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extern/stdcxx/4.2.1/doc/stdlibug/36-3.html
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extern/stdcxx/4.2.1/doc/stdlibug/36-3.html
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<HTML>
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<HEAD>
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<TITLE>Another Look at the Date Format String</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="36-2.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="36-4.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>36.3 Another Look at the Date Format String</H2>
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<A NAME="idx891"><!></A>
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<P>We would like to store the date format string in the iostream storage through <SAMP>std::ios_base::iword()</SAMP> and <SAMP>std::ios_base::pword()</SAMP>. In this way, the input and output operations for <SAMP>date</SAMP> objects can access the format string for parsing and formatting. Format parameters are often set with manipulators (see <A HREF="28-3.html#2832">Section 28.3.2</A>), so we should add a manipulator that sets the date format string. It could be used like this:</P>
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<UL><PRE>
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date today;
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std::ofstream ostr;
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// ...
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ostr << setfmt("%D") << today;
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</PRE></UL>
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<A NAME="idx892"><!></A>
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<P>Here is a suggested implementation for such a manipulator (note that the name of the base class is implementation-defined):</P>
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<UL><PRE>
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class setfmt : public <I>smanip</I><const char*>
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{
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public:
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setfmt(const char* fmt)
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: <I>smanip</I><const char*>(setfmt_,fmt) {}
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private:
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static const int datfmtIdx; //1
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static std::ios_base& setfmt_(std::ios_base& str,
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const char* fmt)
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{
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str.pword(datfmtIdx) = const_cast<char*> (fmt); //2
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return str;
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}
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template<class charT, class Traits>
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friend std::basic_ostream<charT, Traits>& //3
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operator<<(basic_ostream<charT, Traits>& os,
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const date& dat);
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};
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const int setfmt::datfmtIdx = std::ios_base::xalloc(); //4
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</PRE></UL>
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<A NAME="idx893"><!></A>
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<P>The technique applied to implement the manipulator is described in detail in Example 2 of <A HREF="33-3.html">Section 33.3</A>, so we won't repeat it here. But regarding this manipulator and the private use of iostream storage, there are other interesting details:</P>
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<TABLE CELLPADDING="3">
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<TR VALIGN="top"><TD><SAMP>//1</SAMP></TD><TD>The manipulator class owns the index of the element in the iostream storage where we want to store the format string. It is initialized in <SAMP>//4</SAMP> by a call to <SAMP>xalloc()</SAMP>.
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<TR VALIGN="top"><TD><SAMP>//2</SAMP></TD><TD>The manipulator accesses the array <SAMP>pword()</SAMP> using the index <SAMP>datfmtIdx</SAMP>, and stores the pointer to the date format string. [For brevity, error handling is omitted in the example. If allocation fails, then <SAMP>std::ios_base::badbit</SAMP> is set.] Note that the reference returned by <SAMP>pword()</SAMP> is only used for <I>storing</I> the pointer to the date format string. Generally, you should never store a reference returned by <SAMP>iword()</SAMP> or <SAMP>pword()</SAMP> in order to access the stored data through this reference later on. This is because these references can become invalid once the array is reallocated or copied. (See the <A HREF="../stdlibref/noframes.html"><I>Apache C++ Standard Library Reference Guide</I></A> for more details.)
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<TR VALIGN="top"><TD><SAMP>//3</SAMP></TD><TD>The inserter for <SAMP>date</SAMP> objects needs to access the index into the array of pointers, so that it can read the format string and use it. Therefore, the inserter must be declared as a friend. In principle, the extractor must be a friend, too; however, the standard C++ locale falls short of supporting the use of format strings like the ones used by the standard C function <SAMP>strptime()</SAMP>. Hence, the implementation of a date extractor that supports date format strings would be a lot more complicated than the implementation for the inserter, which can use the stream's locale. We have omitted the extractor for the sake of brevity.
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<TR VALIGN="top"><TD><SAMP>//4</SAMP></TD><TD>Initializes the index of elements in istream storage where the format string is kept.
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</TABLE>
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<A NAME="idx894"><!></A>
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<P>The inserter for <SAMP>date</SAMP> objects given below is almost identical to the one we described in <A HREF="32-5.html#3251">Section 32.5.1</A>:</P>
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<UL><PRE>
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template<class charT, class Traits>
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std::basic_ostream<charT, Traits> &
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operator << (std::basic_ostream<charT, Traits >& os,
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const date& dat)
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{
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std::ios_base::iostate err = 0;
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charT* fmt = 0;
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try {
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typename std::basic_ostream<charT, Traits>::sentry opfx(os);
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if(opfx)
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{
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char* patt = static_cast<char*>
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(os.pword(setfmt.datfmtIdx); //1
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std::size_t len = std::strlen(patt);
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fmt = new charT[len];
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std::use_facet<std::ctype<charT> >(os.getloc()).
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widen(patt, patt+len, fmt);
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if (std::use_facet<std::time_put<charT,
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std::ostreambuf_iterator<charT,Traits> > >
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(os.getloc())
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.put(os,os,os.fill(),&dat.tm_date,fmt,fmt+len) //2
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.failed()
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)
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err = std::ios_base::badbit;
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os.width(0);
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}
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}
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catch(...)
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{
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delete [] fmt;
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bool flag = false;
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try {
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os.setstate(std::ios_base::failbit);
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}
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catch(std::ios_base::failure) {
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flag = true;
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}
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if (flag)
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throw;
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}
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delete [] fmt;
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if ( err )
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os.setstate(err);
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return os;
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}
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</PRE></UL>
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<P>The only change from the previous inserter is that the format string here is read from the iostream storage (in statement <SAMP>//1</SAMP>) instead of being the fixed string <SAMP>"%x"</SAMP>. The format string is then provided to the locale's time formatting facet (in statement <SAMP>//2</SAMP>).</P>
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