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<A NAME="dual_use"></A>
<H2>2.2.9. Dual-Use Filters</H2>


<P>
    It can be confusing to maintain two separate filters &#8212; one for input and the other for output &#8212; 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">&lt;boost/iostreams/char_traits.hpp&gt;</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">&lt;boost/iostreams/concepts.hpp&gt;</SPAN></A>    <SPAN CLASS="comment">// input_filter</SPAN>
<SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/operations.hpp"><SPAN CLASS="literal">&lt;boost/iostreams/operations.hpp&gt;</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>&lt;<SPAN CLASS="keyword">typename</SPAN> Source&gt;
    <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> get(Source&amp; src)
    {
        <SPAN CLASS='comment'>/* same as shell_comments_input_filter::get() */</SPAN>
    }

    <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Sink&gt;
    <SPAN CLASS="keyword"><SPAN CLASS="keyword">bool</SPAN></SPAN> put(Sink&amp; 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>&lt;<SPAN CLASS="keyword">typename</SPAN> Device&gt;
    <SPAN CLASS="keyword">void</SPAN> close(Device&amp;, 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.
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    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>)
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