From: Vicente J. Botet Escriba <vicente.botet@wanadoo.fr>
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