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405 lines
12 KiB
HTML
405 lines
12 KiB
HTML
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<HTML>
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<HEAD>
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<H1>
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Notes on the messages implementation.
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</H1>
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</HEAD>
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<I>
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prepared by Benjamin Kosnik (bkoz@redhat.com) on August 8, 2001
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</I>
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<P>
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<H2>
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1. Abstract
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</H2>
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<P>
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The std::messages facet implements message retrieval functionality
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equivalent to Java's java.text.MessageFormat .using either GNU gettext
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or IEEE 1003.1-200 functions.
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</P>
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<P>
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<H2>
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2. What the standard says
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</H2>
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The std::messages facet is probably the most vaguely defined facet in
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the standard library. It's assumed that this facility was built into
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the standard library in order to convert string literals from one
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locale to the other. For instance, converting the "C" locale's
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<TT>const char* c = "please"</TT> to a German-localized <TT>"bitte"</TT>
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during program execution.
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<BLOCKQUOTE>
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22.2.7.1 - Template class messages [lib.locale.messages]
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</BLOCKQUOTE>
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This class has three public member functions, which directly
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correspond to three protected virtual member functions.
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The public member functions are:
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<P>
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<TT>catalog open(const basic_string<char>&, const locale&) const</TT>
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<P>
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<TT>string_type get(catalog, int, int, const string_type&) const</TT>
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<P>
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<TT>void close(catalog) const</TT>
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<P>
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While the virtual functions are:
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<P>
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<TT>catalog do_open(const basic_string<char>&, const locale&) const</TT>
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<BLOCKQUOTE>
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<I>
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-1- Returns: A value that may be passed to get() to retrieve a
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message, from the message catalog identified by the string name
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according to an implementation-defined mapping. The result can be used
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until it is passed to close(). Returns a value less than 0 if no such
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catalog can be opened.
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</I>
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</BLOCKQUOTE>
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<P>
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<TT>string_type do_get(catalog, int, int, const string_type&) const</TT>
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<BLOCKQUOTE>
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<I>
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-3- Requires: A catalog cat obtained from open() and not yet closed.
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-4- Returns: A message identified by arguments set, msgid, and dfault,
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according to an implementation-defined mapping. If no such message can
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be found, returns dfault.
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</I>
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</BLOCKQUOTE>
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<P>
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<TT>void do_close(catalog) const</TT>
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<BLOCKQUOTE>
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<I>
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-5- Requires: A catalog cat obtained from open() and not yet closed.
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-6- Effects: Releases unspecified resources associated with cat.
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-7- Notes: The limit on such resources, if any, is implementation-defined.
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</I>
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</BLOCKQUOTE>
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<P>
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<H2>
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3. Problems with "C" messages: thread safety,
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over-specification, and assumptions.
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</H2>
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A couple of notes on the standard.
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<p>
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First, why is <TT>messages_base::catalog</TT> specified as a typedef
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to int? This makes sense for implementations that use
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<TT>catopen</TT>, but not for others. Fortunately, it's not heavily
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used and so only a minor irritant.
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<p>
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Second, by making the member functions <TT>const</TT>, it is
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impossible to save state in them. Thus, storing away information used
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in the 'open' member function for use in 'get' is impossible. This is
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unfortunate.
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<p>
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The 'open' member function in particular seems to be oddly
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designed. The signature seems quite peculiar. Why specify a <TT>const
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string& </TT> argument, for instance, instead of just <TT>const
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char*</TT>? Or, why specify a <TT>const locale&</TT> argument that is
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to be used in the 'get' member function? How, exactly, is this locale
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argument useful? What was the intent? It might make sense if a locale
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argument was associated with a given default message string in the
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'open' member function, for instance. Quite murky and unclear, on
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reflection.
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<p>
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Lastly, it seems odd that messages, which explicitly require code
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conversion, don't use the codecvt facet. Because the messages facet
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has only one template parameter, it is assumed that ctype, and not
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codecvt, is to be used to convert between character sets.
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<p>
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It is implicitly assumed that the locale for the default message
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string in 'get' is in the "C" locale. Thus, all source code is assumed
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to be written in English, so translations are always from "en_US" to
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other, explicitly named locales.
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<P>
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<H2>
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4. Design and Implementation Details
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</H2>
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This is a relatively simple class, on the face of it. The standard
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specifies very little in concrete terms, so generic implementations
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that are conforming yet do very little are the norm. Adding
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functionality that would be useful to programmers and comparable to
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Java's java.text.MessageFormat takes a bit of work, and is highly
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dependent on the capabilities of the underlying operating system.
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<P>
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Three different mechanisms have been provided, selectable via
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configure flags:
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<UL>
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<LI> generic
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<P>
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This model does very little, and is what is used by default.
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</P>
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<LI> gnu
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<P>
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The gnu model is complete and fully tested. It's based on the
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GNU gettext package, which is part of glibc. It uses the functions
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<TT>textdomain, bindtextdomain, gettext</TT>
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to implement full functionality. Creating message
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catalogs is a relatively straight-forward process and is
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lightly documented below, and fully documented in gettext's
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distributed documentation.
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</P>
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<LI> ieee_1003.1-200x
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<P>
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This is a complete, though untested, implementation based on
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the IEEE standard. The functions
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<TT>catopen, catgets, catclose</TT>
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are used to retrieve locale-specific messages given the
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appropriate message catalogs that have been constructed for
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their use. Note, the script <TT> po2msg.sed</TT> that is part
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of the gettext distribution can convert gettext catalogs into
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catalogs that <TT>catopen</TT> can use.
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</P>
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</UL>
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<P>
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A new, standards-conformant non-virtual member function signature was
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added for 'open' so that a directory could be specified with a given
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message catalog. This simplifies calling conventions for the gnu
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model.
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<P>
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The rest of this document discusses details of the GNU model.
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<P>
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The messages facet, because it is retrieving and converting between
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characters sets, depends on the ctype and perhaps the codecvt facet in
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a given locale. In addition, underlying "C" library locale support is
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necessary for more than just the <TT>LC_MESSAGES</TT> mask:
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<TT>LC_CTYPE</TT> is also necessary. To avoid any unpleasantness, all
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bits of the "C" mask (ie <TT>LC_ALL</TT>) are set before retrieving
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messages.
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<p>
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Making the message catalogs can be initially tricky, but become quite
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simple with practice. For complete info, see the gettext
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documentation. Here's an idea of what is required:
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<UL>
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<LI > Make a source file with the required string literals
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that need to be translated. See
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<TT>intl/string_literals.cc</TT> for an example.
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<p>
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<LI> Make initial catalog (see "4 Making the PO Template File"
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from the gettext docs).
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<p>
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<TT> xgettext --c++ --debug string_literals.cc -o libstdc++.pot </TT>
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<p>
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<LI> Make language and country-specific locale catalogs.
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<p>
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<TT>cp libstdc++.pot fr_FR.po</TT>
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<p>
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<TT>cp libstdc++.pot de_DE.po</TT>
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<p>
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<LI> Edit localized catalogs in emacs so that strings are
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translated.
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<p>
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<TT>emacs fr_FR.po</TT>
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<P>
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<LI> Make the binary mo files.
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<p>
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<TT>msgfmt fr_FR.po -o fr_FR.mo</TT>
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<p>
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<TT>msgfmt de_DE.po -o de_DE.mo</TT>
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<P>
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<LI> Copy the binary files into the correct directory structure.
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<p>
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<TT>cp fr_FR.mo (dir)/fr_FR/LC_MESSAGES/libstdc++-v3.mo</TT>
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<p>
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<TT>cp de_DE.mo (dir)/de_DE/LC_MESSAGES/libstdc++-v3.mo</TT>
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<P>
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<LI> Use the new message catalogs.
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<p>
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<TT>locale loc_de("de_DE");</TT>
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<p>
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<TT>
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use_facet<messages<char> >(loc_de).open("libstdc++", locale(), dir);
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</TT>
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</UL>
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<P>
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<H2>
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5. Examples
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</H2>
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<UL>
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<LI> message converting, simple example using the GNU model.
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<pre>
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#include <locale>
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void test01()
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{
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using namespace std;
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typedef std::messages<char>::catalog catalog;
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// Set to the root directory of the libstdc++.mo catalogs.
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const char* dir = LOCALEDIR;
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locale loc_de("de_DE");
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// Cache the messages facet.
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const messages<char>& mssg_de = use_facet<messages<char> >(loc_de);
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// Check German (de_DE) locale.
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catalog cat_de = mssg_de.open("libstdc++", loc_c, dir);
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string s01 = mssg_de.get(cat_de, 0, 0, "please");
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string s02 = mssg_de.get(cat_de, 0, 0, "thank you");
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// s01 == "bitte"
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// s02 == "danke"
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mssg_de.close(cat_de);
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}
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</pre>
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</UL>
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More information can be found in the following testcases:
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<UL>
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<LI> testsuite/22_locale/messages.cc
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<LI> testsuite/22_locale/messages_byname.cc
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<LI> testsuite/22_locale/messages_char_members.cc
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</UL>
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<P>
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<H2>
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6. Unresolved Issues
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</H2>
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<UL>
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<LI> Things that are sketchy, or remain unimplemented:
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<UL>
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<LI>_M_convert_from_char, _M_convert_to_char are in
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flux, depending on how the library ends up doing
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character set conversions. It might not be possible to
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do a real character set based conversion, due to the
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fact that the template parameter for messages is not
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enough to instantiate the codecvt facet (1 supplied,
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need at least 2 but would prefer 3).
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<LI> There are issues with gettext needing the global
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locale set to extract a message. This dependence on
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the global locale makes the current "gnu" model non
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MT-safe. Future versions of glibc, ie glibc 2.3.x will
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fix this, and the C++ library bits are already in
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place.
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</UL>
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<p>
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<LI> Development versions of the GNU "C" library, glibc 2.3 will allow
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a more efficient, MT implementation of std::messages, and will
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allow the removal of the _M_name_messages data member. If this
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is done, it will change the library ABI. The C++ parts to
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support glibc 2.3 have already been coded, but are not in use:
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once this version of the "C" library is released, the marked
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parts of the messages implementation can be switched over to
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the new "C" library functionality.
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<p>
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<LI> At some point in the near future, std::numpunct will probably use
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std::messages facilities to implement truename/falename
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correctly. This is currently not done, but entries in
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libstdc++.pot have already been made for "true" and "false"
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string literals, so all that remains is the std::numpunct
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coding and the configure/make hassles to make the installed
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library search its own catalog. Currently the libstdc++.mo
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catalog is only searched for the testsuite cases involving
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messages members.
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<p>
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<LI> The following member functions:
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<p>
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<TT>
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catalog
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open(const basic_string<char>& __s, const locale& __loc) const
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</TT>
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<p>
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<TT>
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catalog
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open(const basic_string<char>&, const locale&, const char*) const;
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</TT>
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<p>
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Don't actually return a "value less than 0 if no such catalog
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can be opened" as required by the standard in the "gnu"
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model. As of this writing, it is unknown how to query to see
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if a specified message catalog exists using the gettext
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package.
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</UL>
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<P>
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<H2>
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7. Acknowledgments
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</H2>
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Ulrich Drepper for the character set explanations, gettext details,
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and patient answering of late-night questions, Tom Tromey for the java details.
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<P>
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<H2>
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8. Bibliography / Referenced Documents
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</H2>
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Drepper, Ulrich, GNU libc (glibc) 2.2 manual. In particular, Chapters
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"7 Locales and Internationalization"
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<P>
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Drepper, Ulrich, Thread-Aware Locale Model, A proposal. This is a
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draft document describing the design of glibc 2.3 MT locale
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functionality.
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<P>
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Drepper, Ulrich, Numerous, late-night email correspondence
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<P>
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ISO/IEC 9899:1999 Programming languages - C
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<P>
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ISO/IEC 14882:1998 Programming languages - C++
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<P>
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Java 2 Platform, Standard Edition, v 1.3.1 API Specification. In
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particular, java.util.Properties, java.text.MessageFormat,
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java.util.Locale, java.util.ResourceBundle.
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http://java.sun.com/j2se/1.3/docs/api
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<P>
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System Interface Definitions, Issue 7 (IEEE Std. 1003.1-200x)
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The Open Group/The Institute of Electrical and Electronics Engineers, Inc.
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In particular see lines 5268-5427.
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http://www.opennc.org/austin/docreg.html
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<P> GNU gettext tools, version 0.10.38, Native Language Support
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|
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Library and Tools.
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http://sources.redhat.com/gettext
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<P>
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Langer, Angelika and Klaus Kreft, Standard C++ IOStreams and Locales,
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Advanced Programmer's Guide and Reference, Addison Wesley Longman,
|
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|
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Inc. 2000. See page 725, Internationalized Messages.
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<P>
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Stroustrup, Bjarne, Appendix D, The C++ Programming Language, Special Edition, Addison Wesley, Inc. 2000
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