmailBuffer.front(); mEmailBuffer.pop_front(); } } if (nrOfItems> 0) { bool sent = false; while(!sent) { try { ..... Do something with the message ..... { boost::this_thread::disable_interruption di; boost::lock_guard<boost::mutex> lock(mLoggerMutex); mLogFile<< emailItem.mMessage<< std::endl; } sent = true; } catch (const std::exception&e) { // Unable to send mail, an exception occurred. Retry sending it after some time sent = false; boost::this_thread::sleep(boost::posix_time::seconds(LOG_WAITBEFORE_RETRY)); } } } } The function log() adds a new message to the deque (mEmailBuffer) as follows: { boost::lock_guard<boost::mutex> lock(mMutex); mEmailBuffer.push_back(e); mCond.notify_one(); } When the main program exits, the destructor of the logger object is called. This is where it goes wrong, the application crashes with an error: /usr/include/boost/thread/pthread/mutex.hpp:45: boost::mutex::~mutex(): Assertion `!pthread_mutex_destroy(&m)' failed. The destructor merely calls an interrupt on the thread and then joins it: mQueueThread.interrupt(); mQueueThread.join(); In the main program, I use multiple different classes which make use of boost threading and mutex as well, could this cause this behaviour? Not calling the destructor of the logger object results in no errors, as does using the logger object and not doing anything else. My guess is I am doing something very wrong, or there is a bug in the threading library when using multiple threads divided over several classes. Does anyone have a idea what the reason for this error might be? Regards, Tim van den Kerkhof

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boost::units implicit conversions
by Michael Powell 25 Jul '11

25 Jul '11

Boost::units users, I have gotten my feet wet with boost::units over the past several days and starting to get the hang of it, dimensions, quantities, and units, and so forth. Now I would like to start thinking more in terms of how these units-based calculations should be used by the rest of our application. Of primary concern is to receive a quantity of dimension from the application in a specified unit and to be able to perform the calculations. One concept I've been wondering about is whether there is any value in converting units to a base unit for purposes of IPC (interprocess communication), saving to or loading from files, or that sort of thing. In this way we will know that if we see a quantity of length dimension, for instance, we can always expect the value to be presented in terms of, let's say, meters. My question concerning calculations is something along these lines: how does boost::units treat a quantity<dimension> when facilitating dimensioned calculations? Or is it on us as developers to ensure that an appropriate unit-conversion is taking place somewhere along the way when we are ready to interpret the result for display, report, or whatever? Let me know if I can clarify at all, because I am a little fuzzy along these lines at the moment. Thank you... Regards, Michael

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[Fusion] issues when adapting an existing structure
by Sean Farrow 25 Jul '11

25 Jul '11

Hi: I'm trying to adapt a structure for use with spirit. I have the following code: struct JAWSFunction { std::wstring FunctionType; std::wstring Description; std::wstring Synopsis; std::string ReturnType; std::vector<std::pair<std::wstring, std::wstring> > Parameters; }; //tell boost.Fusion about the JAWSFunction structure to allow it to be used with spirit. BOOST_FUSION_ADAPT_STRUCT(JAWSFunction, (std::wstring, FunctionType)(std::wstring, Description)(std::wstring, Synopsis)(std::wstring, ReturnType)(std::vector<std::pair<std::wstring, std::wstring> >, Parameters)) If I compile this with the std::vector<std::pair<std::wstring, std::wstring> > taken out, this works fine. However replacing the vector of pairs doesn't compile. How can I get this to work? Any help appreciated. Cheers Sean.

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boost::units us system
by Michael Powell 25 Jul '11

25 Jul '11

Boost::units users, I am looking for a system which gathers the U.S. units of measure. I notice, for instance, that SI has a system. Are there any plans in the works to introduce a US system? If not, how hard would that be? There are base units, for instance. So it would be a matter of setting up the appropriate systems? **or** what approach have boost::units users taken in the past when broaching the US-SI unit conversion question? Thank you... Regards, Michael

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boost::units unit proposal
by Michael Powell 25 Jul '11

25 Jul '11

Boost::units developers / users, I need to specify a U.S. Barrel (bbl) for purposes of performing conversions and so forth. First, does this definition look about right? Second, how does one propose an addition to the units code base? Note, I'm not quite sure how the 2^3 scaling from gallon to pints works in terms of scaling from barrel to gallons, or pints for that matter, so I am assuming that gallon can be the base unit for barrel and call it good. Thoughts, questions, concerns? namespace us { typedef scaled_base_unit<gallon_base_unit, scale<42> > barrel_base_unit; } // namespace us template<> struct base_unit_info<us::barrel_base_unit> { static const char* name() { return("bbl (U.S.)"); } static const char* symbol() { return("bbl"); } }; Thank you... Regards, Michael

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url encoding
by Kraus Philipp 25 Jul '11

25 Jul '11

Hello, does boost support any url encoding structures? I would like to transform a std::string with spaces and other chars to the correct url representation (space will be changed to %20 ...). Thanks Phil

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is leaf() be defined at boost/filesystem/operations.hpp ?
by Eric Lin 24 Jul '11

24 Jul '11

Dear advanced boost/c/g++ programers: I copied and try a simple (Reading a directory with boost) program , from page 383, Example 10-19, of book(C++ cookbook) ----------you can download the code to try by yourself at, http://examples.oreilly.com/9780596007614/ // Example 10-19. Reading a directory #include <iostream> #include <cstdlib> #include <boost/filesystem/operations.hpp> #include <boost/filesystem/fstream.hpp> using namespace boost::filesystem; int main(int argc, char** argv) { if (argc < 2) { std::cerr << "Usage: " << argv[0] << " [dir name]\n"; return(EXIT_FAILURE); } path fullPath = // Create the full, absolute path name system_complete(path(argv[1])); if (!exists(fullPath)) { std::cerr << "Error: the directory " << fullPath.string() << " does not exist.\n"; return(EXIT_FAILURE); } if (!is_directory(fullPath)) { std::cout << fullPath.string() << " is not a directory!\n"; return(EXIT_SUCCESS); } directory_iterator end; for (directory_iterator it(fullPath); it != end; ++it) { // Iterate through each // element in the dir, std::cout << it->leaf(); // almost as you would if (is_directory(*it)) // an STL container std::cout << " (dir)"; std::cout << '\n'; } return(EXIT_SUCCESS); } ----------------------------------------------------------------------------- my compiler g++ 4.5.2 not pass it, even I link --------- eric@eric-laptop:~/cppcookbook/ch10$ g++ -lboost_system -lboost_filesystem Example10-19.cpp Example10-19.cpp: In function ‘int main(int, char**)’: Example10-19.cpp:34:24: error: ‘class boost::filesystem3::directory_entry’ has no member named ‘leaf’ eric@eric-laptop:~/cppcookbook/ch10$ --------- I am using 1.46.1 of boost looking to hear from your help and thanks a lot in advance, Eric

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Boost.Iterator questions
by John M. Dlugosz 24 Jul '11

24 Jul '11

Consider something similar to what Glib::ustring does, only better made. If I have a sequence of characters (say, in a normal std::string) and want to use an adaptor to decode UTF-8 code points, the resulting adapted iterator returns a different type (element is char32_t), not an lvalue, and consumes a variable number of underlying elements. It's not just a transform_iterator because it consumes a variable number of underlying elements. It's not writable because a new value might encode a different number of underlying elements than the one it is replacing. I think I need to implement it directly from iterator_facade, not one of the more specialized classes already provided. Am I mistaken? What is the traversal type? Random access is _possible_ just not efficient. To move forward n locations it has to increment n times. ++ and −− are fine. There is indeed a well defined distance between two iterators, but I don't want to encourage that kind of thing and hide the inefficiency. But, I see that relational operators are defined for random access (only), not for bidirectional? Why is that? Note that relational operators for the proposed type _are_ efficient, as it can defer to the underlying iterator. I knows which is greater, just not by how much exactly. The template argument for Difference, in the tutorial, mentions "distance between any two addresses in memory". But the distance between two locations is not the same as the difference in bytes. Is this being implied? After all, the example is a linked list and the distance from one node to another, in hops, has no relation to the address at all. Is Difference something I need to worry about?

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[iterator] iterator_core_access and namespaces
by Nathan Ridge 24 Jul '11

24 Jul '11

Hello, I came across what seems like an interesting oddity: #include <boost/iterator/iterator_facade.hpp> using boost::iterator_core_access; namespace ns { struct range { class iterator : public boost::iterator_facade<iterator, int, boost::single_pass_traversal_tag> { friend class iterator_core_access; int& dereference() const; void increment(); bool equal(const iterator& other) const; }; typedef iterator iterator; iterator begin(); iterator end(); }; } int main() { ns::range::iterator b; ++b; } This code goes not compile with gcc (I tried with 4.3 through 4.7), giving the following error: test.cpp: In static member function 'static void boost::iterator_core_access::increment(Facade&) [with Facade = ns::range::iterator]': boost/boost/iterator/iterator_facade.hpp:664:11: instantiated from 'Derived& boost::iterator_facade<I, V, TC, R, D>::operator++() [with Derived = ns::range::iterator, Value = int, CategoryOrTraversal = boost::single_pass_traversal_tag, Reference = int&, Difference = int]' test.cpp:29:7: instantiated from here test.cpp:16:18: error: 'void ns::range::iterator::increment()' is private boost/boost/iterator/iterator_facade.hpp:523:11: error: within this context If I explicitly qualify iterator_core_access in the friend declaration (i.e. "friend class boost::iterator_core_access"), or if I move the "using boost::iterator_core_access" inside the namespace, it works fine. Is this a compiler bug, or am I bumping into a shadowy corner of the Standard involving friends and namespaces? Thanks, Nate.

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