25 Localization library [localization]

25.4 Standard locale categories [locale.categories]

25.4.7 The message retrieval category [category.messages]

25.4.7.1 Class template messages [locale.messages]

namespace std {
  class messages_base {
  public:
    using catalog = unspecified signed integer type;
  };

  template<class charT>
    class messages : public locale::facet, public messages_base {
    public:
      using char_type   = charT;
      using string_type = basic_string<charT>;

      explicit messages(size_t refs = 0);

      catalog open(const basic_string<char>& fn, const locale&) const;
      string_type get(catalog c, int set, int msgid,
                       const string_type& dfault) const;
      void close(catalog c) const;

      static locale::id id;

    protected:
      ~messages();
      virtual catalog do_open(const basic_string<char>&, const locale&) const;
      virtual string_type do_get(catalog, int set, int msgid,
                                 const string_type& dfault) const;
      virtual void do_close(catalog) const;
    };
}
Values of type messages_­base​::​catalog usable as arguments to members get and close can be obtained only by calling member open.

25.4.7.1.1 messages members [locale.messages.members]

catalog open(const basic_string<char>& name, const locale& loc) const;
Returns: do_­open(name, loc).
string_type get(catalog cat, int set, int msgid, const string_type& dfault) const;
Returns: do_­get(cat, set, msgid, dfault).
void close(catalog cat) const;
Effects: Calls do_­close(cat).

25.4.7.1.2 messages virtual functions [locale.messages.virtuals]

catalog do_open(const basic_string<char>& name, const locale& loc) const;
Returns: A value that may be passed to get() to retrieve a message from the message catalog identified by the string name according to an implementation-defined mapping.
The result can be used until it is passed to close().
Returns a value less than 0 if no such catalog can be opened.
Remarks: The locale argument loc is used for character set code conversion when retrieving messages, if needed.
string_type do_get(catalog cat, int set, int msgid, const string_type& dfault) const;
Requires: cat shall be a catalog obtained from open() and not yet closed.
Returns: A message identified by arguments set, msgid, and dfault, according to an implementation-defined mapping.
If no such message can be found, returns dfault.
void do_close(catalog cat) const;
Requires: cat shall be a catalog obtained from open() and not yet closed.
Effects: Releases unspecified resources associated with cat.
Remarks: The limit on such resources, if any, is implementation-defined.