doc/stdlibug/38-1.html - stdcxx - Git at Google

 <!--
     Licensed to the Apache Software  Foundation (ASF) under one or more
     contributor  license agreements.  See  the NOTICE  file distributed
     with  this  work  for  additional information  regarding  copyright
     ownership.   The ASF  licenses this  file to  you under  the Apache
     License, Version  2.0 (the  License); you may  not use  this file
     except in  compliance with the License.   You may obtain  a copy of
     the License at

     http://www.apache.org/licenses/LICENSE-2.0

     Unless required by applicable law or agreed to in writing, software
     distributed under the  License is distributed on an  "AS IS" BASIS,
     WITHOUT  WARRANTIES OR CONDITIONS  OF ANY  KIND, either  express or
     implied.   See  the License  for  the  specific language  governing
     permissions and limitations under the License.

     Copyright 1999-2007 Rogue Wave Software, Inc.
 -->

 <HTML>
 <HEAD>
 <TITLE>Deriving a New Stream Type</TITLE>
 <LINK REL=StyleSheet HREF="../rw.css" TYPE="text/css" TITLE="Apache stdcxx Stylesheet"></HEAD>
 <BODY BGCOLOR=#FFFFFF>
 <A HREF="38.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="38-2.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>38.1 Deriving a New Stream Type</H2>
 <A NAME="idx899"><!></A>
 <P>Sometimes it is useful to derive a stream type from the standard iostreams. This is the case when you want to add data members or functions, or modify the behavior of a stream's I/O operations.</P>
 <P>In <A HREF="36.html">Chapter&nbsp;36</A> we learned that additional data can be added to a stream object by using <SAMP>std::os_base::xalloc()</SAMP>, <SAMP>std::ios_base::iword()</SAMP>, and <SAMP>std::ios_base::pword()</SAMP>. However, this solution has a certain weakness in that only a pointer to the additional data can be stored and someone else has to worry about the actual memory.</P>
 <A NAME="idx900"><!></A>
 <P>This weakness can be overcome by deriving a new stream type that stores the additional data as a data member. Let's consider again the example of the <SAMP>date</SAMP> inserter and the <SAMP>setfmt</SAMP> manipulator from <A HREF="36-3.html">Section&nbsp;36.3</A>. Here let's derive a new stream that has an additional data member for storing the format string together with a corresponding member function for setting the date format specification. [This, of course, is only an example. You would probably never derive a new class for adding only one data member. However, it keeps the example simple and allows us to demonstrate the principle of deriving new stream classes.]</P>
 <P>Again, we confine the example to the inserter of the <SAMP>date</SAMP> object and omit the extractor. Instead of inserting into an output stream, as we did before, we now use a new type of stream called <SAMP>odatstream</SAMP>:</P>

 <UL><PRE>
 date today;
 odatstream ostr(std::cout);
 // ...
 ostr &lt;&lt; setfmt("%D") &lt;&lt; today;
 </PRE></UL>
 <P>In the next sections, we explore how we can implement such a derived stream type.</P>

 <BR>
 <HR>
 <A HREF="38.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="38-2.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>
	<!--
	Licensed to the Apache Software Foundation (ASF) under one or more
	contributor license agreements. See the NOTICE file distributed
	with this work for additional information regarding copyright
	ownership. The ASF licenses this file to you under the Apache
	License, Version 2.0 (the License); you may not use this file
	except in compliance with the License. You may obtain a copy of
	the License at

	http://www.apache.org/licenses/LICENSE-2.0

	Unless required by applicable law or agreed to in writing, software
	distributed under the License is distributed on an "AS IS" BASIS,
	WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
	implied. See the License for the specific language governing
	permissions and limitations under the License.

	Copyright 1999-2007 Rogue Wave Software, Inc.
	-->

	<HTML>
	<HEAD>
	<TITLE>Deriving a New Stream Type</TITLE>
	<LINK REL=StyleSheet HREF="../rw.css" TYPE="text/css" TITLE="Apache stdcxx Stylesheet"></HEAD>
	<BODY BGCOLOR=#FFFFFF>
	<A HREF="38.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="38-2.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>38.1 Deriving a New Stream Type</H2>
	<A NAME="idx899"><!></A>
	<P>Sometimes it is useful to derive a stream type from the standard iostreams. This is the case when you want to add data members or functions, or modify the behavior of a stream's I/O operations.</P>
	<P>In <A HREF="36.html">Chapter 36</A> we learned that additional data can be added to a stream object by using <SAMP>std::os_base::xalloc()</SAMP>, <SAMP>std::ios_base::iword()</SAMP>, and <SAMP>std::ios_base::pword()</SAMP>. However, this solution has a certain weakness in that only a pointer to the additional data can be stored and someone else has to worry about the actual memory.</P>
	<A NAME="idx900"><!></A>
	<P>This weakness can be overcome by deriving a new stream type that stores the additional data as a data member. Let's consider again the example of the <SAMP>date</SAMP> inserter and the <SAMP>setfmt</SAMP> manipulator from <A HREF="36-3.html">Section 36.3</A>. Here let's derive a new stream that has an additional data member for storing the format string together with a corresponding member function for setting the date format specification. [This, of course, is only an example. You would probably never derive a new class for adding only one data member. However, it keeps the example simple and allows us to demonstrate the principle of deriving new stream classes.]</P>
	<P>Again, we confine the example to the inserter of the <SAMP>date</SAMP> object and omit the extractor. Instead of inserting into an output stream, as we did before, we now use a new type of stream called <SAMP>odatstream</SAMP>:</P>

	<UL><PRE>
	date today;
	odatstream ostr(std::cout);
	// ...
	ostr << setfmt("%D") << today;
	</PRE></UL>
	<P>In the next sections, we explore how we can implement such a derived stream type.</P>

	<BR>
	<HR>
	<A HREF="38.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="38-2.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>