<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <HTML> <HEAD> <TITLE>Tutorial</TITLE> <LINK REL="stylesheet" HREF="../../../../boost.css"> <LINK REL="stylesheet" HREF="../theme/iostreams.css"> </HEAD> <BODY> <!-- Begin Banner --> <H1 CLASS="title">Tutorial</H1> <HR CLASS="banner"> <!-- End Banner --> <!-- Begin Nav --> <DIV CLASS='nav'> <A HREF='multichar_filters.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/prev.png'></A> <A HREF='tutorial.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/up.png'></A> <A HREF='finite_state_filters.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/next.png'></A> </DIV> <!-- End Nav --> <A NAME="dual_use"></A> <H2>2.2.9. Dual-Use Filters</H2> <P> It can be confusing to maintain two separate filters — one for input and the other for output — to implement the same filtering algorithm. One way to avoid this is to use the <A HREF="writing_filters.html#filter_helpers">filter helpers</A> mentioned above. The filter helpers produce filters which can be used for either input or output, but not both simultaneously. Such filters are called <A HREF="../concepts/dual_use_filter.html">Dual-Use Filters</A>. </P> <P> A <A HREF="../concepts/dual_use_filter.html">Dual-Use Filter</A> can be produced by merging the implementations of an <A HREF="../concepts/input_filter.html">InputFilter</A> and an <A HREF="../concepts/output_filter.html">OutputFilter</A>, and giving the resulting class a <A HREF="../guide/traits.html#category">category</A> convertible to <A HREF="../guide/traits.html#category_tags"><CODE>dual_use_filter_tag</CODE></A>. For example, you can implement a <A HREF="shell_comments_filters.html">shell comments Filter</A> as a Dual-Use Filter as follows: </P> <PRE CLASS="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/char_traits.hpp"><SPAN CLASS="literal"><boost/iostreams/char_traits.hpp></SPAN></A> <SPAN CLASS="comment">// EOF, WOULD_BLOCK</SPAN> <SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/concepts.hpp"><SPAN CLASS="literal"><boost/iostreams/concepts.hpp></SPAN></A> <SPAN CLASS="comment">// input_filter</SPAN> <SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/operations.hpp"><SPAN CLASS="literal"><boost/iostreams/operations.hpp></SPAN></A> <SPAN CLASS="comment">// get</SPAN> <SPAN CLASS='keyword'>namespace</SPAN> boost { <SPAN CLASS='keyword'>namespace</SPAN> iostreams { <SPAN CLASS='keyword'>namespace</SPAN> example { <SPAN CLASS="keyword">class</SPAN> shell_comments_input_filter : <SPAN CLASS="keyword"><SPAN CLASS="keyword"><SPAN CLASS="keyword">public</SPAN></SPAN></SPAN> dual_use_filter { <SPAN CLASS="keyword">public</SPAN>: <SPAN CLASS="keyword">explicit</SPAN> shell_comments_input_filter(<SPAN CLASS="keyword">char</SPAN> comment_char = <SPAN CLASS="literal">'#'</SPAN>) : comment_char_(comment_char), skip_(<SPAN CLASS="keyword">false</SPAN>) { } <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Source> <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> get(Source& src) { <SPAN CLASS='comment'>/* same as shell_comments_input_filter::get() */</SPAN> } <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Sink> <SPAN CLASS="keyword"><SPAN CLASS="keyword">bool</SPAN></SPAN> put(Sink& dest, <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c) { <SPAN CLASS='comment'>/* same as shell_comments_output_filter::put() */</SPAN> } <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Device> <SPAN CLASS="keyword">void</SPAN> close(Device&, std::ios_base::openmode) { skip_ = <SPAN CLASS="keyword">false</SPAN>; } <SPAN CLASS="keyword">private</SPAN>: <SPAN CLASS="keyword">char</SPAN> comment_char_; <SPAN CLASS="keyword"><SPAN CLASS="keyword">bool</SPAN></SPAN> skip_; }; } } } <SPAN CLASS="comment">// End namespace boost::iostreams:example</SPAN></PRE> <P> When a <A HREF="../guide/filtering_streams.html">filtering stream or stream buffer</A> containing a <A HREF="../concepts/closable.html">Closable</A> <A HREF="../concepts/dual_use_filter.html">Dual-Use Filter</A> is closed, the Filter's member function <CODE>close</CODE> is called with one of the values <CODE>std::ios_base::in</CODE> or <CODE>std::ios_base::out</CODE> as the second function argument, indicating whether the Filter was used for input or output. </P> <!-- Begin Nav --> <DIV CLASS='nav'> <A HREF='multichar_filters.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/prev.png'></A> <A HREF='tutorial.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/up.png'></A> <A HREF='finite_state_filters.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/next.png'></A> </DIV> <!-- End Nav --> <!-- Begin Footer --> <HR> <P CLASS="copyright">Revised 02 Feb 2008</P> <P CLASS="copyright">© Copyright 2008 <a href="http://www.coderage.com/" target="_top">CodeRage, LLC</a><br/>© Copyright 2004-2007 <a href="http://www.coderage.com/turkanis/" target="_top">Jonathan Turkanis</a></P> <P CLASS="copyright"> Use, modification, and distribution are subject to the Boost Software License, Version 2.0. (See accompanying file <A HREF="../../../../LICENSE_1_0.txt">LICENSE_1_0.txt</A> or copy at <A HREF="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</A>) </P> <!-- End Footer --> </BODY>