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 — 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. 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"><iosfwd></SPAN> <SPAN CLASS='comment'>// streamsize</SPAN> 41<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 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><<SPAN CLASS="keyword">typename</SPAN> Source> 52 std::streamsize read(Source& 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"><iosfwd></SPAN> <SPAN CLASS='comment'>// streamsize</SPAN> 81<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> 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><<SPAN CLASS="keyword">typename</SPAN> Source> 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"><boost/iostreams/char_traits.hpp></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"><boost/iostreams/concepts.hpp></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"><boost/iostreams/operations.hpp></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><<SPAN CLASS="keyword">typename</SPAN> Source> 111 std::streamsize read(Source& 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 < 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><<SPAN CLASS="keyword">typename</SPAN> Source> 134 <SPAN CLASS="keyword">void</SPAN> close(Source&) { 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"><iosfwd></SPAN> <SPAN CLASS='comment'>// streamsize</SPAN> 152<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 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><<SPAN CLASS="keyword">typename</SPAN> Sink> 163 std::streamsize write(Sink& 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"><iosfwd></SPAN> <SPAN CLASS='comment'>// streamsize</SPAN> 192<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> 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><<SPAN CLASS="keyword">typename</SPAN> Sink> 197 std::streamsize write(Sink& 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"><boost/iostreams/char_traits.hpp></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"><boost/iostreams/concepts.hpp></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"><boost/iostreams/operations.hpp></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><<SPAN CLASS="keyword">typename</SPAN> Sink> 222 std::streamsize write(Sink& 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 < 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><<SPAN CLASS="keyword">typename</SPAN> Source> 241 <SPAN CLASS="keyword">void</SPAN> close(Source&) { 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">© Copyright 2008 <a href="http://www.coderage.com/" target="_top">CodeRage, LLC</a><br/>© Copyright 2004-2007 <a href="https://www.boost.org/users/people/jonathan_turkanis.html" 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>
