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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&amp; 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&amp; 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&amp;) { 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&amp; 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&amp; 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&amp; 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&amp;) { 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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