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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 us 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& 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& 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&) { 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& 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& 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& 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&) { 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>
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267<P CLASS="copyright">Revised
268<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
26920 May, 2004
270<!--webbot bot="Timestamp" endspan i-checksum="38504" -->
271</P>
272
273<P CLASS="copyright">&copy; Copyright <A HREF="http://www.kangaroologic.com" TARGET="_top">Jonathan Turkanis</A>, 2004</P>
274<P CLASS="copyright"> 
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