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="https://www.boost.org/users/people/jonathan_turkanis.html" 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>