ceph/src/boost/libs/iostreams/doc/concepts/input_filter.html

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   4     <TITLE>InputFilter</TITLE>
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  11
  12     <H1 CLASS="title">InputFilter</H1>
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  14
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  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">&lt;ctype.h&gt;</SPAN>                 <SPAN CLASS="comment">// isalpha</SPAN>
  56     <SPAN CLASS="preprocessor">#include</SPAN> <SPAN CLASS="literal">&lt;cstdio.h&gt;</SPAN>                <SPAN CLASS="comment">// EOF</SPAN>
  57     <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">// input_filter_tag</SPAN>
  58     <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, 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>&lt;<SPAN CLASS="keyword">typename</SPAN> Source&gt;
  68         <SPAN CLASS="keyword">int</SPAN> get(Source&amp; src)
  69         {
  70             <SPAN CLASS="keyword">int</SPAN> c;
  71             <SPAN CLASS="keyword">while</SPAN> ( (c = boost::iostreams::get(src)) != EOF &amp;&amp;
  72                      c != WOULD_BLOCK &amp;&amp;
  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>&lt;boost/iostreams/char_traits.hpp&gt;</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">&lt;ctype.h&gt;</SPAN>                 <SPAN CLASS="comment">// isalpha</SPAN>
  94     <SPAN CLASS="preprocessor">#include</SPAN> <SPAN CLASS="literal">&lt;cstdio.h&gt;</SPAN>                <SPAN CLASS="comment">// EOF</SPAN>
  95     <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">// input_filter_tag</SPAN>
  96     <SPAN CLASS="preprocessor">#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/concepts.hpp"><SPAN CLASS="literal">&lt;boost/iostreams/operations.hpp&gt;</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>&lt;<SPAN CLASS="keyword">typename</SPAN> Source&gt;
 106         streamsize read(Source&amp; 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 &amp;&amp;
 112                         (c = boost::iostreams::get(src)) != EOF &amp;&amp;
 113                          c != WOULD_BLOCK &amp;&amp;
 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>&lt;streamsize&gt;(first - s);
 119                 <SPAN CLASS="keyword">return</SPAN> result == <SPAN CLASS='numeric_literal'>0</SPAN> &amp;&amp; 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&lt;Ch&gt;</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>&lt;F&gt;::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>&lt;F&gt;::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 -->
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 246
 247 <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>
 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>)
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