The std::messages facet is probably the most vaguely defined facet in the standard library. It's assumed that this facility was built into the standard library in order to convert string literals from one locale to the other. For instance, converting the "C" locale's const char* c = "please" to a German-localized "bitte" during program execution.

This class has three public member functions, which directly correspond to three protected virtual member functions.

The public member functions are:

catalog open(const string&, const locale&) const

string_type get(catalog, int, int, const string_type&) const

void close(catalog) const

While the virtual functions are:

catalog do_open(const string&, const locale&) const

string_type do_get(catalog, int, int, const string_type&) const

void do_close(catalog) const

A couple of notes on the standard.

First, why is messages_base::catalog specified as a typedef to int? This makes sense for implementations that use catopen, but not for others. Fortunately, it's not heavily used and so only a minor irritant.

Second, by making the member functions const, it is impossible to save state in them. Thus, storing away information used in the 'open' member function for use in 'get' is impossible. This is unfortunate.

The 'open' member function in particular seems to be oddly designed. The signature seems quite peculiar. Why specify a const string& argument, for instance, instead of just const char*? Or, why specify a const locale& argument that is to be used in the 'get' member function? How, exactly, is this locale argument useful? What was the intent? It might make sense if a locale argument was associated with a given default message string in the 'open' member function, for instance. Quite murky and unclear, on reflection.

Lastly, it seems odd that messages, which explicitly require code conversion, don't use the codecvt facet. Because the messages facet has only one template parameter, it is assumed that ctype, and not codecvt, is to be used to convert between character sets.

It is implicitly assumed that the locale for the default message string in 'get' is in the "C" locale. Thus, all source code is assumed to be written in English, so translations are always from "en_US" to other, explicitly named locales.

A simple example using the GNU model of message conversion.

#include <iostream>
#include <locale>
using namespace std;

void test01()
{
  typedef messages<char>::catalog catalog;
  const char* dir =
  "/mnt/egcs/build/i686-pc-linux-gnu/libstdc++/po/share/locale";  
  const locale loc_de("de_DE");
  const messages<char>& mssg_de = use_facet<messages<char> >(loc_de); 

  catalog cat_de = mssg_de.open("libstdc++", loc_de, dir);
  string s01 = mssg_de.get(cat_de, 0, 0, "please");
  string s02 = mssg_de.get(cat_de, 0, 0, "thank you");
  cout << "please in german:" << s01 << '\n';
  cout << "thank you in german:" << s02 << '\n';
  mssg_de.close(cat_de);
}