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 <A NAME="multichar"></A>
 <H2>2.2.8. Multi-Character Filters</H2>

 <P>
     All the Filters you've seen so far &#8212; except for those that derive from <CODE>stdio_filter</CODE> &#8212; process characters one at a time. If you instead process several characters at once, you can often reduce the number of function calls it takes to filter a character sequence, resulting in more efficient code. This is what <A HREF="../concepts/multi_character.html">Multi-Character Filters</A> allow you to do.
 </P>

 <A NAME="multichar_input_filters"></A>
 <H4>Multi-Character InputFilters</H4>

 <P>A typical narrow-character <A HREF="../concepts/multi_character.html">Multi-Character</A> <A HREF="../concepts/input_filter.html">InputFilter</A> looks like this:<P>

 <PRE CLASS="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <SPAN CLASS="literal">&lt;iosfwd&gt;</SPAN>                          <SPAN CLASS='comment'>// streamsize</SPAN>
 <SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="HEADER" HREF="../../../../boost/iostreams/categories.hpp"><SPAN CLASS='literal'>&lt;boost/iostreams/categories.hpp&gt;</SPAN></A>  <SPAN CLASS='comment'>// tags
 </SPAN>
 <SPAN CLASS='keyword'>class</SPAN> my_input_filter {
 <SPAN CLASS='keyword'>public</SPAN>:
     <SPAN CLASS='keyword'>typedef</SPAN> <SPAN CLASS='keyword'>char</SPAN> char_type;
     <SPAN CLASS='keyword'>struct</SPAN> category
         : input_filter_tag,
           multichar_tag
         { };

     <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Source&gt;
     std::streamsize read(Source& src, <SPAN CLASS='keyword'>char</SPAN>* s, std::streamsize n)
     {
         <SPAN CLASS='comment'>// Read up to n filtered characters into the buffer s,</SPAN>
         <SPAN CLASS='comment'>// returning the number of characters read or -1 for EOF.</SPAN>
         <SPAN CLASS='comment'>// Use src to access the unfiltered character sequence</SPAN>
     }

     <SPAN CLASS='comment'>/* Other members */</SPAN>
 };</PRE>

 <P>
     Notice that the member type category is a <CODE>struct</CODE> convertible to <CODE>input_filter_tag</CODE> and to <CODE>multichar_tag</CODE>. This tells the Iostream library that <CODE>my_input_filter</CODE> is a <A HREF="../concepts/multi_character.html">Multi-Character Filter</A> and an <A HREF="../concepts/input_filter.html">InputFilter</A>. You could have achieved the same effect several ways. <I>E.g.</I>,
 </P>

 <PRE CLASS="broken_ie">    <SPAN CLASS='keyword'>struct</SPAN> category
         : input,
           filter_tag,
           multichar_tag
         { };

         <SPAN CLASS='comment'>/* or */</SPAN>

     <SPAN CLASS='keyword'>typedef</SPAN> multichar_input_filter_tag category;</PRE>

 <P>(For details, <I>see</I> <A HREF="../guide/modes.html#mode_tags">Mode Tags</A> and <A HREF="../guide/traits.html#category_tags">Category Tags</A>.)

 <P> You could also write the above example as follows:</P>

 <PRE CLASS="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <SPAN CLASS="literal">&lt;iosfwd&gt;</SPAN>                       <SPAN CLASS='comment'>// streamsize</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'>// multichar_input_filter</SPAN>

 <SPAN CLASS='keyword'>class</SPAN> my_input_filter : <SPAN CLASS='keyword'>public</SPAN> multichar_input_filter {
 <SPAN CLASS='keyword'>public</SPAN>:
     <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Source&gt;
     std::streamsize read(<SPAN CLASS='keyword'>char</SPAN>* s, std::streamsize n);

     <SPAN CLASS='comment'>/* Other members */</SPAN>
 };</PRE>

 <P>Here <A HREF="../classes/filter.html#synopsis"><CODE>multichar_input_filter</CODE></A> is a convenience base class which provides the member types <CODE>char_type</CODE> and <CODE>category</CODE>, as well as no-op implementations of member functions <CODE>close</CODE> and <CODE>imbue</CODE>.

 <A NAME="shell_comments_multichar_input_filter"></A>
 <H4><CODE>shell_comments_multichar_input_filter</CODE></H4>

 <P>You can express a <A HREF="shell_comments_filters.html">shell comments Filter</A> as an <A HREF="../concepts/multi_character.html">Multi-Character</A> <A HREF="../concepts/input_filter.html">InputFilter</A> 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">// multichar_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_multichar_input_filter : <SPAN CLASS="keyword">public</SPAN> multichar_input_filter {
 <SPAN CLASS="keyword">public</SPAN>:
     <SPAN CLASS="keyword">explicit</SPAN> shell_comments_multichar_input_filter(<SPAN CLASS="keyword">char</SPAN> comment_char = '#')
         : comment_char_(comment_char), skip_(<SPAN CLASS="keyword">false</SPAN>)
         { }

     <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Source&gt;
     std::streamsize read(Source& src, <SPAN CLASS="keyword">char</SPAN>* s, std::streamsize n)
     {
         <SPAN CLASS="keyword">for</SPAN> (std::streamsize z = <SPAN CLASS='numeric_literal'>0</SPAN>; z &lt; n; ++z) {
             <SPAN CLASS="keyword">int</SPAN> c;
             <SPAN CLASS="keyword">while</SPAN> (true) {
                 <SPAN CLASS="keyword">if</SPAN> ((c = boost::iostreams::get(src)) == <SPAN CLASS='numeric_literal'>EOF</SPAN>)
                     <SPAN CLASS="keyword">return</SPAN> z != <SPAN CLASS='numeric_literal'>0</SPAN> ? z : <SPAN CLASS='numeric_literal'>-1</SPAN>;
                 <SPAN CLASS="keyword">else</SPAN> <SPAN CLASS="keyword">if</SPAN> (c == WOULD_BLOCK)
                     <SPAN CLASS="keyword">return</SPAN> z;
                 skip_ = c == comment_char_ ?
                     <SPAN CLASS="keyword">true</SPAN> :
                     c == '\n' ?
                         <SPAN CLASS="keyword">false</SPAN> :
                         skip_;
                 <SPAN CLASS="keyword">if</SPAN> (!skip_)
                     <SPAN CLASS="keyword">break</SPAN>;
             }
             s[z] = c;
         }
         <SPAN CLASS="keyword">return</SPAN> n;
     }

     <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Source&gt;
     <SPAN CLASS="keyword">void</SPAN> close(Source&) { skip_ = false; }
 <SPAN CLASS="keyword">private</SPAN>:
     <SPAN CLASS="keyword">char</SPAN> comment_char_;
     <SPAN CLASS="keyword">bool</SPAN> skip_;
 };

 } } } <SPAN CLASS="comment">// End namespace boost::iostreams:example</SPAN></PRE>

 <P>
     Note that the implementation of <CODE>read</CODE> is very similar to what you would get if you put the implementation of <A HREF="shell_comments_filters.html#shell_comments_input_filter"><CODE>shell_comments_input_filter::get</CODE></A> inside a <CODE>for</CODE> loop iterating from <CODE>0</CODE> to <CODE>n</CODE>. InputFilters which call themselves recursively, such as <A HREF="tab_expanding_filters.html#tab_expanding_input_filter"><CODE>tab_expanding_input_filter</CODE></A>,  are much harder to transform into Multi-Character filters.
 </P>

 <A NAME="multichar_output_filters"></A>
 <H4>Multi-Character OutputFilters</H4>

 <P>A typical narrow-character <A HREF="../concepts/multi_character.html">Multi-Character</A> <A HREF="../concepts/output_filter.html">OutputFilter</A> looks like this:<P>

 <PRE CLASS="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <SPAN CLASS="literal">&lt;iosfwd&gt;</SPAN>                          <SPAN CLASS='comment'>// streamsize</SPAN>
 <SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="HEADER" HREF="../../../../boost/iostreams/categories.hpp"><SPAN CLASS='literal'>&lt;boost/iostreams/categories.hpp&gt;</SPAN></A>  <SPAN CLASS='comment'>// tags
 </SPAN>
 <SPAN CLASS='keyword'>class</SPAN> my_output_filter {
 <SPAN CLASS='keyword'>public</SPAN>:
     <SPAN CLASS='keyword'>typedef</SPAN> <SPAN CLASS='keyword'>char</SPAN> char_type;
     <SPAN CLASS='keyword'>struct</SPAN> category
         : output_filter_tag,
           multichar_tag
         { };

     <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Sink&gt;
     std::streamsize write(Sink& dest, <SPAN CLASS='keyword'>const</SPAN> <SPAN CLASS='keyword'>char</SPAN>* s, std::streamsize n)
     {
         <SPAN CLASS='comment'>// Consume up to n filtered characters from the buffer s,</SPAN>
         <SPAN CLASS='comment'>// writing filtered characters to dest. Return the number</SPAN>
         <SPAN CLASS='comment'>// of characters consumed.</SPAN>
     }

     <SPAN CLASS='comment'>/* Other members */</SPAN>
 };</PRE>

 <P>
     Notice that the member type category is a <CODE>struct</CODE> convertible to <CODE>keyword</CODE> and to <CODE>multichar_tag</CODE>. This tells the Iostream library that <CODE>my_output_filter</CODE> is a <A HREF="../concepts/multi_character.html">Multi-Character Filter</A> and an <A HREF="../concepts/output_filter.html">OutputFilter</A>. As with <A HREF="../concepts/multi_character.html">Multi-Character</A> <A HREF="../concepts/input_filter.html">InputFilters</A>, you could have achieved the same effect several different ways. <I>E.g.</I>,
 </P>

 <PRE CLASS="broken_ie">    <SPAN CLASS='keyword'>struct</SPAN> category
         : output,
           filter_tag,
           multichar_tag
         { };

         <SPAN CLASS='comment'>/* or */</SPAN>

     <SPAN CLASS='keyword'>typedef</SPAN> multichar_output_filter_tag category;</PRE>

 <P>(For details, <I>see</I> <A HREF="../guide/modes.html#mode_tags">Mode Tags</A> and <A HREF="../guide/traits.html#category_tags">Category Tags</A>.)

 <P> You could also write the above example as follows:</P>

 <PRE CLASS="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <SPAN CLASS="literal">&lt;iosfwd&gt;</SPAN>                       <SPAN CLASS='comment'>// streamsize</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'>// multichar_output_filter</SPAN>

 <SPAN CLASS='keyword'>class</SPAN> my_output_filter : <SPAN CLASS='keyword'>public</SPAN> multichar_output_filter {
 <SPAN CLASS='keyword'>public</SPAN>:
     <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Sink&gt;
     std::streamsize write(Sink& dest, <SPAN CLASS='keyword'>const</SPAN> <SPAN CLASS='keyword'>char</SPAN>* s, std::streamsize n);

     <SPAN CLASS='comment'>/* Other members */</SPAN>
 };</PRE>

 <P>Here <A HREF="../classes/filter.html#synopsis"><CODE>multichar_output_filter</CODE></A> is a convenience base class which provides the member types <CODE>char_type</CODE> and <CODE>category</CODE>, as well as no-op implementations of member functions <CODE>close</CODE> and <CODE>imbue</CODE>.

 <A NAME="shell_comments_multichar_ouput_filter"></A>
 <H4><CODE>shell_comments_multichar_output_filter</CODE></H4>

 <P>You can express a <A HREF="shell_comments_filters.html">shell comments Filter</A> as an <A HREF="../concepts/multi_character.html">Multi-Character</A> <A HREF="../concepts/output_filter.html">OutputFilter</A> 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">// multichar_output_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_multichar_output_filter : <SPAN CLASS="keyword">public</SPAN> multichar_output_filter {
 <SPAN CLASS="keyword">public</SPAN>:
     <SPAN CLASS="keyword">explicit</SPAN> shell_comments_multichar_output_filter(<SPAN CLASS="keyword">char</SPAN> comment_char = '#')
         : comment_char_(comment_char), skip_(<SPAN CLASS="keyword">false</SPAN>)
         { }

     <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Sink&gt;
     std::streamsize write(Sink& dest, <SPAN CLASS="keyword">const</SPAN> char* s, std::streamsize n)
     {
         std::streamsize z;
         <SPAN CLASS="keyword">for</SPAN> (z = <SPAN CLASS='numeric_literal'>0</SPAN>; z &lt; n; ++z) {
             <SPAN CLASS="keyword">int</SPAN> c = s[z];
             skip_ = c == comment_char_ ?
                 <SPAN CLASS="keyword">true</SPAN> :
                 c == <SPAN CLASS='literal'>'\n'</SPAN> ?
                     <SPAN CLASS="keyword">false</SPAN> :
                     skip_;
             <SPAN CLASS="keyword">if</SPAN> (skip_)
                 <SPAN CLASS="keyword">continue</SPAN>;
             <SPAN CLASS="keyword">if</SPAN> (!iostreams::put(dest, c))
                 <SPAN CLASS="keyword">break</SPAN>;
         }
         <SPAN CLASS="keyword">return</SPAN> z;
     }

     <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Source&gt;
     <SPAN CLASS="keyword">void</SPAN> close(Source&) { skip_ = <SPAN CLASS="keyword">false</SPAN>; }
 <SPAN CLASS="keyword">private</SPAN>:
     <SPAN CLASS="keyword">char</SPAN> comment_char_;
     <SPAN CLASS="keyword">bool</SPAN> skip_;
 };

 } } } <SPAN CLASS="comment">// End namespace boost::iostreams:example</SPAN></PRE>

 <P>
     Note that the implementation of <CODE>write</CODE> is very similar to what you would get if you put the implementation of <A HREF="shell_comments_filters.html#shell_comments_output_filter"><CODE>shell_comments_output_filter::put</CODE></A> inside a <CODE>for</CODE> loop iterating from <CODE>0</CODE> to <CODE>n</CODE>. OutputFilters which call themselves recursively, such as <A HREF="unix2dos_filters.html#unix2dos_output_filter"><CODE>unix2dos_output_filter</CODE></A>,  are much harder to transform into Multi-Character filters.
 </P>

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 <P CLASS="copyright">Revised 02 Feb 2008</P>

 <P CLASS="copyright">&copy; Copyright 2008 <a href="http://www.coderage.com/" target="_top">CodeRage, LLC</a><br/>&copy; 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>)
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	<A NAME="multichar"></A>
	<H2>2.2.8. Multi-Character Filters</H2>

	<P>
	All the Filters you've seen so far — except for those that derive from <CODE>stdio_filter</CODE> — process characters one at a time. If you instead process several characters at once, you can often reduce the number of function calls it takes to filter a character sequence, resulting in more efficient code. This is what <A HREF="../concepts/multi_character.html">Multi-Character Filters</A> allow you to do.
	</P>

	<A NAME="multichar_input_filters"></A>
	<H4>Multi-Character InputFilters</H4>

	<P>A typical narrow-character <A HREF="../concepts/multi_character.html">Multi-Character</A> <A HREF="../concepts/input_filter.html">InputFilter</A> looks like this:<P>

	<PRE CLASS="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <SPAN CLASS="literal"><iosfwd></SPAN> <SPAN CLASS='comment'>// streamsize</SPAN>
	<SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="HEADER" HREF="../../../../boost/iostreams/categories.hpp"><SPAN CLASS='literal'><boost/iostreams/categories.hpp></SPAN></A> <SPAN CLASS='comment'>// tags
	</SPAN>
	<SPAN CLASS='keyword'>class</SPAN> my_input_filter {
	<SPAN CLASS='keyword'>public</SPAN>:
	<SPAN CLASS='keyword'>typedef</SPAN> <SPAN CLASS='keyword'>char</SPAN> char_type;
	<SPAN CLASS='keyword'>struct</SPAN> category
	: input_filter_tag,
	multichar_tag
	{ };

	<SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Source>
	std::streamsize read(Source& src, <SPAN CLASS='keyword'>char</SPAN>* s, std::streamsize n)
	{
	<SPAN CLASS='comment'>// Read up to n filtered characters into the buffer s,</SPAN>
	<SPAN CLASS='comment'>// returning the number of characters read or -1 for EOF.</SPAN>
	<SPAN CLASS='comment'>// Use src to access the unfiltered character sequence</SPAN>
	}

	<SPAN CLASS='comment'>/* Other members */</SPAN>
	};</PRE>

	<P>
	Notice that the member type category is a <CODE>struct</CODE> convertible to <CODE>input_filter_tag</CODE> and to <CODE>multichar_tag</CODE>. This tells the Iostream library that <CODE>my_input_filter</CODE> is a <A HREF="../concepts/multi_character.html">Multi-Character Filter</A> and an <A HREF="../concepts/input_filter.html">InputFilter</A>. You could have achieved the same effect several ways. <I>E.g.</I>,
	</P>

	<PRE CLASS="broken_ie"> <SPAN CLASS='keyword'>struct</SPAN> category
	: input,
	filter_tag,
	multichar_tag
	{ };

	<SPAN CLASS='comment'>/* or */</SPAN>

	<SPAN CLASS='keyword'>typedef</SPAN> multichar_input_filter_tag category;</PRE>

	<P>(For details, <I>see</I> <A HREF="../guide/modes.html#mode_tags">Mode Tags</A> and <A HREF="../guide/traits.html#category_tags">Category Tags</A>.)

	<P> You could also write the above example as follows:</P>

	<PRE CLASS="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <SPAN CLASS="literal"><iosfwd></SPAN> <SPAN CLASS='comment'>// streamsize</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'>// multichar_input_filter</SPAN>

	<SPAN CLASS='keyword'>class</SPAN> my_input_filter : <SPAN CLASS='keyword'>public</SPAN> multichar_input_filter {
	<SPAN CLASS='keyword'>public</SPAN>:
	<SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Source>
	std::streamsize read(<SPAN CLASS='keyword'>char</SPAN>* s, std::streamsize n);

	<SPAN CLASS='comment'>/* Other members */</SPAN>
	};</PRE>

	<P>Here <A HREF="../classes/filter.html#synopsis"><CODE>multichar_input_filter</CODE></A> is a convenience base class which provides the member types <CODE>char_type</CODE> and <CODE>category</CODE>, as well as no-op implementations of member functions <CODE>close</CODE> and <CODE>imbue</CODE>.

	<A NAME="shell_comments_multichar_input_filter"></A>
	<H4><CODE>shell_comments_multichar_input_filter</CODE></H4>

	<P>You can express a <A HREF="shell_comments_filters.html">shell comments Filter</A> as an <A HREF="../concepts/multi_character.html">Multi-Character</A> <A HREF="../concepts/input_filter.html">InputFilter</A> 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">// multichar_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_multichar_input_filter : <SPAN CLASS="keyword">public</SPAN> multichar_input_filter {
	<SPAN CLASS="keyword">public</SPAN>:
	<SPAN CLASS="keyword">explicit</SPAN> shell_comments_multichar_input_filter(<SPAN CLASS="keyword">char</SPAN> comment_char = '#')
	: comment_char_(comment_char), skip_(<SPAN CLASS="keyword">false</SPAN>)
	{ }

	<SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Source>
	std::streamsize read(Source& src, <SPAN CLASS="keyword">char</SPAN>* s, std::streamsize n)
	{
	<SPAN CLASS="keyword">for</SPAN> (std::streamsize z = <SPAN CLASS='numeric_literal'>0</SPAN>; z < n; ++z) {
	<SPAN CLASS="keyword">int</SPAN> c;
	<SPAN CLASS="keyword">while</SPAN> (true) {
	<SPAN CLASS="keyword">if</SPAN> ((c = boost::iostreams::get(src)) == <SPAN CLASS='numeric_literal'>EOF</SPAN>)
	<SPAN CLASS="keyword">return</SPAN> z != <SPAN CLASS='numeric_literal'>0</SPAN> ? z : <SPAN CLASS='numeric_literal'>-1</SPAN>;
	<SPAN CLASS="keyword">else</SPAN> <SPAN CLASS="keyword">if</SPAN> (c == WOULD_BLOCK)
	<SPAN CLASS="keyword">return</SPAN> z;
	skip_ = c == comment_char_ ?
	<SPAN CLASS="keyword">true</SPAN> :
	c == '\n' ?
	<SPAN CLASS="keyword">false</SPAN> :
	skip_;
	<SPAN CLASS="keyword">if</SPAN> (!skip_)
	<SPAN CLASS="keyword">break</SPAN>;
	}
	s[z] = c;
	}
	<SPAN CLASS="keyword">return</SPAN> n;
	}

	<SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Source>
	<SPAN CLASS="keyword">void</SPAN> close(Source&) { skip_ = false; }
	<SPAN CLASS="keyword">private</SPAN>:
	<SPAN CLASS="keyword">char</SPAN> comment_char_;
	<SPAN CLASS="keyword">bool</SPAN> skip_;
	};

	} } } <SPAN CLASS="comment">// End namespace boost::iostreams:example</SPAN></PRE>

	<P>
	Note that the implementation of <CODE>read</CODE> is very similar to what you would get if you put the implementation of <A HREF="shell_comments_filters.html#shell_comments_input_filter"><CODE>shell_comments_input_filter::get</CODE></A> inside a <CODE>for</CODE> loop iterating from <CODE>0</CODE> to <CODE>n</CODE>. InputFilters which call themselves recursively, such as <A HREF="tab_expanding_filters.html#tab_expanding_input_filter"><CODE>tab_expanding_input_filter</CODE></A>, are much harder to transform into Multi-Character filters.
	</P>

	<A NAME="multichar_output_filters"></A>
	<H4>Multi-Character OutputFilters</H4>

	<P>A typical narrow-character <A HREF="../concepts/multi_character.html">Multi-Character</A> <A HREF="../concepts/output_filter.html">OutputFilter</A> looks like this:<P>

	<PRE CLASS="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <SPAN CLASS="literal"><iosfwd></SPAN> <SPAN CLASS='comment'>// streamsize</SPAN>
	<SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="HEADER" HREF="../../../../boost/iostreams/categories.hpp"><SPAN CLASS='literal'><boost/iostreams/categories.hpp></SPAN></A> <SPAN CLASS='comment'>// tags
	</SPAN>
	<SPAN CLASS='keyword'>class</SPAN> my_output_filter {
	<SPAN CLASS='keyword'>public</SPAN>:
	<SPAN CLASS='keyword'>typedef</SPAN> <SPAN CLASS='keyword'>char</SPAN> char_type;
	<SPAN CLASS='keyword'>struct</SPAN> category
	: output_filter_tag,
	multichar_tag
	{ };

	<SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Sink>
	std::streamsize write(Sink& dest, <SPAN CLASS='keyword'>const</SPAN> <SPAN CLASS='keyword'>char</SPAN>* s, std::streamsize n)
	{
	<SPAN CLASS='comment'>// Consume up to n filtered characters from the buffer s,</SPAN>
	<SPAN CLASS='comment'>// writing filtered characters to dest. Return the number</SPAN>
	<SPAN CLASS='comment'>// of characters consumed.</SPAN>
	}

	<SPAN CLASS='comment'>/* Other members */</SPAN>
	};</PRE>

	<P>
	Notice that the member type category is a <CODE>struct</CODE> convertible to <CODE>keyword</CODE> and to <CODE>multichar_tag</CODE>. This tells the Iostream library that <CODE>my_output_filter</CODE> is a <A HREF="../concepts/multi_character.html">Multi-Character Filter</A> and an <A HREF="../concepts/output_filter.html">OutputFilter</A>. As with <A HREF="../concepts/multi_character.html">Multi-Character</A> <A HREF="../concepts/input_filter.html">InputFilters</A>, you could have achieved the same effect several different ways. <I>E.g.</I>,
	</P>

	<PRE CLASS="broken_ie"> <SPAN CLASS='keyword'>struct</SPAN> category
	: output,
	filter_tag,
	multichar_tag
	{ };

	<SPAN CLASS='comment'>/* or */</SPAN>

	<SPAN CLASS='keyword'>typedef</SPAN> multichar_output_filter_tag category;</PRE>

	<P>(For details, <I>see</I> <A HREF="../guide/modes.html#mode_tags">Mode Tags</A> and <A HREF="../guide/traits.html#category_tags">Category Tags</A>.)

	<P> You could also write the above example as follows:</P>

	<PRE CLASS="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <SPAN CLASS="literal"><iosfwd></SPAN> <SPAN CLASS='comment'>// streamsize</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'>// multichar_output_filter</SPAN>

	<SPAN CLASS='keyword'>class</SPAN> my_output_filter : <SPAN CLASS='keyword'>public</SPAN> multichar_output_filter {
	<SPAN CLASS='keyword'>public</SPAN>:
	<SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Sink>
	std::streamsize write(Sink& dest, <SPAN CLASS='keyword'>const</SPAN> <SPAN CLASS='keyword'>char</SPAN>* s, std::streamsize n);

	<SPAN CLASS='comment'>/* Other members */</SPAN>
	};</PRE>

	<P>Here <A HREF="../classes/filter.html#synopsis"><CODE>multichar_output_filter</CODE></A> is a convenience base class which provides the member types <CODE>char_type</CODE> and <CODE>category</CODE>, as well as no-op implementations of member functions <CODE>close</CODE> and <CODE>imbue</CODE>.

	<A NAME="shell_comments_multichar_ouput_filter"></A>
	<H4><CODE>shell_comments_multichar_output_filter</CODE></H4>

	<P>You can express a <A HREF="shell_comments_filters.html">shell comments Filter</A> as an <A HREF="../concepts/multi_character.html">Multi-Character</A> <A HREF="../concepts/output_filter.html">OutputFilter</A> 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">// multichar_output_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_multichar_output_filter : <SPAN CLASS="keyword">public</SPAN> multichar_output_filter {
	<SPAN CLASS="keyword">public</SPAN>:
	<SPAN CLASS="keyword">explicit</SPAN> shell_comments_multichar_output_filter(<SPAN CLASS="keyword">char</SPAN> comment_char = '#')
	: comment_char_(comment_char), skip_(<SPAN CLASS="keyword">false</SPAN>)
	{ }

	<SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Sink>
	std::streamsize write(Sink& dest, <SPAN CLASS="keyword">const</SPAN> char* s, std::streamsize n)
	{
	std::streamsize z;
	<SPAN CLASS="keyword">for</SPAN> (z = <SPAN CLASS='numeric_literal'>0</SPAN>; z < n; ++z) {
	<SPAN CLASS="keyword">int</SPAN> c = s[z];
	skip_ = c == comment_char_ ?
	<SPAN CLASS="keyword">true</SPAN> :
	c == <SPAN CLASS='literal'>'\n'</SPAN> ?
	<SPAN CLASS="keyword">false</SPAN> :
	skip_;
	<SPAN CLASS="keyword">if</SPAN> (skip_)
	<SPAN CLASS="keyword">continue</SPAN>;
	<SPAN CLASS="keyword">if</SPAN> (!iostreams::put(dest, c))
	<SPAN CLASS="keyword">break</SPAN>;
	}
	<SPAN CLASS="keyword">return</SPAN> z;
	}

	<SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Source>
	<SPAN CLASS="keyword">void</SPAN> close(Source&) { skip_ = <SPAN CLASS="keyword">false</SPAN>; }
	<SPAN CLASS="keyword">private</SPAN>:
	<SPAN CLASS="keyword">char</SPAN> comment_char_;
	<SPAN CLASS="keyword">bool</SPAN> skip_;
	};

	} } } <SPAN CLASS="comment">// End namespace boost::iostreams:example</SPAN></PRE>

	<P>
	Note that the implementation of <CODE>write</CODE> is very similar to what you would get if you put the implementation of <A HREF="shell_comments_filters.html#shell_comments_output_filter"><CODE>shell_comments_output_filter::put</CODE></A> inside a <CODE>for</CODE> loop iterating from <CODE>0</CODE> to <CODE>n</CODE>. OutputFilters which call themselves recursively, such as <A HREF="unix2dos_filters.html#unix2dos_output_filter"><CODE>unix2dos_output_filter</CODE></A>, are much harder to transform into Multi-Character filters.
	</P>

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	<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>
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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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