doc/stdlibug/36-3.html - stdcxx - Git at Google

 <HTML>
 <HEAD>
 <TITLE>Another Look at the Date Format String</TITLE>
 <LINK REL=StyleSheet HREF="../rw.css" TYPE="text/css" TITLE="Apache stdcxx Stylesheet"></HEAD>
 <BODY BGCOLOR=#FFFFFF>
 <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>
 <H2>36.3 Another Look at the Date Format String</H2>
 <A NAME="idx891"><!></A>
 <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&nbsp;28.3.2</A>), so we should add a manipulator that sets the date format string. It could be used like this:</P>

 <UL><PRE>
 date today;
 std::ofstream ostr;
 // ...
 ostr &lt;&lt; setfmt("%D") &lt;&lt; today;
 </PRE></UL>
 <A NAME="idx892"><!></A>
 <P>Here is a suggested implementation for such a manipulator (note that the name of the base class is implementation-defined):</P>

 <UL><PRE>
 class setfmt : public <I>smanip</I>&lt;const char*&gt;
 {
  public:
   setfmt(const char* fmt)
   : <I>smanip</I>&lt;const char*&gt;(setfmt_,fmt) {}
  private:
   static const int datfmtIdx;                                 //1

   static std::ios_base&amp; setfmt_(std::ios_base&amp; str,
                                 const char* fmt)
   {
     str.pword(datfmtIdx) = const_cast&lt;char*&gt; (fmt);           //2
     return str;
   }

   template&lt;class charT, class Traits&gt;
   friend std::basic_ostream&lt;charT, Traits&gt;&amp;                   //3
   operator&lt;&lt;(basic_ostream&lt;charT, Traits&gt;&amp; os,
              const date&amp; dat);
 };

 const int setfmt::datfmtIdx = std::ios_base::xalloc();        //4
 </PRE></UL>
 <A NAME="idx893"><!></A>
 <P>The technique applied to implement the manipulator is described in detail in Example 2 of <A HREF="33-3.html">Section&nbsp;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>
 <TABLE CELLPADDING="3">

 <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>.
 <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>C++ Standard Library Module Reference Guide</I></A> for more details.)
 <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.
 <TR VALIGN="top"><TD><SAMP>//4</SAMP></TD><TD>Initializes the index of elements in istream storage where the format string is kept.
 </TABLE>
 <A NAME="idx894"><!></A>
 <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&nbsp;32.5.1</A>:</P>

 <UL><PRE>
 template&lt;class charT, class Traits&gt;
 std::basic_ostream&lt;charT, Traits&gt; &amp;
 operator &lt;&lt; (std::basic_ostream&lt;charT, Traits &gt;&amp; os,
              const date&amp; dat)
 {
  std::ios_base::iostate err = 0;
  charT* fmt  = 0;

  try {
   typename std::basic_ostream&lt;charT, Traits&gt;::sentry opfx(os);

    if(opfx)
    {
      char* patt      = static_cast&lt;char*&gt;
                          (os.pword(setfmt.datfmtIdx);         //1
      std::size_t len = std::strlen(patt);
      fmt             = new charT[len];

      std::use_facet&lt;std::ctype&lt;charT&gt; &gt;(os.getloc()).
          widen(patt, patt+len, fmt);

      if (std::use_facet&lt;std::time_put&lt;charT,
                   std::ostreambuf_iterator&lt;charT,Traits&gt; &gt; &gt;
           (os.getloc())
           .put(os,os,os.fill(),&amp;dat.tm_date,fmt,fmt+len)      //2
           .failed()
         )
           err = std::ios_base::badbit;
      os.width(0);
    }
  }
  catch(...)
  {
    delete [] fmt;
    bool flag = false;
    try {
      os.setstate(std::ios_base::failbit);
    }
    catch(std::ios_base::failure) {
      flag = true;
    }
    if (flag)
      throw;
  }

  delete [] fmt;
  if ( err )
    os.setstate(err);
  return os;
 }
 </PRE></UL>
 <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>

 <BR>
 <HR>
 <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=20 HEIGHT=21 ALT="Next file" BORDER=O></A>

 <!-- Google Analytics tracking code -->
 <script src="http://www.google-analytics.com/urchin.js" type="text/javascript">
 </script>
 <script type="text/javascript">
     _uacct = "UA-1775151-1";
     urchinTracker();
 </script>
 <!-- end of Google Analytics tracking code -->

 </BODY>
 </HTML>
	<HTML>
	<HEAD>
	<TITLE>Another Look at the Date Format String</TITLE>
	<LINK REL=StyleSheet HREF="../rw.css" TYPE="text/css" TITLE="Apache stdcxx Stylesheet"></HEAD>
	<BODY BGCOLOR=#FFFFFF>
	<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>
	<H2>36.3 Another Look at the Date Format String</H2>
	<A NAME="idx891"><!></A>
	<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>

	<UL><PRE>
	date today;
	std::ofstream ostr;
	// ...
	ostr << setfmt("%D") << today;
	</PRE></UL>
	<A NAME="idx892"><!></A>
	<P>Here is a suggested implementation for such a manipulator (note that the name of the base class is implementation-defined):</P>

	<UL><PRE>
	class setfmt : public <I>smanip</I><const char*>
	{
	public:
	setfmt(const char* fmt)
	: <I>smanip</I><const char*>(setfmt_,fmt) {}
	private:
	static const int datfmtIdx; //1

	static std::ios_base& setfmt_(std::ios_base& str,
	const char* fmt)
	{
	str.pword(datfmtIdx) = const_cast<char*> (fmt); //2
	return str;
	}

	template<class charT, class Traits>
	friend std::basic_ostream<charT, Traits>& //3
	operator<<(basic_ostream<charT, Traits>& os,
	const date& dat);
	};

	const int setfmt::datfmtIdx = std::ios_base::xalloc(); //4
	</PRE></UL>
	<A NAME="idx893"><!></A>
	<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>
	<TABLE CELLPADDING="3">

	<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>.
	<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>C++ Standard Library Module Reference Guide</I></A> for more details.)
	<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.
	<TR VALIGN="top"><TD><SAMP>//4</SAMP></TD><TD>Initializes the index of elements in istream storage where the format string is kept.
	</TABLE>
	<A NAME="idx894"><!></A>
	<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>

	<UL><PRE>
	template<class charT, class Traits>
	std::basic_ostream<charT, Traits> &
	operator << (std::basic_ostream<charT, Traits >& os,
	const date& dat)
	{
	std::ios_base::iostate err = 0;
	charT* fmt = 0;

	try {
	typename std::basic_ostream<charT, Traits>::sentry opfx(os);

	if(opfx)
	{
	char* patt = static_cast<char*>
	(os.pword(setfmt.datfmtIdx); //1
	std::size_t len = std::strlen(patt);
	fmt = new charT[len];

	std::use_facet<std::ctype<charT> >(os.getloc()).
	widen(patt, patt+len, fmt);

	if (std::use_facet<std::time_put<charT,
	std::ostreambuf_iterator<charT,Traits> > >
	(os.getloc())
	.put(os,os,os.fill(),&dat.tm_date,fmt,fmt+len) //2
	.failed()
	)
	err = std::ios_base::badbit;
	os.width(0);
	}
	}
	catch(...)
	{
	delete [] fmt;
	bool flag = false;
	try {
	os.setstate(std::ios_base::failbit);
	}
	catch(std::ios_base::failure) {
	flag = true;
	}
	if (flag)
	throw;
	}

	delete [] fmt;
	if ( err )
	os.setstate(err);
	return os;
	}
	</PRE></UL>
	<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>

	<BR>
	<HR>
	<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=20 HEIGHT=21 ALT="Next file" BORDER=O></A>

	<!-- Google Analytics tracking code -->
	<script src="http://www.google-analytics.com/urchin.js" type="text/javascript">
	</script>
	<script type="text/javascript">
	_uacct = "UA-1775151-1";
	urchinTracker();
	</script>
	<!-- end of Google Analytics tracking code -->

	</BODY>
	</HTML>