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9.6 KiB
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145 lines
9.6 KiB
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<HTML>
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<HEAD>
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<TITLE>Modifying a Standard Facet's Behavior</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="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>
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<H2>25.5 Modifying a Standard Facet's Behavior</H2>
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<A NAME="idx577"><!></A>
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<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>
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<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>
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<UL><PRE>
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int main(int argc, char** argv)
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{
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bool any_arguments = (argc > 1); //1
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std::cout.setf(ios_base::boolalpha); //2
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std::cout << any_arguments << '\n'; //3
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// ...
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}
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</PRE></UL>
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<TABLE CELLPADDING="3">
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<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.
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<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.
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<TR VALIGN="top"><TD><SAMP>//3</SAMP></TD><TD>Here either the string <SAMP>"true"</SAMP> or the string <SAMP>"false"</SAMP> are printed.
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</TABLE>
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<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>
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<A NAME="idx578"><!></A>
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<P>This is the interface of facet <B><I><A HREF="../stdlibref/numpunct.html">numpunct</A></I></B>:</P>
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<UL><PRE>
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namespace std{
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template <class charT>
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class numpunct : public locale::facet {
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public:
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typedef charT char_type;
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typedef basic_string<charT> string_type;
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explicit numpunct(size_t refs = 0);
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char_type decimal_point() const;
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char_type thousands_sep() const;
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string grouping() const;
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string_type <I>truename</I>() const;
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string_type <I>falsename</I>() const;
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static locale::id id;
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// ...
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};
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}
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</PRE></UL>
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<A NAME="idx579"><!></A>
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<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>
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<UL><PRE>
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template <class charT> //1
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class change_bool_names
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: public std::numpunct_byname<charT> //2
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{
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public:
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typedef std::basic_string<charT> string_type;
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explicit change_bool_names (const char* name, //3
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const charT* t,
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const charT* f, size_t refs=0)
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: std::numpunct_byname<charT> (name,refs),
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true_string(t), false_string(f) { }
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protected:
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virtual string_type do_truename() const { //4
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return true_string;
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}
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virtual string_type do_falsename() const {
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return false_string;
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}
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private:
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string_type true_string, false_string;
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};
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</PRE></UL>
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<TABLE CELLPADDING="3">
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<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.
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<TR VALIGN="top"><TD><SAMP>//2</SAMP></TD><TD>The new facet is derived from the <SAMP>std::numpunct_byname<charT></SAMP> facet.
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<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>.
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<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.
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<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.
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</TABLE>
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<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>
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<A NAME="idx580"><!></A>
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<H4><A NAME="Figure 11">Figure 11: Replacing the std::numpunct<char> facet object</A></H4>
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<P><IMG SRC="images/locfig12.gif" WIDTH=476 HEIGHT=311></P>
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<A NAME="idx581"><!></A>
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<P>The code looks like this:</P>
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<UL><PRE>
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void main(int argc, char** argv)
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{
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std::locale loc(std::locale("de_DE"), //1
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new change_bool_names<char>("de_DE","Ja.","Nein.")); //2
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std::cout.imbue(loc); //3
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std::cout << "Argumente vorhanden? " //Any arguments?
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<< std::boolalpha << (argc > 1) << std::endl; //4
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}
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</PRE></UL>
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<TABLE CELLPADDING="3">
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<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>
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<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.").
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<TR VALIGN="top"><TD><SAMP>//3</SAMP></TD><TD>The standard output stream <SAMP>std::cout</SAMP> is imbued with the newly created locale.
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<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:
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<BR><BR><SAMP>Argument vorhanden? Ja.</SAMP>
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</TABLE>
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<BR>
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