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[general question] on threading and double checked locking pattern
by Ovanes Markarian 22 Jan '07

22 Jan '07

I might be wrong here, but I would like to hear some thoughts of C++ professionals. I read in lots of sources about DCLP and how it is broken in conjunction with Singleton pattern and my question is of a very simple nature. Is it possible to solve this problem by introducing function calls via function pointers? Or is there still a way, that these calls might be inlined and if yes then how could this look like? This example is not about freeing the singleton, it is just about creating it. class Singleton; typedef Singleton* (*singleton_getter)(); class Singleton : boost::noncopyable { static Singleton* pInstance; static singleton_getter getter; //other internal data... Singleton(); static Singleton* creating_singleton_getter(); static Singleton* non_creating_singleton_getter(); public: static Singleton* instance(); }; //implementation Singleton* Singleton::pInstance = NULL; singleton_getter Singleton::getter = &creating_singleton_getter; Singleton::Singleton() {} Singleton* Singleton::creating_singleton_getter() { //aquire mutex here if(Singleton::pInstance==NULL) Singleton::pInstance = new Singleton; Singleton::getter = &non_creating_singleton_getter; return Singleton::pInstance; } //mutex unlocked Singleton* Singleton::non_creating_singleton_getter() { //no mutex anymore return Singleton::pInstance; } Singleton* Singleton::instance() { return (Singleton::getter)(); } With Kind Regards, Ovanes Markarian

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[date_time] Misparses POSIX timezone specs
by Stephen Turner 22 Jan '07

22 Jan '07

If I'm being stupid, tell me; but I believe the date_time library misreads all POSIX timezone specs by swapping + and -. It uses negative for west of Greenwich, but POSIX (admittedly unintuitively) uses negative for east of Greenwich. Compare this Linux box: [chiark:~] (1)% date Fri Jan 19 16:05:53 GMT 2007 [chiark:~] (2)% setenv TZ PST8 [chiark:~] (3)% date Fri Jan 19 08:06:04 PST 2007 with this code which wraps the date_time library (the internals aren't important): DWORD dw1 = 1149163201; // 1 June 2006 12:00:01 GMT CMyTime::set_timezone_posix("PST8"); CMyTime t = dw1; h = t.GetHour(); // h = 20 -- Stephen Turner, Cambridge, UK http://homepage.ntlworld.com/adelie/stephen/ "The internet is a reflection of our society. If we do not like what we see, the problem is not to fix the mirror, we have to fix society." (Vint Cerf)

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[bind] Problem compiling when passing instantiation of class
by Paul Giaccone 22 Jan '07

22 Jan '07

I am using Boost bind in conjunction with Boost unit test, as I have many times before, but I can't get my code to compile. I've looked at http://boost.org/libs/bind/bind.html#Troubleshooting but don't see anything that covers my case. Here is the line that is failing (with disguised variable names): test_suite -> add(BOOST_TEST_CASE(boost::bind(my_test, obj1, obj2, obj3, 1.0, obj4, 10, 1E-9, results))); and here is the error that Microsoft Visual C++ .NET is generating: main.cpp(152): error C2665: 'boost::bind' : none of the 2 overloads can convert parameter 4 from type 'Obj3Type' The variables obj3 and obj4 are of type Obj3Type and Obj4Type respectively (see definitions below): The function my_test is defined as: void my_test(boost::shared_array<Vector>& a, boost::shared_array<Vector>& b, Obj3Type& c, const double d, const Obj4Type& e, const unsigned int f, const double g, boost::shared_array<std::pair<double, std::valarray<double> > >& h) where Obj3Type and Obj4Type are defined as: class Obj3Type { bool var1; unsigned int var2; unsigned int var3; double var4; double var5; MyType var6; boost::scoped_array<int> var7; boost::scoped_array<boost::scoped_array<double> > var8; }; struct Obj4Type { MyType var1; double var2; double var3; double var4; double var5; double var6; Vector var7; Vector var8; double var9; bool var10; } where MyType is an enumerated type and Vector is another struct of mine. (Where I have used structs rather than classes, this is for convenience.) Note that obj4 does not generate a compiler error, even when I move it to appear before obj3 in the parameter list. I wondered if it was necessary to overload operator= for Obj3Type as, presumably, the compiler cannot generate it for the scoped arrays in that struct but can for everything in Obj4Type, including Vector, for which I have defined operator=. I wrote an operator= for Obj3Type, but it made no difference to the compiler error. This was to be expected really, as obj3 is passed by reference rather than by value. Does it have anything to do with using structs instead of classes? I can't see how that could be the case. So what am I missing here? Thanks for any help.

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Glib::ustring and tokenizer
by Manuel Jung 22 Jan '07

22 Jan '07

Hi everybody, I want to use Glib::ustring for UTF8 support. I was using boost::tokenizer up to this moment for breaking up some strings, which now would be ustrings. In the tokenizer Documentation you can find: template < class TokenizerFunc = char_delimiters_separator<char>, class Iterator = std::string::const_iterator, class Type = std::string > class tokenizer Im never used template programming, so can somebody give me a hint, if and how i can modify this to Glib::ustring? Maybe something like this? template < class TokenizerFunc = char_delimiters_separator<char>, class Iterator = Glib::ustring::const_iterator, class Type = Glib::ustring > class utokenizer Can i use wchar_t now? im a liddle bit confused, if it not works i have just to write some code myself, but i have hope getting it working. Thanks for reading and maybe some answers. Greetings from stormy germany Manuel Jung

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Re: [Boost-users] [filesystem] code pages support
by Nat Goodspeed 22 Jan '07

22 Jan '07

________________________________________ From: boost-users-bounces(a)lists.boost.org [mailto:boost-users-bounces@lists.boost.org] On Behalf Of ????? ???????? Sent: Friday, January 19, 2007 10:41 AM To: boost-users(a)lists.boost.org Subject: [Boost-users] [filesystem] code pages support namespace fs = boost :: filesystem; boost :: intmax_t filesize = fs :: file_size (argv [1]); This causes an exception if the name consists of cyrillic symbols. (Windows XP, MinGW). Does boost :: filesystem support these names. [Nat] Maybe try: boost :: intmax_t filesize = fs :: file_size (fs::path(argv [1], fs::no_check)); This uses an explicit conversion to fs::path rather than an implicit one, which allows you to explicitly disable the path syntax checker. I always disable the path syntax checker. It doesn't even support ASCII names well. ;-)

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Newby question re (boost::shared_ptr) enable_shared_from_this
by ethann.castell＠caliton.com 22 Jan '07

22 Jan '07

Hi, I'm still fairly new to C++ and I'm in the process of converting pointers to boost shared pointers. I have a question regarding the use of enable_shared_from_this. I would like to store a self-referencing pointer within the object. Something like: class someclass : public boost::enable_shared_from_this<someclass> { protected: boost::shared_ptr<someclass> m_ptr_this; someclass() { m_ptr_this = shared_from_this(); } } And then access m_ptr_this in classes derived from someclass, and pass as a parameter to methods of other (derived or non-derived) classes. eg. class someclass2 : public someclass { ... some_func(){ someclass3* f = new someclass3(); someclass3->do_something( m_ptr_this ); } } I understand that using a shared_ptr (without enable_shared_from_this) to self-reference will lead to problems because the reference count of the pointer will never reach zero and therefore the memory will never get released. But as enable_shared_from_this uses a weak pointer internally, does this get around the problem? I'm getting a little confused because it gets typecast from weak_ptr to shared_ptr and I'm not sure what this leads to. I've also considered having a member function return the pointer: boost::shared_ptr<someclass> someclass::get_this() { return shared_from_this(); } and then calling the routine like this; someclass3->do_something( get_this() ); but I think that this will cause a problem because the documentation says not to use boost pointers which are only created as temporary variables. i.e. store all boost pointers in vars. What I'm hoping to do is avoid having to declare extra shared_ptr vars all over the place. Any help will be most appreciated. And I'll owe you a beer! Regards Ethann Castell

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program_options: Why it throw ambiguous_option?
by Peng Yu 22 Jan '07

22 Jan '07

Hi, I got the following error for the program list below. Where program_options considers "help" and "help_abc" as ambiguous? $ ./main --help terminate called after throwing an instance of 'boost::program_options::ambiguous_option' what(): ambiguous option help Aborted Thanks, Peng #include <boost/program_options.hpp> namespace po = boost::program_options; int main(int ac, char* av[]) { po::options_description desc("Allowed options"); desc.add_options() ("help", "produce help message") ("help_abc", po::value<int>(), "set compression level") ; po::variables_map vm; po::store(po::parse_command_line(ac, av, desc), vm); po::notify(vm); return 0; }

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[Fusion] General comments
by Sohail Somani 22 Jan '07

22 Jan '07

Hi, FWIW. I think Fusion is a really good library. Having the results of algorithms be lazily evaluated was genius. Now I need to just find more excuses to use it :) One minor doc suggestion: A full TOC not unlike the MPL TOC would be very helpful for looking up stuff. One has to click through maybe 3 levels before finding out they went to the wrong place (Ok, doesn't happen that often anymore.. but still) Is it possible to get quickbook(?) to do that? Thanks, Sohail

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[fusion] accumulate documentation
by Chris Weed 22 Jan '07

22 Jan '07

Hi, I've tried some code similar to the example here: <http://spirit.sourceforge.net/dl_more/fusion_v2/libs/fusion/doc/html/fusion…> but to get it to compile I have to make the inner function class 'result' instead of 'apply'. Chris

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Does boost::thread_group::create_thread have memory leak?
by pete＠mu.org 22 Jan '07

22 Jan '07

I've been trying to track down memory leak being reported by valgrind. There are others out there who appear to have the same problem, but I can't find any reference to it in the boost list archives. I know valgrind can be inaccurate, but it seems pretty accurate every where else in my program. I've narrowed it down to something like this: ----------------- #include <boost/bind.hpp> #include <boost/thread.hpp> #include <iostream> void fn(){ return; } int main() { boost::thread_group thrd; thrd.create_thread(boost::bind(&fn)); thrd.join_all(); return 0; } ----------------- Valgrind is showing this: ... by 0x4053D76: boost::thread_group::create_thread(boost::function0<void, std::allocator<boost::function_base> > const&) (in /usr/lib/libboost_thread-mt.so.1.33.1) LEAK SUMMARY: ==22004== definitely lost: 0 bytes in 0 blocks. ==22004== possibly lost: 144 bytes in 1 blocks. ==22004== still reachable: 0 bytes in 0 blocks. ==22004== suppressed: 0 bytes in 0 blocks. Any clues? Thoughts? Thanks, Pete

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