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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>C++ Standard Library Module 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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	<HTML>
	<HEAD>
	<TITLE>Modifying a Standard Facet's Behavior</TITLE>
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	<BODY BGCOLOR=#FFFFFF>
	<A HREF="25-4.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="25-6.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>
	<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 > 1); //1
	std::cout.setf(ios_base::boolalpha); //2
	std::cout << any_arguments << '\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 > 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 <class charT>
	class numpunct : public locale::facet {
	public:
	typedef charT char_type;
	typedef basic_string<charT> 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 <class charT> //1
	class change_bool_names
	: public std::numpunct_byname<charT> //2
	{
	public:
	typedef std::basic_string<charT> string_type;
	explicit change_bool_names (const char* name, //3
	const charT* t,
	const charT* f, size_t refs=0)
	: std::numpunct_byname<charT> (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<charT></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>C++ Standard Library Module Reference Guide</I></A> for further details.
	</TABLE>
	<P>Now let's create a German locale in which the <SAMP>std::numpunct<char></SAMP> facet has been replaced by an object of our new derived facet type, as shown in <A HREF="25-5.html#Figure 11">Figure 11</A>:</P>
	<A NAME="idx580"><!></A>
	<H4><A NAME="Figure 11">Figure 11: Replacing the std::numpunct<char> 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<char>("de_DE","Ja.","Nein.")); //2
	std::cout.imbue(loc); //3
	std::cout << "Argumente vorhanden? " //Any arguments?
	<< std::boolalpha << (argc > 1) << 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 > 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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