ceph/src/boost/libs/iostreams/doc/tutorial/multichar_filters.html

   1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
   2 <HTML>
   3 <HEAD>
   4     <TITLE>Tutorial</TITLE>
   5     <LINK REL="stylesheet" HREF="../../../../boost.css">
   6     <LINK REL="stylesheet" HREF="../theme/iostreams.css">
   7 </HEAD>
   8 <BODY>
   9
  10 <!-- Begin Banner -->
  11
  12     <H1 CLASS="title">Tutorial</H1>
  13     <HR CLASS="banner">
  14
  15 <!-- End Banner -->
  16
  17 <!-- Begin Nav -->
  18
  19 <DIV CLASS='nav'>
  20     <A HREF='unix2dos_filters.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/prev.png'></A>
  21     <A HREF='tutorial.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/up.png'></A>
  22     <A HREF='dual_use_filters.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/next.png'></A>
  23 </DIV>
  24
  25 <!-- End Nav -->
  26
  27
  28 <A NAME="multichar"></A>
  29 <H2>2.2.8. Multi-Character Filters</H2>
  30
  31 <P>
  32     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.
  33 </P>
  34
  35 <A NAME="multichar_input_filters"></A>
  36 <H4>Multi-Character InputFilters</H4>
  37
  38 <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>
  39
  40 <PRE CLASS="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <SPAN CLASS="literal">&lt;iosfwd&gt;</SPAN>                          <SPAN CLASS='comment'>// streamsize</SPAN>
  41 <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
  42 </SPAN>
  43 <SPAN CLASS='keyword'>class</SPAN> my_input_filter {
  44 <SPAN CLASS='keyword'>public</SPAN>:
  45     <SPAN CLASS='keyword'>typedef</SPAN> <SPAN CLASS='keyword'>char</SPAN> char_type;
  46     <SPAN CLASS='keyword'>struct</SPAN> category
  47         : input_filter_tag,
  48           multichar_tag
  49         { };
  50
  51     <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Source&gt;
  52     std::streamsize read(Source&amp; src, <SPAN CLASS='keyword'>char</SPAN>* s, std::streamsize n)
  53     {
  54         <SPAN CLASS='comment'>// Read up to n filtered characters into the buffer s,</SPAN>
  55         <SPAN CLASS='comment'>// returning the number of characters read or -1 for EOF.</SPAN>
  56         <SPAN CLASS='comment'>// Use src to access the unfiltered character sequence</SPAN>
  57     }
  58
  59     <SPAN CLASS='comment'>/* Other members */</SPAN>
  60 };</PRE>
  61
  62 <P>
  63     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>,
  64 </P>
  65
  66 <PRE CLASS="broken_ie">    <SPAN CLASS='keyword'>struct</SPAN> category
  67         : input,
  68           filter_tag,
  69           multichar_tag
  70         { };
  71
  72         <SPAN CLASS='comment'>/* or */</SPAN>
  73
  74     <SPAN CLASS='keyword'>typedef</SPAN> multichar_input_filter_tag category;</PRE>
  75
  76 <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>.)
  77
  78 <P> You could also write the above example as follows:</P>
  79
  80 <PRE CLASS="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <SPAN CLASS="literal">&lt;iosfwd&gt;</SPAN>                       <SPAN CLASS='comment'>// streamsize</SPAN>
  81 <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>
  82
  83 <SPAN CLASS='keyword'>class</SPAN> my_input_filter : <SPAN CLASS='keyword'>public</SPAN> multichar_input_filter {
  84 <SPAN CLASS='keyword'>public</SPAN>:
  85     <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Source&gt;
  86     std::streamsize read(<SPAN CLASS='keyword'>char</SPAN>* s, std::streamsize n);
  87
  88     <SPAN CLASS='comment'>/* Other members */</SPAN>
  89 };</PRE>
  90
  91 <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>.
  92
  93 <A NAME="shell_comments_multichar_input_filter"></A>
  94 <H4><CODE>shell_comments_multichar_input_filter</CODE></H4>
  95
  96 <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>
  97
  98 <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>
  99 <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>
 100 <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>
 101
 102 <SPAN CLASS='keyword'>namespace</SPAN> boost { <SPAN CLASS='keyword'>namespace</SPAN> iostreams { <SPAN CLASS='keyword'>namespace</SPAN> example {
 103
 104 <SPAN CLASS="keyword">class</SPAN> shell_comments_multichar_input_filter : <SPAN CLASS="keyword">public</SPAN> multichar_input_filter {
 105 <SPAN CLASS="keyword">public</SPAN>:
 106     <SPAN CLASS="keyword">explicit</SPAN> shell_comments_multichar_input_filter(<SPAN CLASS="keyword">char</SPAN> comment_char = '#')
 107         : comment_char_(comment_char), skip_(<SPAN CLASS="keyword">false</SPAN>)
 108         { }
 109
 110     <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Source&gt;
 111     std::streamsize read(Source&amp; src, <SPAN CLASS="keyword">char</SPAN>* s, std::streamsize n)
 112     {
 113         <SPAN CLASS="keyword">for</SPAN> (std::streamsize z = <SPAN CLASS='numeric_literal'>0</SPAN>; z &lt; n; ++z) {
 114             <SPAN CLASS="keyword">int</SPAN> c;
 115             <SPAN CLASS="keyword">while</SPAN> (true) {
 116                 <SPAN CLASS="keyword">if</SPAN> ((c = boost::iostreams::get(src)) == <SPAN CLASS='numeric_literal'>EOF</SPAN>)
 117                     <SPAN CLASS="keyword">return</SPAN> z != <SPAN CLASS='numeric_literal'>0</SPAN> ? z : <SPAN CLASS='numeric_literal'>-1</SPAN>;
 118                 <SPAN CLASS="keyword">else</SPAN> <SPAN CLASS="keyword">if</SPAN> (c == WOULD_BLOCK)
 119                     <SPAN CLASS="keyword">return</SPAN> z;
 120                 skip_ = c == comment_char_ ?
 121                     <SPAN CLASS="keyword">true</SPAN> :
 122                     c == '\n' ?
 123                         <SPAN CLASS="keyword">false</SPAN> :
 124                         skip_;
 125                 <SPAN CLASS="keyword">if</SPAN> (!skip_)
 126                     <SPAN CLASS="keyword">break</SPAN>;
 127             }
 128             s[z] = c;
 129         }
 130         <SPAN CLASS="keyword">return</SPAN> n;
 131     }
 132
 133     <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Source&gt;
 134     <SPAN CLASS="keyword">void</SPAN> close(Source&amp;) { skip_ = false; }
 135 <SPAN CLASS="keyword">private</SPAN>:
 136     <SPAN CLASS="keyword">char</SPAN> comment_char_;
 137     <SPAN CLASS="keyword">bool</SPAN> skip_;
 138 };
 139
 140 } } } <SPAN CLASS="comment">// End namespace boost::iostreams:example</SPAN></PRE>
 141
 142 <P>
 143     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.
 144 </P>
 145
 146 <A NAME="multichar_output_filters"></A>
 147 <H4>Multi-Character OutputFilters</H4>
 148
 149 <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>
 150
 151 <PRE CLASS="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <SPAN CLASS="literal">&lt;iosfwd&gt;</SPAN>                          <SPAN CLASS='comment'>// streamsize</SPAN>
 152 <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
 153 </SPAN>
 154 <SPAN CLASS='keyword'>class</SPAN> my_output_filter {
 155 <SPAN CLASS='keyword'>public</SPAN>:
 156     <SPAN CLASS='keyword'>typedef</SPAN> <SPAN CLASS='keyword'>char</SPAN> char_type;
 157     <SPAN CLASS='keyword'>struct</SPAN> category
 158         : output_filter_tag,
 159           multichar_tag
 160         { };
 161
 162     <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Sink&gt;
 163     std::streamsize write(Sink&amp; dest, <SPAN CLASS='keyword'>const</SPAN> <SPAN CLASS='keyword'>char</SPAN>* s, std::streamsize n)
 164     {
 165         <SPAN CLASS='comment'>// Consume up to n filtered characters from the buffer s,</SPAN>
 166         <SPAN CLASS='comment'>// writing filtered characters to dest. Return the number</SPAN>
 167         <SPAN CLASS='comment'>// of characters consumed.</SPAN>
 168     }
 169
 170     <SPAN CLASS='comment'>/* Other members */</SPAN>
 171 };</PRE>
 172
 173 <P>
 174     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>,
 175 </P>
 176
 177 <PRE CLASS="broken_ie">    <SPAN CLASS='keyword'>struct</SPAN> category
 178         : output,
 179           filter_tag,
 180           multichar_tag
 181         { };
 182
 183         <SPAN CLASS='comment'>/* or */</SPAN>
 184
 185     <SPAN CLASS='keyword'>typedef</SPAN> multichar_output_filter_tag category;</PRE>
 186
 187 <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>.)
 188
 189 <P> You could also write the above example as follows:</P>
 190
 191 <PRE CLASS="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <SPAN CLASS="literal">&lt;iosfwd&gt;</SPAN>                       <SPAN CLASS='comment'>// streamsize</SPAN>
 192 <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>
 193
 194 <SPAN CLASS='keyword'>class</SPAN> my_output_filter : <SPAN CLASS='keyword'>public</SPAN> multichar_output_filter {
 195 <SPAN CLASS='keyword'>public</SPAN>:
 196     <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Sink&gt;
 197     std::streamsize write(Sink&amp; dest, <SPAN CLASS='keyword'>const</SPAN> <SPAN CLASS='keyword'>char</SPAN>* s, std::streamsize n);
 198
 199     <SPAN CLASS='comment'>/* Other members */</SPAN>
 200 };</PRE>
 201
 202 <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>.
 203
 204 <A NAME="shell_comments_multichar_ouput_filter"></A>
 205 <H4><CODE>shell_comments_multichar_output_filter</CODE></H4>
 206
 207 <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>
 208
 209 <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>
 210 <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>
 211 <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>
 212
 213 <SPAN CLASS='keyword'>namespace</SPAN> boost { <SPAN CLASS='keyword'>namespace</SPAN> iostreams { <SPAN CLASS='keyword'>namespace</SPAN> example {
 214
 215 <SPAN CLASS="keyword">class</SPAN> shell_comments_multichar_output_filter : <SPAN CLASS="keyword">public</SPAN> multichar_output_filter {
 216 <SPAN CLASS="keyword">public</SPAN>:
 217     <SPAN CLASS="keyword">explicit</SPAN> shell_comments_multichar_output_filter(<SPAN CLASS="keyword">char</SPAN> comment_char = '#')
 218         : comment_char_(comment_char), skip_(<SPAN CLASS="keyword">false</SPAN>)
 219         { }
 220
 221     <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Sink&gt;
 222     std::streamsize write(Sink&amp; dest, <SPAN CLASS="keyword">const</SPAN> char* s, std::streamsize n)
 223     {
 224         std::streamsize z;
 225         <SPAN CLASS="keyword">for</SPAN> (z = <SPAN CLASS='numeric_literal'>0</SPAN>; z &lt; n; ++z) {
 226             <SPAN CLASS="keyword">int</SPAN> c = s[z];
 227             skip_ = c == comment_char_ ?
 228                 <SPAN CLASS="keyword">true</SPAN> :
 229                 c == <SPAN CLASS='literal'>'\n'</SPAN> ?
 230                     <SPAN CLASS="keyword">false</SPAN> :
 231                     skip_;
 232             <SPAN CLASS="keyword">if</SPAN> (skip_)
 233                 <SPAN CLASS="keyword">continue</SPAN>;
 234             <SPAN CLASS="keyword">if</SPAN> (!iostreams::put(dest, c))
 235                 <SPAN CLASS="keyword">break</SPAN>;
 236         }
 237         <SPAN CLASS="keyword">return</SPAN> z;
 238     }
 239
 240     <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Source&gt;
 241     <SPAN CLASS="keyword">void</SPAN> close(Source&amp;) { skip_ = <SPAN CLASS="keyword">false</SPAN>; }
 242 <SPAN CLASS="keyword">private</SPAN>:
 243     <SPAN CLASS="keyword">char</SPAN> comment_char_;
 244     <SPAN CLASS="keyword">bool</SPAN> skip_;
 245 };
 246
 247 } } } <SPAN CLASS="comment">// End namespace boost::iostreams:example</SPAN></PRE>
 248
 249 <P>
 250     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.
 251 </P>
 252
 253 <!-- Begin Nav -->
 254
 255 <DIV CLASS='nav'>
 256     <A HREF='unix2dos_filters.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/prev.png'></A>
 257     <A HREF='tutorial.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/up.png'></A>
 258     <A HREF='dual_use_filters.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/next.png'></A>
 259 </DIV>
 260
 261 <!-- End Nav -->
 262
 263 <!-- Begin Footer -->
 264
 265 <HR>
 266
 267
 268 <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>
 269 <P CLASS="copyright">
 270     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>)
 271 </P>
 272 <!-- End Footer -->
 273
 274 </BODY>