567 lines
16 KiB
XML
567 lines
16 KiB
XML
<section xmlns="http://docbook.org/ns/docbook" version="5.0"
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xml:id="manual.localization.facet.messages" xreflabel="Messages">
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<?dbhtml filename="messages.html"?>
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<info><title>messages</title>
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<keywordset>
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<keyword>
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ISO C++
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</keyword>
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<keyword>
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messages
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</keyword>
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</keywordset>
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</info>
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<para>
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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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</para>
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<section xml:id="facet.messages.req"><info><title>Requirements</title></info>
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<para>
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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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<code>const char* c = "please"</code> to a German-localized <code>"bitte"</code>
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during program execution.
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</para>
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<blockquote>
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<para>
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22.2.7.1 - Template class messages [lib.locale.messages]
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</para>
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</blockquote>
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<para>
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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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</para>
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<para>
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The public member functions are:
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</para>
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<para>
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<code>catalog open(const string&, const locale&) const</code>
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</para>
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<para>
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<code>string_type get(catalog, int, int, const string_type&) const</code>
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</para>
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<para>
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<code>void close(catalog) const</code>
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</para>
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<para>
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While the virtual functions are:
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</para>
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<para>
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<code>catalog do_open(const string&, const locale&) const</code>
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</para>
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<blockquote>
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<para>
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<emphasis>
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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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</emphasis>
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</para>
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</blockquote>
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<para>
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<code>string_type do_get(catalog, int, int, const string_type&) const</code>
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</para>
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<blockquote>
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<para>
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<emphasis>
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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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</emphasis>
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</para>
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</blockquote>
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<para>
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<code>void do_close(catalog) const</code>
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</para>
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<blockquote>
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<para>
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<emphasis>
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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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</emphasis>
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</para>
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</blockquote>
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</section>
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<section xml:id="facet.messages.design"><info><title>Design</title></info>
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<para>
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A couple of notes on the standard.
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</para>
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<para>
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First, why is <code>messages_base::catalog</code> specified as a typedef
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to int? This makes sense for implementations that use
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<code>catopen</code>, but not for others. Fortunately, it's not heavily
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used and so only a minor irritant.
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</para>
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<para>
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Second, by making the member functions <code>const</code>, 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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</para>
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<para>
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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 <code>const
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string& </code> argument, for instance, instead of just <code>const
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char*</code>? Or, why specify a <code>const locale&</code> 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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</para>
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<para>
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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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</para>
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<para>
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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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</para>
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</section>
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<section xml:id="facet.messages.impl"><info><title>Implementation</title></info>
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<section xml:id="messages.impl.models"><info><title>Models</title></info>
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<para>
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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
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implementations that are conforming yet do very little are the
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norm. Adding functionality that would be useful to programmers and
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comparable to Java's java.text.MessageFormat takes a bit of work,
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and is highly dependent on the capabilities of the underlying
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operating system.
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</para>
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<para>
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Three different mechanisms have been provided, selectable via
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configure flags:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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generic
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</para>
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<para>
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This model does very little, and is what is used by default.
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</para>
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</listitem>
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<listitem>
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<para>
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gnu
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</para>
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<para>
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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
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functions <code>textdomain, bindtextdomain, gettext</code> to
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implement full functionality. Creating message catalogs is a
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relatively straight-forward process and is lightly documented
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below, and fully documented in gettext's distributed
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documentation.
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</para>
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</listitem>
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<listitem>
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<para>
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ieee_1003.1-200x
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</para>
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<para>
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This is a complete, though untested, implementation based on
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the IEEE standard. The functions <code>catopen, catgets,
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catclose</code> are used to retrieve locale-specific messages
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given the appropriate message catalogs that have been
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constructed for their use. Note, the script <code>
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po2msg.sed</code> that is part of the gettext distribution can
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convert gettext catalogs into catalogs that
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<code>catopen</code> can use.
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</para>
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</listitem>
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</itemizedlist>
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<para>
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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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</para>
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</section>
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<section xml:id="messages.impl.gnu"><info><title>The GNU Model</title></info>
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<para>
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The messages facet, because it is retrieving and converting
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between characters sets, depends on the ctype and perhaps the
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codecvt facet in a given locale. In addition, underlying "C"
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library locale support is necessary for more than just the
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<code>LC_MESSAGES</code> mask: <code>LC_CTYPE</code> is also
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necessary. To avoid any unpleasantness, all bits of the "C" mask
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(i.e. <code>LC_ALL</code>) are set before retrieving messages.
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</para>
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<para>
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Making the message catalogs can be initially tricky, but become
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quite 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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</para>
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<itemizedlist>
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<listitem>
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<para>
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Make a source file with the required string literals that need
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to be translated. See <code>intl/string_literals.cc</code> for
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an example.
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</para>
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</listitem>
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<listitem>
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<para>
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Make initial catalog (see "4 Making the PO Template File" from
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the gettext docs).</para>
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<para>
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<code> xgettext --c++ --debug string_literals.cc -o libstdc++.pot </code>
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</para>
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</listitem>
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<listitem>
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<para>Make language and country-specific locale catalogs.</para>
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<para>
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<code>cp libstdc++.pot fr_FR.po</code>
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</para>
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<para>
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<code>cp libstdc++.pot de_DE.po</code>
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</para>
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</listitem>
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<listitem>
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<para>
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Edit localized catalogs in emacs so that strings are
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translated.
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</para>
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<para>
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<code>emacs fr_FR.po</code>
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</para>
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</listitem>
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<listitem>
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<para>Make the binary mo files.</para>
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<para>
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<code>msgfmt fr_FR.po -o fr_FR.mo</code>
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</para>
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<para>
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<code>msgfmt de_DE.po -o de_DE.mo</code>
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</para>
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</listitem>
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<listitem>
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<para>Copy the binary files into the correct directory structure.</para>
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<para>
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<code>cp fr_FR.mo (dir)/fr_FR/LC_MESSAGES/libstdc++.mo</code>
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</para>
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<para>
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<code>cp de_DE.mo (dir)/de_DE/LC_MESSAGES/libstdc++.mo</code>
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</para>
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</listitem>
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<listitem>
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<para>Use the new message catalogs.</para>
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<para>
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<code>locale loc_de("de_DE");</code>
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</para>
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<para>
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<code>
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use_facet<messages<char> >(loc_de).open("libstdc++", locale(), dir);
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</code>
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</para>
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</listitem>
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</itemizedlist>
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</section>
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</section>
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<section xml:id="facet.messages.use"><info><title>Use</title></info>
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<para>
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A simple example using the GNU model of message conversion.
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</para>
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<programlisting>
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#include <iostream>
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#include <locale>
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using namespace std;
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void test01()
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{
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typedef messages<char>::catalog catalog;
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const char* dir =
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"/mnt/egcs/build/i686-pc-linux-gnu/libstdc++/po/share/locale";
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const locale loc_de("de_DE");
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const messages<char>& mssg_de = use_facet<messages<char> >(loc_de);
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catalog cat_de = mssg_de.open("libstdc++", loc_de, 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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cout << "please in german:" << s01 << '\n';
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cout << "thank you in german:" << s02 << '\n';
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mssg_de.close(cat_de);
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}
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</programlisting>
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</section>
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<section xml:id="facet.messages.future"><info><title>Future</title></info>
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<itemizedlist>
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<listitem>
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<para>
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Things that are sketchy, or remain unimplemented:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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_M_convert_from_char, _M_convert_to_char are in flux,
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depending on how the library ends up doing character set
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conversions. It might not be possible to do a real character
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set based conversion, due to the fact that the template
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parameter for messages is not enough to instantiate the
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codecvt facet (1 supplied, need at least 2 but would prefer
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3).
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</para>
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</listitem>
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<listitem>
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<para>
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There are issues with gettext needing the global locale set
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to extract a message. This dependence on the global locale
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makes the current "gnu" model non MT-safe. Future versions
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of glibc, i.e. glibc 2.3.x will fix this, and the C++ library
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bits are already in place.
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</para>
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</listitem>
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</itemizedlist>
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</listitem>
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<listitem>
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<para>
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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 is
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done, it will change the library ABI. The C++ parts to support
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glibc 2.3 have already been coded, but are not in use: once this
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version of the "C" library is released, the marked parts of the
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messages implementation can be switched over to the new "C"
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library functionality.
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</para>
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</listitem>
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<listitem>
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<para>
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At some point in the near future, std::numpunct will probably use
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std::messages facilities to implement truename/falsename
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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" string
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literals, so all that remains is the std::numpunct coding and the
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configure/make hassles to make the installed library search its
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own catalog. Currently the libstdc++.mo catalog is only searched
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for the testsuite cases involving messages members.
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</para>
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</listitem>
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<listitem>
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<para> The following member functions:</para>
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<para>
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<code>
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catalog
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open(const basic_string<char>& __s, const locale& __loc) const
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</code>
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</para>
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<para>
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<code>
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catalog
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open(const basic_string<char>&, const locale&, const char*) const;
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</code>
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</para>
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<para>
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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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</para>
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</listitem>
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</itemizedlist>
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</section>
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<bibliography xml:id="facet.messages.biblio"><info><title>Bibliography</title></info>
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<biblioentry>
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<citetitle>
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The GNU C Library
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</citetitle>
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<author><personname><surname>McGrath</surname><firstname>Roland</firstname></personname></author>
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<author><personname><surname>Drepper</surname><firstname>Ulrich</firstname></personname></author>
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<copyright>
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<year>2007</year>
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<holder>FSF</holder>
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</copyright>
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<pagenums>Chapters 6 Character Set Handling, and 7 Locales and Internationalization
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</pagenums>
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</biblioentry>
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<biblioentry>
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<citetitle>
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Correspondence
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</citetitle>
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<author><personname><surname>Drepper</surname><firstname>Ulrich</firstname></personname></author>
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<copyright>
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<year>2002</year>
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<holder/>
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</copyright>
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</biblioentry>
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<biblioentry>
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<citetitle>
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ISO/IEC 14882:1998 Programming languages - C++
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</citetitle>
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<copyright>
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<year>1998</year>
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<holder>ISO</holder>
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</copyright>
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</biblioentry>
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<biblioentry>
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<citetitle>
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ISO/IEC 9899:1999 Programming languages - C
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</citetitle>
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<copyright>
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<year>1999</year>
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<holder>ISO</holder>
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</copyright>
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</biblioentry>
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<biblioentry>
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<biblioid xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.opengroup.org/austin" class="uri">
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</biblioid>
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<citetitle>
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System Interface Definitions, Issue 7 (IEEE Std. 1003.1-2008)
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</citetitle>
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<copyright>
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<year>2008</year>
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<holder>
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The Open Group/The Institute of Electrical and Electronics
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Engineers, Inc.
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</holder>
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</copyright>
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</biblioentry>
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|
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<biblioentry>
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<citetitle>
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The C++ Programming Language, Special Edition
|
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</citetitle>
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<author><personname><surname>Stroustrup</surname><firstname>Bjarne</firstname></personname></author>
|
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<copyright>
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<year>2000</year>
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<holder>Addison Wesley, Inc.</holder>
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</copyright>
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<pagenums>Appendix D</pagenums>
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<publisher>
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<publishername>
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Addison Wesley
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</publishername>
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</publisher>
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</biblioentry>
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<biblioentry>
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<citetitle>
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Standard C++ IOStreams and Locales
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</citetitle>
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<subtitle>
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Advanced Programmer's Guide and Reference
|
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</subtitle>
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<author><personname><surname>Langer</surname><firstname>Angelika</firstname></personname></author>
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<author><personname><surname>Kreft</surname><firstname>Klaus</firstname></personname></author>
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<copyright>
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<year>2000</year>
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<holder>Addison Wesley Longman, Inc.</holder>
|
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</copyright>
|
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<publisher>
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<publishername>
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Addison Wesley Longman
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</publishername>
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</publisher>
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</biblioentry>
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<biblioentry>
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<biblioid xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://java.sun.com/reference/api/index.html" class="uri">
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</biblioid>
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<citetitle>
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API Specifications, Java Platform
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</citetitle>
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<pagenums>java.util.Properties, java.text.MessageFormat,
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java.util.Locale, java.util.ResourceBundle
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</pagenums>
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</biblioentry>
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<biblioentry>
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<biblioid xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.gnu.org/software/gettext/" class="uri">
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</biblioid>
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<citetitle>
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GNU gettext tools, version 0.10.38, Native Language Support
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Library and Tools.
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</citetitle>
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</biblioentry>
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</bibliography>
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</section>
|