]>
Commit | Line | Data |
---|---|---|
7c673cae FG |
1 | <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> |
2 | <HTML> | |
3 | <HEAD> | |
4 | <TITLE>InputFilter</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">InputFilter</H1> | |
13 | <HR CLASS="banner"> | |
14 | ||
15 | <!-- End Banner --> | |
16 | ||
17 | <DL CLASS="page-index" STYLE="margin-top:0"> | |
18 | <DT><A HREF="#definition">Definition</A> | |
19 | <DT><A HREF="#description">Description</A> | |
20 | <DT><A HREF="#examples">Examples</A> | |
21 | <DT><A HREF="#detailed_specification">Detailed Specification</A> | |
22 | </DL> | |
23 | ||
24 | <A NAME="definition"></A> | |
25 | <H2>Definition</H2> | |
26 | ||
27 | <P> | |
28 | An InputFilter is a <A HREF="filter.html">Filter</A> whose <A HREF="../guide/modes.html">mode</A> refines <A HREF="../guide/modes.html#input">input</A>. | |
29 | </P> | |
30 | ||
31 | <A NAME="description"></A> | |
32 | <H2>Description</H2> | |
33 | ||
34 | <P> | |
35 | An InputFilter operates on the character sequence controlled by a <A HREF="source.html">Source</A>, providing access to a filtered sequence having the same character type. It may expose the filtered sequence in two ways: | |
36 | <OL> | |
37 | <LI STYLE="list-style-type:lower-roman"> | |
38 | by defining a member function <CODE>get</CODE> | |
39 | </LI> | |
40 | <LI STYLE="list-style-type:lower-roman"> | |
41 | by defining a member function <CODE>read</CODE> | |
42 | </OL> | |
43 | The second alternative is provided for enhanced performance. InputFilters implementing this alternative are referred to as <I>Multi-Character</I>. (<I>See</I> <A HREF="multi_character.html">Multi-Character Filter</A>.) | |
44 | </P> | |
45 | ||
46 | <A NAME="examples"></A> | |
47 | <H2>Examples</H2> | |
48 | ||
49 | <H4>I. Ordinary InputFilter</H4> | |
50 | ||
51 | <P> | |
52 | The following example shows an InputFilter which removes all non-alphabetic characters from a sequence. | |
53 | </P> | |
54 | ||
55 | <PRE CLASS="broken_ie"> <SPAN CLASS="preprocessor">#include</SPAN> <SPAN CLASS="literal"><ctype.h></SPAN> <SPAN CLASS="comment">// isalpha</SPAN> | |
56 | <SPAN CLASS="preprocessor">#include</SPAN> <SPAN CLASS="literal"><cstdio.h></SPAN> <SPAN CLASS="comment">// EOF</SPAN> | |
57 | <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">// input_filter_tag</SPAN> | |
58 | <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, WOULD_BLOCK</SPAN> | |
59 | ||
60 | <SPAN CLASS="keyword">using</SPAN> <SPAN CLASS="keyword">namespace</SPAN> std; | |
61 | <SPAN CLASS="keyword">using</SPAN> <SPAN CLASS="keyword">namespace</SPAN> boost::iostreams; | |
62 | ||
63 | <SPAN CLASS="keyword">struct</SPAN> alphabetic_input_filter { | |
64 | <SPAN CLASS="keyword">typedef</SPAN> <SPAN CLASS="keyword">char</SPAN> char_type; | |
65 | <SPAN CLASS="keyword">typedef</SPAN> input_filter_tag category; | |
66 | ||
67 | <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Source> | |
68 | <SPAN CLASS="keyword">int</SPAN> get(Source& src) | |
69 | { | |
70 | <SPAN CLASS="keyword">int</SPAN> c; | |
71 | <SPAN CLASS="keyword">while</SPAN> ( (c = boost::iostreams::get(src)) != EOF && | |
72 | c != WOULD_BLOCK && | |
73 | !isalpha((<SPAN CLASS="keyword">unsigned</SPAN> <SPAN CLASS="keyword">char</SPAN>) c) ) | |
74 | ; | |
75 | <SPAN CLASS="keyword">return</SPAN> c; | |
76 | } | |
77 | };</PRE> | |
78 | ||
79 | <P> | |
80 | Here <CODE>char_type</CODE> is the <A HREF="../guide/traits.html#char_type">character type</A> of the Filter, <CODE>input_filter_tag</CODE> is a <A HREF="../guide/traits.html#category_tags">category tag</A> identifying the Filter as a model of InputFilter, and the function <A HREF="../functions/get.html"><CODE>boost::iostreams::get</CODE></A> reads a character from an arbitrary Source.<A CLASS="footnote_ref" NAME="note_1_ref" HREF="#note_1"><SUP>[1]</SUP></A> The constant <A HREF="../classes/char_traits.html#WOULD_BLOCK"><CODE>WOULD_BLOCK</CODE></A>, defined in the header <A HREF="../../../../boost/iostreams/char_traits.hpp"><CODE><boost/iostreams/char_traits.hpp></CODE></A>, is used to indicate that input is temporarily unavilable. | |
81 | </P> | |
82 | ||
83 | <P> | |
84 | The Iostreams library defines two convenience classes, <A HREF="../classes/filter.html#synopsis"><CODE>input_filter</CODE></A> and <A HREF="../classes/filter.html#synopsis"><CODE>input_wfilter</CODE></A>, which provide member <CODE>typedef</CODE>s <CODE>char_type</CODE> and <CODE>category</CODE> as well as default implementations of several member functions. When defining a new model of InputFilter, it is often sufficient to derive from <CODE>input_filter</CODE> or <CODE>input_wfilter</CODE> and to define a member function <CODE>get</CODE>. | |
85 | </P> | |
86 | ||
87 | <H4>II. Multi-Character InputFilter</H4> | |
88 | ||
89 | <P> | |
90 | The following example shows a Multi-Character InputFilter which performs the same filtering operation as the Filter in Example I. | |
91 | </P> | |
92 | ||
93 | <PRE CLASS="broken_ie"> <SPAN CLASS="preprocessor">#include</SPAN> <SPAN CLASS="literal"><ctype.h></SPAN> <SPAN CLASS="comment">// isalpha</SPAN> | |
94 | <SPAN CLASS="preprocessor">#include</SPAN> <SPAN CLASS="literal"><cstdio.h></SPAN> <SPAN CLASS="comment">// EOF</SPAN> | |
95 | <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">// input_filter_tag</SPAN> | |
96 | <SPAN CLASS="preprocessor">#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/concepts.hpp"><SPAN CLASS="literal"><boost/iostreams/operations.hpp></SPAN></A> <SPAN CLASS="comment">// get</SPAN> | |
97 | ||
98 | <SPAN CLASS="keyword">using</SPAN> <SPAN CLASS="keyword">namespace</SPAN> std; | |
99 | <SPAN CLASS="keyword">using</SPAN> <SPAN CLASS="keyword">namespace</SPAN> boost::io; | |
100 | ||
101 | <SPAN CLASS="keyword">struct</SPAN> alphabetic_input_filter { | |
102 | <SPAN CLASS="keyword">typedef</SPAN> <SPAN CLASS="keyword">char</SPAN> char_type; | |
103 | <SPAN CLASS="keyword">typedef</SPAN> multichar_input_filter_tag category; | |
104 | ||
105 | <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Source> | |
106 | streamsize read(Source& src, <SPAN CLASS="keyword">char</SPAN>* s, streamsize n) | |
107 | { | |
108 | <SPAN CLASS="keyword">int</SPAN> c; | |
109 | <SPAN CLASS="keyword">char</SPAN>* first = s; | |
110 | <SPAN CLASS="keyword">char</SPAN>* last = s + n; | |
111 | <SPAN CLASS="keyword">while</SPAN> ( first != last && | |
112 | (c = boost::iostreams::get(src)) != EOF && | |
113 | c != WOULD_BLOCK && | |
114 | isalpha((<SPAN CLASS="keyword">unsigned</SPAN> <SPAN CLASS="keyword">char</SPAN>) c) ) | |
115 | { | |
116 | *first++ = c; | |
117 | } | |
118 | streamsize result = <SPAN CLASS="keyword">static_cast</SPAN><streamsize>(first - s); | |
119 | <SPAN CLASS="keyword">return</SPAN> result == <SPAN CLASS='numeric_literal'>0</SPAN> && c != WOULD_BLOCK ? | |
120 | <SPAN CLASS='numeric_literal'>-1</SPAN> : | |
121 | result; | |
122 | } | |
123 | };</PRE> | |
124 | ||
125 | <P> | |
126 | Here <CODE>multichar_input_filter_tag</CODE> is a <A HREF="../guide/traits.html#category">category tag</A> identifying the Filter as a Multi-Character InputFilter. | |
127 | </P> | |
128 | <P> | |
129 | The Iostreams library defines two convenience classes, <A HREF="../classes/filter.html#synopsis"><CODE>multichar_input_filter</CODE></A> and <A HREF="../classes/filter.html#synopsis"><CODE>multichar_input_wfilter</CODE></A>, which provide the member <CODE>typedef</CODE>s <CODE>char_type</CODE> and <CODE>category</CODE> as well as default implementations of several member functions. When defining a new Multi-Character InputFilter, it is often sufficient to derive from <CODE>multichar_input_filter</CODE> or <CODE>multichar_input_wfilter</CODE> and to define a member function <CODE>read</CODE>. | |
130 | </P> | |
131 | ||
132 | <A NAME="detailed_specification"></A> | |
133 | <H2>Refinement of</H2> | |
134 | ||
135 | <P><A HREF="filter.html">Filter</A>.</P> | |
136 | ||
137 | <H2>Associated Types</H2> | |
138 | ||
139 | <TABLE CELLPADDING="5" BORDER="1"> | |
140 | <TR><TD>Character type</TD><TD>The type of the characters in the filtered sequences</TD></TR> | |
141 | <TR> | |
142 | <TD>Category</TD> | |
143 | <TD> | |
144 | A type convertible to <A HREF="../guide/traits.html#category_tags"><CODE>filter_tag</CODE></A> and to <A HREF="../guide/modes.html#input"><CODE>input</CODE></A> | |
145 | </TD> | |
146 | </TR> | |
147 | <TR> | |
148 | <TD>Mode</TD> | |
149 | <TD> | |
150 | The unique <I>most-derived</I> <A HREF="../guide/modes.html#mode_tags">mode tag</A> to which Category is convertible | |
151 | </TD> | |
152 | </TR> | |
153 | </TABLE> | |
154 | ||
155 | <H2>Notation</H2> | |
156 | ||
157 | <TABLE CELLPADDING="2"> | |
158 | <TR><TD><CODE>F</CODE></TD><TD>- A type which is a model of InputFilter</TD></TR> | |
159 | <TR><TD><CODE>D</CODE></TD><TD>- A type which is a model of <A HREF="device.html">Device</A>, with the same character type as <CODE>F</CODE> and with mode refining the mode of <CODE>F</CODE></TD></TR> | |
160 | <TR><TD><CODE>Ch</CODE></TD><TD>- The character type of <CODE>F</CODE></TD></TR> | |
161 | <TR><TD><CODE>Tr</CODE></A></TD><TD>- <A HREF="../classes/char_traits.html"><CODE>boost::iostreams::char_traits<Ch></CODE></A></TD></TR> | |
162 | <TR><TD><CODE>f</CODE></TD><TD>- Object of type <CODE>F</CODE></TD></TR> | |
163 | <TR><TD><CODE>d</CODE></TD><TD>- Object of type <CODE>D</CODE></TD></TR> | |
164 | <TR><TD><CODE>s</CODE></TD><TD>- Object of type <CODE>Ch*</CODE></TD></TR> | |
165 | <TR><TD><CODE>n</CODE></TD><TD>- Object of type <CODE>std::streamsize</CODE></TD></TR> | |
166 | <TR><TD><CODE>io</CODE></TD><TD>- Alias for namespace <CODE>boost::iostreams</CODE></TD></TR> | |
167 | </TABLE> | |
168 | ||
169 | <A NAME="semantics"></A> | |
170 | <H2>Valid Expressions / Semantics</H2> | |
171 | ||
172 | <TABLE CELLPADDING="5" BORDER="1"> | |
173 | <TR><TH>Expression</TH><TH>Expression Type</TH><TH>Category Precondition</TH><TH>Semantics</TH></TR> | |
174 | <TR> | |
175 | <TD> | |
176 | <PRE CLASS="plain_code"><CODE>typename <A HREF="../guide/traits.html#char_type_of_ref">char_type_of</A><F>::type</CODE></PRE> | |
177 | </TD> | |
178 | <TD><CODE>typename</CODE> of the character type</TD> | |
179 | <TD ALIGN="center">-</TD><TD ALIGN="center">-</TD> | |
180 | </TR> | |
181 | <TR> | |
182 | <TD> | |
183 | <PRE CLASS="plain_code"><CODE>typename <A HREF="../guide/traits.html#category_ref">category_of</A><F>::type</CODE></PRE> | |
184 | </TD> | |
185 | <TD><CODE>typename</CODE> of the category</TD> | |
186 | <TD ALIGN="center">-</TD><TD ALIGN="center">-</TD> | |
187 | </TR> | |
188 | <TR> | |
189 | <TD><PRE CLASS="plain_code"><CODE>f.get(d)</CODE></PRE></TD> | |
190 | <TD><CODE>Tr::int_type</CODE></TD> | |
191 | <TD> | |
192 | Convertible to <A HREF="../guide/modes.html#mode_tags"><CODE>input</CODE></A> but not to <A HREF="../guide/traits.html#category_tags"><CODE>multichar_tag</CODE></A> | |
193 | </TD> | |
194 | <TD> | |
195 | Returns the next character in the input sequence controlled by <CODE>f</CODE>, <CODE>Tr::eof()</CODE> if the end of the sequence has been reached or <CODE>Tr::would_block()</CODE> if input is temporarily unavilable because a call to <CODE>d</CODE> has produced fewer characters than requested. The input sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/get.html"><CODE>io::get</CODE></A>, <A HREF="../functions/read.html"><CODE>io::read</CODE></A> and <A HREF="../functions/putback.html"><CODE>io::putback</CODE></A>. | |
196 | </TD> | |
197 | </TR> | |
198 | <TR> | |
199 | <TD><PRE CLASS="plain_code"><CODE>f.read(d, s, n)</CODE></PRE></TD> | |
200 | <TD><PRE CLASS="plain_code"><CODE>std::streamsize</CODE></PRE></TD> | |
201 | <TD> | |
202 | Convertible to <A HREF="../guide/modes.html#mode_tags"><CODE>input</CODE></A> and to <A HREF="../guide/traits.html#category_tags"><CODE>multichar_tag</CODE></A> | |
203 | </TD> | |
204 | <TD> | |
205 | Reads up to <CODE>n</CODE> characters from the input sequence controlled by <CODE>f</CODE> into the buffer <CODE>s</CODE>, returning the number of characters read or <CODE>-1</CODE> to indicate end-of-sequence. A value less than <CODE>n</CODE> may be returned only at end-of-sequence or if input is temporarily unavilable because a call to <CODE>d</CODE> has produced fewer characters than requested. The input sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/get.html"><CODE>io::get</CODE></A>, <A HREF="../functions/read.html"><CODE>io::read</CODE></A> and <A HREF="../functions/putback.html"><CODE>io::putback</CODE></A>. | |
206 | </TD> | |
207 | </TR> | |
208 | </TABLE> | |
209 | ||
210 | <H2>Exceptions</H2> | |
211 | ||
212 | <P> | |
213 | Errors which occur during the execution of <CODE>get</CODE> and <CODE>read</CODE> are indicated by throwing exceptions. Reaching the end of the sequence is not an error. | |
214 | </P> | |
215 | ||
216 | <P> | |
217 | After an exception is thrown, an InputFilter must be in a consistent state; further i/o operations may throw exceptions but must have well-defined behaviour. Furthermore, unless it is <A HREF="closable.html">Closable</A>, it must be ready to begin processing a new character sequence. | |
218 | </P> | |
219 | ||
220 | <H2>Models</H2> | |
221 | ||
222 | <UL> | |
223 | <LI>The <A HREF="../guide/text_processing.html">Text Processing Filters</A>. | |
224 | <LI>The compression and decompression filters. | |
225 | </UL> | |
226 | ||
227 | <H2>Acknowledgments</H2> | |
228 | ||
229 | <P> | |
230 | The concept InputFilter was inspired by the <I>extractors</I> of <A CLASS="footnote_ref" HREF="../bibliography.html#kanze">[Kanze]</A>. | |
231 | </P> | |
232 | ||
233 | <!-- Begin Footnotes --> | |
234 | ||
235 | <HR> | |
236 | ||
237 | <P> | |
238 | <A CLASS="footnote_ref" NAME="note_1" HREF="#note_1_ref"><SUP>[1]</SUP></A>Technically, <CODE>boost::iostreams::get</CODE> requires that a Source be <A HREF="../concepts/direct.html"><I>indirect</I></A>. | |
239 | </P> | |
240 | ||
241 | <!-- End Footnotes --> | |
242 | ||
243 | <!-- Begin Footer --> | |
244 | ||
245 | <HR> | |
246 | ||
247 | <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> | |
248 | <P CLASS="copyright"> | |
249 | Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at <A HREF="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</A>) | |
250 | </P> | |
251 | ||
252 | <!-- End Footer --> | |
253 | ||
254 | </BODY> |