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Subject: Re: [boost] [iostreams][io_state_savers] Request for interest in some ios_base basic utilities
From: Artyom Beilis (artyomtnk_at_[hidden])
Date: 2011-11-10 17:18:01

> From: Vicente J. Botet Escriba <vicente.botet_at_[hidden]> >> >>     try { >>       foo const&f =use_facet<foo>(f); >>       // do required >>     } >>     catch(std::bad_cast const&) { >>       // do default >>     } > I'm missing surely something. Recall, I'm a beginner. How do you ensure > that any stream has associated a locale that contains your facet? You don't. For example     std::ostream &operator<<(std::ostream &out,my_time const &tm)     {          std::locale l = out.getloc();          if(std::has_facet<my_time_facet>(loc)) {             my_time_facet const &f=std::use_facet<my_time_facet>(loc);             f.print_time(tm,out);          }          else { // user had not installed anything interesting so fallback for something.             my_time::print_iso_time(tm,out);          }          return out;     } It is not YOUR job to install facets. Ideally. std::locale should include "Classic" version of my_time_facet. But without it you can do what I had shown above. >> No actually, it is not. facets are not generated frequently. >> locale is cheap to copy because facets are reference counted >> so you create facet once and use it for long time. For example >> messages facet can load dictionaries convert their encoding, >> it is by no means cheap. But you create it once and then >> reuse it. >> >> > How do you reach to reuse them. > > For example if we use a string stream we will need to add the facet. No, you don't. You don't add facet unless you need to bring some locale specific data. > I guess that this operation will not take too much time, and in any case I > expect it to load a dictionary that has already been loaded. > So I > suspect that the facet should only contain a key identifying the > dictionary or a reference to it. > > Vicente > The process works this way (with Boost.Locale and even with std::locale. Boost Locale. ------------ std::locale l=generator(""); // load dictionaries creates facets etc std::cout.imbue(l); l << as::time << time(0); With std::locale ------------------ std::locale l(""); // load dictionaries create facets etc. std::cout.imbue(l); std::use_facet<std::time_put>(std::cout.getloc()).put(std::cout,std::cout,' ',localtime(time),'c',0); All the data created when you create locale object. You don't alter it afterwards. Now as C++ standard provided by default all facets even if l is C locale it will have facet you need. But from user point of view you can fallback to something if custom locale facet is not installed. I hope it is clear or we talk about different things Artyom

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