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[mpl] Any mpl tricks to use default tags and get around function template argument deduction with function defaults?
by jesseperla 10 Jul '09

10 Jul '09

I am building a make_ function for a complicated type with many generic parameters that all have defaults. I believe the following doesn't work in C++03 (but might in C++0X?) template<class T> void myfunc(T t = default_type()) {} if called with no arguments. Here is pseudocode for what I would like to do (unworking, so please bear with any mpl mistakes) struct default_tag{}; struct default_type // metafunction to return the default type to keep it centralized. { typedef int type; type construct() //A construction function? { return type(); } }; template<class T> class myclass { myclass(const T& t){....} //Requires a type }; template<class T> myclass< mpl::if_<mpl::is_same<T, default_tag>, //returns a my_class<> default_type, //call this metafunction if default T > //otherwise pass it in. > make_myclass(const T& t = default_tag()) //Note use of the default_tag { typename myclass< mpl::if_<mpl::is_same<T, default_tag>, default_type, //call this metafunction T >::type resolved_type; //this is what we want to use //Here I am confused of how to conditionally construct a type of default_type::type to pass in? // I thought it might help to have a default_type.construct function but wasn't sure what to do? //Note that T is often too large to do a full 'copy'. It won't be an int. //Pass in t directly or a constructed version of resolved_type if T if 'default'? return my_class<resolved_type>( ...); } The usage I would like to enable: auto m1 = make_myclass(); //Uses the default auto m2 = make_myclass( default_tag() ); //Uses the default as well. Why do I need this? Will actually have a number of these tags where you may want to conditionally choose some as defaults. int n = 5; auto m3 = make_myclass(n); Any ideas on the best idioms to support this and get around the template argument detection problems? Is this possible? If not, can I drop the need for the make_myclass(); usage and still keep the ability to call with a default and conditionally constructs the default object based ont eh type of a metafunction? Thanks, Jesse

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Re: [Boost-users] [optional] How to make boost::optional throw if trying to access uninitialized value
by Anders Dalvander 10 Jul '09

10 Jul '09

dariomt(a)gmail.com wrote: > Thanks for the info! > > I think it is not possible to have a specific boost::assertion_failed handler > for boost::optional and other handlers for other assertion failures in other > places, right? > > That would mean that the exception I throw from the handler cannot mention the > fact that I am accessing an uninitialized value in a boost::optional. > > Maybe I should create my own my::safe_optional based on boost::optional but > checking for uninitialized access. > > Any ideas? > You could create your own functions for this: template <typename T> T const& safe_get(boost::optional<T> const& opt) { if (!opt) throw some_exception(); return *opt; } template <typename T> T& safe_get(boost::optional<T>& opt) { if (!opt) throw some_exception(); return *opt; } Regards, Anders Dalvander

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boost::asio::async_read_until composite completion condition
by Etienne Philip Pretorius 10 Jul '09

10 Jul '09

Hello List, I need to implement a completion condition for boost::asio::async_read_until where either a boost::regex is matched or boost::asio::transfer_at_least is matched. Any suggestions will be much appreciated. Kindly, Etienne

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A question on data change with lambda expression. Can any one with kindness help me?
by fmingu 10 Jul '09

10 Jul '09

I am using Dev-C++ and lambda expression.I wrote the following lines: .................. typedef std::map<int, int> IIPrimemap; IIPrimemap primemapvec; ..................... void a(){ primemapvec[1]=3; bind(&IIPrimemap::value_type::second,(*bind(static_cast<IIPrimemap::iterator (IIPrimemap::*)()> (&IIPrimemap::end),var(primemapvec))))++; return; } .................... but after the program ran, the primemapvec[1] is still 3, not 4. I am puzzled about that. How can I change primemapvec to 4 ? Can anyone with kindness help me?

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[Thread] (Newb) shared memory in multithread process
by gabe_rosser 10 Jul '09

10 Jul '09

Dear All, Apologies for my ignorance but I would be extremely grateful if someone would kindly point me in the right direction. I am trying to parallelize my Monte Carlo simulation. The bottleneck in the process is the generation of Normally distributed random numbers (using boost.random libraries), so I was hoping to substantially improve the efficiency of the process by setting 2 or 3 cores of my quad core processor to the the task of generating these numbers while one core gets on with the rest of the algorithm. The simplest way I can envisage doing this is by allocating shared memory that is simultaneously written (by rand number generating threads) and read (by main thread), with some method of ensuring that the main thread doesn't read too far ahead (perhaps locking or just a simple while loop?). Boost.thread alone doesn't seem to be enough - an attempt to do exactly this resulted in a substantial slow down (with or without locking - without may result in undefined behaviour but I tried it anyway to see if it was causing the slowdown). Do I need to investigate shared pointers? I really need some large container in shared memory which I can fill in any order, much as would be the case if I were appending to and reading from a file, but hopefully faster! Any suggestions please? Sorry for the lack of understanding! Thanks, Gabe -- View this message in context: http://www.nabble.com/-Thread--%28Newb%29-shared-memory-in-multithread-proc… Sent from the Boost - Users mailing list archive at Nabble.com.

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[Boost.Program_options] extra_parser for config files
by Marti Cuquet 10 Jul '09

10 Jul '09

Hi, I want to implement the --switch / --no-switch behavior. I defined a bool option "switch" and then I parse the command line with an extra parser that sets "switch" to "false" if it finds a --no-switch. myopts.add_options() ("switch", po::value<bool>()->implicit_value(true), "Use switch.") ; po::store (po::command_line_parser(argc, argv).options(all).extra_parser(parse_no_options).run(), vm); The extra parser is: std::pair<std::string, std::string> parse_no_options (const std::string& s, const std::string name) { if (s.find("--switch") != s.npos) { if (s.substr (2, 3) == "no-") return std::make_pair (s.substr (5), std::string("false")); else return std::make_pair (s.substr (2), std::string("true")); } return std::make_pair (std::string(), std::string()); } This works fine for the command line. I also need to implement this for the config file, but I do not know how to pass the extra parser to the parse_config_file. Is there a way? Thanks, Marti

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[Threads] Test for running thread
by Krzysiek 10 Jul '09

10 Jul '09

Hi, I use following code to test running thread. bool isRunning = !thread.timed_join ( boost::posix_time::milliseconds ( 0 ) ); Is it safe and portable? Thanks, Krzysiek

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[interprocess] Container
by Moritz 10 Jul '09

10 Jul '09

Hi, is there a possibility to put something like this into the shared_memory? class A { int a; double d; // ... boost::interprocess string s; } Thanks in advance.

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[asio] Is io_service::run() reentrant?
by bearclaw 10 Jul '09

10 Jul '09

Hi, I cannot figure out from the documentation if I can call io_service::run() from an asynchronous handler, called itsels by run() of the same io_service. If it is possible, should I call reset() or not? I tried and it seems to work under linux, but I need to call run() in a loop as it returns prematurely, even with an io_service::work(), and with/without a reset(). The reason I need to do this is because my asynchronous handlers are calling legacy code that needs to perform synchronous operations. Using synchronous socket operations while this lasts as it was suggested to handle this kind of situations would be possible, but a lot more complicated in my setup. A pseudocode example of what I am trying to do in case I'm not clear: io_service io; tcp::socket s(io); onRead2() { // We got our data, stop inner io::run invocation io.stop(); } onRead(socket& s) { s.write("request"); //we need the result of the request synchronously // I know this looks silly, but it makes more sense in my real-world // use case where the async_read is burried deep. s.async_read(&onRead2); // optional io.reset() here, has no effect. io.run(); // <-- ends prematurely } main() { <connect s > s.async_read(&onRead) io.run(); } Regards, Matthieu

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shared_ptr: struct of 3 shared_ptrs: is operator= for struct required?
by Hicham Mouline 10 Jul '09

10 Jul '09

Hello Copying structs by default means memberwise copy. If I have a struct S { shared_ptr<T1> ptr1; shared_ptr<T2> ptr2; shared_ptr<T3> ptr3; }; and i have 2 instances s1 and s2 s1=s2 will call shared_ptr<T.> 's operator=(), right? Regards,

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