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| <H2>41.2 Defining Traits and Facets for User-Defined Types</H2> |
| <A NAME="idx967"><!></A> |
| <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> |
| <A NAME="idx968"><!></A> |
| <P>The following code shows a traits class declaration for <SAMP>Echar</SAMP>:</P> |
| |
| <UL><PRE> |
| struct Etraits |
| { |
| typedef Echar char_type; |
| typedef long int_type; |
| |
| typedef std::streamoff off_type; |
| typedef std::mbstate_t state_type; |
| typedef std::fpos<state_type> pos_type; |
| |
| static void assign (char_type& c1, const char_type& c2); |
| static bool eq(const char_type& c1,const char_type& c2); |
| static bool lt (const char_type& c1, const char_type& c2); |
| static int compare (const char_type* s1, const char_type* s2, |
| std::size_t n); |
| static std::size_t length(const char_type *s); |
| static const char_type* |
| find (const char_type* s, int n, const char_type& a); |
| static char_type* move (char_type* s1, const char_type* s2, |
| std::size_t n); |
| static char_type* copy(char_type *dst,const char_type *src, |
| std::size_t n); |
| static char_type* assign (char_type* s, std::size_t n, |
| const char_type& a); |
| static int_type not_eof(const int_type& c); |
| static char_type to_char_type(const int_type& c); |
| static int_type to_int_type(const char_type& c); |
| static bool eq_int_type(const int_type& c1,const int_type& c2); |
| static state_type get_state(pos_type pos); |
| static int_type eof(); |
| }; |
| </PRE></UL> |
| <P>See the <A HREF="../stdlibref/noframes.html"><I>C++ Standard Library Module 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> |
| <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> |
| <A NAME="idx969"><!></A> |
| <P>A code conversion facet for <SAMP>Echar</SAMP> has a declaration that looks like this:</P> |
| |
| <UL><PRE> |
| class Ecodecvt : public std::codecvt<Echar,char,Estate> |
| { |
| public: |
| explicit Ecodecvt (std::size_t refs = 0) |
| : std::codecvt<Echar,char,Estate>(refs) { } |
| |
| protected: |
| |
| virtual result do_out(Estate& state, const Echar* from, |
| const Echar* from_end, |
| const Echar*& from_next, |
| char* to, char* to_limit, |
| char*& to_next) const; |
| virtual result do_in(Estate& state, |
| const char* from, |
| const char* from_end, |
| const char*& from_next, |
| Echar* to, Echar* to_limit, |
| Echar*& to_next) const; |
| virtual result do_unshift(Estate& state, |
| char* to, char* to_limit, |
| char*& to_next) const; |
| virtual int do_encoding() const throw(); |
| virtual bool do_always_noconv() const throw(); |
| virtual int do_length(Estate& state, const char* from, |
| const char* end, size_t maxm) const; |
| virtual int do_max_length() const throw(); |
| }; |
| </PRE></UL> |
| <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>C++ Standard Library Module Reference Guide</I></A> section on <A HREF="../stdlibref/codecvt.html">codecvt</A> for a complete description of member functions.</P> |
| <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> |
| <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> |
| <A NAME="idx970"><!></A> |
| <P>A character classification facet for <SAMP>Echar</SAMP> has a declaration that looks like this:</P> |
| |
| <UL><PRE> |
| class Ectype : public std::ctype<Echar> |
| { |
| public: |
| typedef Echar char_type; |
| explicit Ectype (std::size_t refs = 0) : |
| std::ctype<Echar>(refs) { } // must pass refs onto |
| // the ctype constructor |
| |
| protected: |
| |
| virtual bool do_is(mask m, Echar c) const; |
| virtual const Echar* do_is( |
| const Echar* low, |
| const Echar* high, |
| mask* vec) const; |
| virtual const Echar* do_scan_is( |
| mask m, const Echar* low, |
| const Echar* high) const; |
| virtual const Echar* do_scan_not( |
| mask m, const Echar* low, |
| const Echar* high) const; |
| virtual Echar do_toupper(Echar e) const; |
| virtual const Echar* do_toupper(Echar* low, |
| const Echar* high) const; |
| virtual Echar do_tolower(Echar) const; |
| virtual const Echar* do_tolower(Echar* low, |
| const Echar* high) const; |
| virtual Echar do_widen(char) const; |
| virtual const char* do_widen(const char* lo, |
| const char* hi, |
| Echar* dest) const; |
| virtual char do_narrow(Echar, char dfault) const; |
| virtual const Echar* do_narrow(const Echar* lo, |
| const Echar* hi,char dfault, |
| char* dest) const; |
| |
| }; |
| </PRE></UL> |
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