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<TITLE>Modifying a Standard Facet's Behavior</TITLE>
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<H2>25.5 Modifying a Standard Facet's Behavior</H2>
<A NAME="idx577"><!></A>
<P>At times you may need to replace a facet object in a locale by another kind of facet object. In the following example, let us derive from one of the standard facet classes, <B><I><A HREF="../stdlibref/numpunct.html">numpunct</A></I></B>, and create a locale object in which the standard <SAMP>std::numpunct</SAMP> facet object is replaced by an instance of our new, derived facet class. </P>
<P>Here is the problem we want to solve. When you print Boolean values, you can choose between the numeric representation of the values <SAMP>"true"</SAMP> and <SAMP>"false"</SAMP>, or their alphanumeric representation. </P>

<UL><PRE>
int main(int argc, char** argv)
{
  bool any_arguments = (argc &gt; 1);                           //1
  std::cout.setf(ios_base::boolalpha);                       //2
  std::cout &lt;&lt; any_arguments &lt;&lt; '\n';                        //3
  // ...
}
</PRE></UL>
<TABLE CELLPADDING="3">

<TR VALIGN="top"><TD><SAMP>//1</SAMP></TD><TD>A variable of type <SAMP>bool</SAMP> is defined. Its initial value is the boolean value of the logical expression <SAMP>(argc &gt; 1)</SAMP>, so the variable <SAMP>any_arguments</SAMP> contains the information, whether the program was called with or without arguments.
<TR VALIGN="top"><TD><SAMP>//2</SAMP></TD><TD>The format flag <SAMP>std::ios_base::boolalpha</SAMP> is set in the predefined output stream <SAMP>cout</SAMP>. The effect is that the string representation of boolean values is printed, instead of their numerical representation <SAMP>0</SAMP> or <SAMP>1</SAMP>, which is the default representation.
<TR VALIGN="top"><TD><SAMP>//3</SAMP></TD><TD>Here either the string <SAMP>"true"</SAMP> or the string <SAMP>"false"</SAMP> are printed.
</TABLE>
<P>Of course, the string representation depends on the language. Hence, the alphanumeric representation of boolean values is provided by a locale. It is the <SAMP>std::numpunct</SAMP> facet of a locale that describes the cultural conventions for numerical formatting. It contains services that return the string representations of the boolean values <SAMP>true</SAMP> and <SAMP>false</SAMP>.</P>
<A NAME="idx578"><!></A>
<P>This is the interface of facet <B><I><A HREF="../stdlibref/numpunct.html">numpunct</A></I></B>:</P>

<UL><PRE>
namespace std{
  template &lt;class charT&gt;
    class numpunct : public locale::facet {
      public:
        typedef charT                    char_type;
        typedef basic_string&lt;charT&gt; string_type;
        explicit numpunct(size_t refs = 0);
        char_type decimal_point()   const;
        char_type thousands_sep()   const;
        string    grouping()        const;
        string_type  <I>truename</I>()     const;
        string_type  <I>falsename</I>()    const;
        static locale::id id;
        // ...
    };
}
</PRE></UL>
<A NAME="idx579"><!></A>
<P>Now let us replace this facet. To make it more exciting, let's use not only a different language, but also different words for <SAMP>true</SAMP> and <SAMP>false</SAMP>, such as <SAMP>Yes!</SAMP> and <SAMP>No!</SAMP>. For just using another language, we would not need a new facet; we would simply use the right native locale, and it would contain the right facet.</P>

<UL><PRE>
template &lt;class charT&gt;                                         //1
class change_bool_names
    : public std::numpunct_byname&lt;charT&gt;                       //2
{
  public:
    typedef std::basic_string&lt;charT&gt; string_type;
    explicit change_bool_names (const char* name,              //3
                                const charT* t,
                                const charT* f, size_t refs=0)
      : std::numpunct_byname&lt;charT&gt; (name,refs),
        true_string(t), false_string(f) { }
   protected:
     virtual string_type do_truename() const {                  //4
       return true_string;
     }
     virtual string_type do_falsename() const {
       return false_string;
     }
   private:
     string_type true_string, false_string;
};
</PRE></UL>
<TABLE CELLPADDING="3">

<TR VALIGN="top"><TD><SAMP>//1</SAMP></TD><TD>The new facet is a class template that takes the character type as a template parameter.
<TR VALIGN="top"><TD><SAMP>//2</SAMP></TD><TD>The new facet is derived from the <SAMP>std::numpunct_byname&lt;charT&gt;</SAMP> facet.
<BR><BR>The byname facets read the respective locale information from the external representation of a C locale. The name provided to construct a byname<I> </I>facet is the name of a locale, as you would use it in a call to <SAMP>setlocale()</SAMP>.
<TR VALIGN="top"><TD><SAMP>//3</SAMP></TD><TD>A constructor is provided that takes a locale name and the new values we want to display for the alpha versions of <SAMP>true</SAMP> and <SAMP>false</SAMP>. The fourth parameter, <SAMP>refs</SAMP>, controls the facet's lifetime, as described in an earlier section.
<TR VALIGN="top"><TD><SAMP>//4</SAMP></TD><TD>The virtual member functions <SAMP>do_truename()</SAMP> and <SAMP>do_falsename()</SAMP> are reimplemented. They are called by the public member functions <SAMP>truename()</SAMP> and <SAMP>falsename()</SAMP>. See the <A HREF="../stdlibref/noframes.html"><I>Apache C++ Standard Library Reference Guide</I></A> for further details.
</TABLE>
<P>Now let's create a German locale in which the <SAMP>std::numpunct&lt;char&gt;</SAMP> facet has been replaced by an object of our new derived facet type, as shown in <A HREF="25-5.html#Figure&nbsp;11">Figure&nbsp;11</A>:</P>
<A NAME="idx580"><!></A>
<H4><A NAME="Figure&nbsp;11">Figure&nbsp;11: Replacing the std::numpunct&lt;char&gt; facet object</A></H4>

<P><IMG SRC="images/locfig12.gif" WIDTH=476 HEIGHT=311></P>
<A NAME="idx581"><!></A>
<P>The code looks like this:</P>

<UL><PRE>
void main(int argc, char** argv)
{
  std::locale loc(std::locale("de_DE"),                        //1
    new change_bool_names&lt;char&gt;("de_DE","Ja.","Nein."));       //2
  std::cout.imbue(loc);                                        //3
  std::cout &lt;&lt; "Argumente vorhanden? "  //Any arguments?
            &lt;&lt; std::boolalpha &lt;&lt; (argc &gt; 1) &lt;&lt; std::endl;      //4
}
</PRE></UL>
<TABLE CELLPADDING="3">

<TR VALIGN="top"><TD><SAMP>//1</SAMP></TD><TD>A locale object is constructed with an instance of the new facet class. The locale object has all facet objects from a German locale object, except that the new facet object <SAMP>change_bool_names</SAMP> substitutes for the <SAMP>std::numpunct</SAMP> facet object<SAMP>.</SAMP>
<TR VALIGN="top"><TD><SAMP>//2</SAMP></TD><TD>The new facet object takes all information from a German <SAMP>numpunct</SAMP> facet object, and replaces the default native names for <SAMP>true</SAMP> and <SAMP>false</SAMP> with the provided strings <SAMP>"Ja."</SAMP> ("Yes.") and <SAMP>"Nein."</SAMP> ("No.").
<TR VALIGN="top"><TD><SAMP>//3</SAMP></TD><TD>The standard output stream <SAMP>std::cout</SAMP> is imbued with the newly created locale.
<TR VALIGN="top"><TD><SAMP>//4</SAMP></TD><TD>The expression <SAMP>(argc &gt; 1)</SAMP> yields a boolean value, which indicates whether the program was called with arguments. This boolean value's alphanumeric representation is printed to the standard output stream. The output might be:
<BR><BR><SAMP>Argument vorhanden? Ja.</SAMP>
</TABLE>

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