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extern/stdcxx/4.2.1/doc/stdlibug/41-2.html
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extern/stdcxx/4.2.1/doc/stdlibug/41-2.html
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Licensed to the Apache Software Foundation (ASF) under one or more
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contributor license agreements. See the NOTICE file distributed
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ownership. The ASF licenses this file to you under the Apache
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License, Version 2.0 (the License); you may not use this file
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except in compliance with the License. You may obtain a copy of
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Unless required by applicable law or agreed to in writing, software
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
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<HTML>
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<HEAD>
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<TITLE>Defining Traits and Facets for User-Defined Types</TITLE>
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<LINK REL=StyleSheet HREF="../rw.css" TYPE="text/css" TITLE="Apache stdcxx Stylesheet"></HEAD>
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<BODY BGCOLOR=#FFFFFF>
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<A HREF="41-1.html"><IMG SRC="images/bprev.gif" WIDTH=20 HEIGHT=21 ALT="Previous file" BORDER=O></A><A HREF="noframes.html"><IMG SRC="images/btop.gif" WIDTH=56 HEIGHT=21 ALT="Top of Document" BORDER=O></A><A HREF="booktoc.html"><IMG SRC="images/btoc.gif" WIDTH=56 HEIGHT=21 ALT="Contents" BORDER=O></A><A HREF="tindex.html"><IMG SRC="images/bindex.gif" WIDTH=56 HEIGHT=21 ALT="Index page" BORDER=O></A><A HREF="41-3.html"><IMG SRC="images/bnext.gif" WIDTH=25 HEIGHT=21 ALT="Next file" BORDER=O></A><DIV CLASS="DOCUMENTNAME"><B>Apache C++ Standard Library User's Guide</B></DIV>
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<H2>41.2 Defining Traits and Facets for User-Defined Types</H2>
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<A NAME="idx967"><!></A>
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<P>A user-defined type requires its own traits class, its own code conversion facet, and its own character classification facet. The traits class defines a conversion <SAMP>state_type</SAMP> and also defines operations such as copying arrays of the type and comparing arrays. The code conversion facet provides conversion to and from simple <SAMP>char</SAMP>s. The <SAMP>ctype</SAMP> facet provides character classification and manipulation routines, including the method for converting the new type to and from simple <SAMP>char</SAMP>s.</P>
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<A NAME="idx968"><!></A>
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<P>The following code shows a traits class declaration for <SAMP>Echar</SAMP>:</P>
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<UL><PRE>
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struct Etraits
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{
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typedef Echar char_type;
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typedef long int_type;
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typedef std::streamoff off_type;
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typedef std::mbstate_t state_type;
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typedef std::fpos<state_type> pos_type;
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static void assign (char_type& c1, const char_type& c2);
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static bool eq(const char_type& c1,const char_type& c2);
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static bool lt (const char_type& c1, const char_type& c2);
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static int compare (const char_type* s1, const char_type* s2,
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std::size_t n);
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static std::size_t length(const char_type *s);
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static const char_type*
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find (const char_type* s, int n, const char_type& a);
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static char_type* move (char_type* s1, const char_type* s2,
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std::size_t n);
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static char_type* copy(char_type *dst,const char_type *src,
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std::size_t n);
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static char_type* assign (char_type* s, std::size_t n,
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const char_type& a);
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static int_type not_eof(const int_type& c);
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static char_type to_char_type(const int_type& c);
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static int_type to_int_type(const char_type& c);
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static bool eq_int_type(const int_type& c1,const int_type& c2);
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static state_type get_state(pos_type pos);
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static int_type eof();
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};
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</PRE></UL>
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<P>See the <A HREF="../stdlibref/noframes.html"><I>Apache C++ Standard Library Reference Guide</I></A> section on <A HREF="../stdlibref/char-traits.html">char_traits</A> for a complete description of the member functions in a traits class.</P>
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<P>To create a new code conversion facet, you must inherit from the <SAMP>std::codecvt</SAMP> template and then override some or all the protected virtual functions. Iostreams calls the protected functions through the public interface defined for <SAMP>std::codecvt</SAMP>. You must also provide a constructor taking a single <SAMP>std::size_t</SAMP> argument, and initialize <SAMP>codecvt</SAMP> with that argument.</P>
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<A NAME="idx969"><!></A>
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<P>A code conversion facet for <SAMP>Echar</SAMP> has a declaration that looks like this:</P>
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<UL><PRE>
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class Ecodecvt : public std::codecvt<Echar,char,Estate>
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{
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public:
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explicit Ecodecvt (std::size_t refs = 0)
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: std::codecvt<Echar,char,Estate>(refs) { }
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protected:
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virtual result do_out(Estate& state, const Echar* from,
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const Echar* from_end,
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const Echar*& from_next,
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char* to, char* to_limit,
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char*& to_next) const;
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virtual result do_in(Estate& state,
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const char* from,
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const char* from_end,
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const char*& from_next,
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Echar* to, Echar* to_limit,
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Echar*& to_next) const;
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virtual result do_unshift(Estate& state,
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char* to, char* to_limit,
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char*& to_next) const;
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virtual int do_encoding() const throw();
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virtual bool do_always_noconv() const throw();
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virtual int do_length(Estate& state, const char* from,
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const char* end, size_t maxm) const;
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virtual int do_max_length() const throw();
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};
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</PRE></UL>
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<P>See <A HREF="40.html">Chapter 40</A> on defining a code conversion facet for more details. Also see the <A HREF="../stdlibref/noframes.html"><I>Apache C++ Standard Library Reference Guide</I></A> section on <A HREF="../stdlibref/codecvt.html">codecvt</A> for a complete description of member functions.</P>
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<P>To create a character classification facet, you must inherit from the <SAMP>std::ctype</SAMP> template and provide implementations for all protected virtual functions. You must also provide a constructor taking a single <SAMP>std::size_t</SAMP> argument, and initialize <SAMP>std::ctype</SAMP> with that argument.</P>
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<P>Note that the <SAMP>widen()</SAMP> functions define conversions from simple <SAMP>char</SAMP>s to the user-defined character type, and the narrow function provides conversions from the user-defined type to simple <SAMP>char</SAMP>s.</P>
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<A NAME="idx970"><!></A>
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<P>A character classification facet for <SAMP>Echar</SAMP> has a declaration that looks like this:</P>
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<UL><PRE>
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class Ectype : public std::ctype<Echar>
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{
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public:
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typedef Echar char_type;
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explicit Ectype (std::size_t refs = 0) :
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std::ctype<Echar>(refs) { } // must pass refs onto
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// the ctype constructor
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protected:
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virtual bool do_is(mask m, Echar c) const;
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virtual const Echar* do_is(
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const Echar* low,
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const Echar* high,
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mask* vec) const;
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virtual const Echar* do_scan_is(
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mask m, const Echar* low,
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const Echar* high) const;
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virtual const Echar* do_scan_not(
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mask m, const Echar* low,
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const Echar* high) const;
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virtual Echar do_toupper(Echar e) const;
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virtual const Echar* do_toupper(Echar* low,
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const Echar* high) const;
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virtual Echar do_tolower(Echar) const;
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virtual const Echar* do_tolower(Echar* low,
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const Echar* high) const;
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virtual Echar do_widen(char) const;
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virtual const char* do_widen(const char* lo,
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const char* hi,
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Echar* dest) const;
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virtual char do_narrow(Echar, char dfault) const;
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virtual const Echar* do_narrow(const Echar* lo,
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const Echar* hi,char dfault,
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char* dest) const;
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};
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</PRE></UL>
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<BR>
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