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

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   4     <TITLE>OutputFilter</TITLE>
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  12     <H1 CLASS="title">OutputFilter</H1>
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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 OutputFilter is a <A HREF="filter.html">Filter</A> whose <A HREF="../guide/modes.html">mode</A> refines <A HREF="../guide/modes.html#output">output</A>.
  29 </P>
  30
  31 <A NAME="description"></A>
  32 <H2>Description</H2>
  33
  34 <P>
  35     An OutputFilter operates on the character sequence controlled by a <A HREF="sink.html">Sink</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>put</CODE>
  39         </LI>
  40         <LI STYLE="list-style-type:lower-roman">
  41             by defining a member function <CODE>write</CODE>
  42     </OL>
  43     The second alternative is provided for enhanced performance. OutputFilters implementing this alternative are referred to as 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 OutputFilter</H4>
  50
  51 <P>
  52     The following example shows an OutputFilter which converts each character in a sequence to uppercase.
  53 </P>
  54
  55 <PRE CLASS="broken_ie">    <SPAN CLASS="preprocessor">#include</SPAN> <SPAN CLASS="literal">&lt;ctype.h&gt;</SPAN>                        <SPAN CLASS="comment">// toupper</SPAN>
  56     <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">// output_filter_tag</SPAN>
  57     <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">// put</SPAN>
  58
  59     <SPAN CLASS="keyword">using</SPAN> <SPAN CLASS="keyword">namespace</SPAN> std;
  60     <SPAN CLASS="keyword">namespace</SPAN> io = boost::iostreams;
  61
  62     <SPAN CLASS="keyword">struct</SPAN> toupper_output_filter {
  63         <SPAN CLASS="keyword">typedef</SPAN> <SPAN CLASS="keyword">char</SPAN>                   char_type;
  64         <SPAN CLASS="keyword">typedef</SPAN> io::output_filter_tag  category;
  65
  66         <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Sink&gt;
  67         <SPAN CLASS="keyword">bool</SPAN> put(Sink&amp; snk, <SPAN CLASS="keyword">char</SPAN> c)
  68         {
  69             <SPAN CLASS="keyword">return</SPAN> io::put(snk, toupper((<SPAN CLASS="keyword">unsigned</SPAN> <SPAN CLASS="keyword">char</SPAN>) c));
  70         }
  71     };</PRE>
  72
  73 <P>
  74     Here <CODE>char_type</CODE> is the <A HREF="../guide/traits.html#char_type">character type</A> of the Filter, <CODE>output_filter_tag</CODE> is a <A HREF="../guide/traits.html#category">category tag</A> identifying the Filter as a model of OutputFilter, and the function <A HREF="../functions/put.html"><CODE>io::put</CODE></A> writes a character to an arbitrary Sink.<A CLASS="footnote_ref" NAME="note_1_ref" HREF="#note_1"><SUP>[1]</SUP></A>
  75 </P>
  76
  77 <P>
  78     The Iostreams library defines two convenience classes, <A HREF="../classes/filter.html#synopsis"><CODE>output_filter</CODE></A> and <A HREF="../classes/filter.html#synopsis"><CODE>output_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 OutputFilter, it is often sufficient to derive from <CODE>output_filter</CODE> or <CODE>output_wfilter</CODE> and to define a member function <CODE>put</CODE>.
  79 </P>
  80
  81 <H4>II. Multi-Character OutputFilter</H4>
  82
  83 <P>
  84     The following example shows a Multi-Character OutputFilter which performs the same filtering operation as the Filter in Example I.
  85 </P>
  86
  87 <PRE CLASS="broken_ie">    <SPAN CLASS="preprocessor">#include</SPAN> <SPAN CLASS="literal">&lt;ctype.h&gt;</SPAN>                        <SPAN CLASS="comment">// toupper</SPAN>
  88     <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">// output_filter_tag</SPAN>
  89     <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">// put</SPAN>
  90
  91     <SPAN CLASS="keyword">using</SPAN> <SPAN CLASS="keyword">namespace</SPAN> std;
  92     <SPAN CLASS="keyword">namespace</SPAN> io = boost::iostreams;
  93
  94     <SPAN CLASS="keyword">struct</SPAN> toupper_filter {
  95         <SPAN CLASS="keyword">typedef</SPAN> <SPAN CLASS="keyword">char</SPAN>                             char_type;
  96         <SPAN CLASS="keyword">typedef</SPAN> io::multichar_output_filter_tag  category;
  97
  98         <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Sink&gt;
  99         <SPAN CLASS="keyword">std::streamsize</SPAN> write(Sink&amp; snk, <SPAN CLASS="keyword">const</SPAN> <SPAN CLASS="keyword">char</SPAN>* s, streamsize n)
 100         {
 101             std::streamsize rest = n;
 102             <SPAN CLASS="keyword">while</SPAN> (rest != 0 &amp;&amp; io::put(snk, toupper((<SPAN CLASS="keyword">unsigned</SPAN> <SPAN CLASS="keyword">char</SPAN>) *s++))
 103                 --rest;
 104             <SPAN CLASS="keyword">return</SPAN> n - rest;
 105         }
 106     };</PRE>
 107
 108 <P>
 109     Here <CODE>multichar_output_filter_tag</CODE> is a <A HREF="../guide/traits.html#category">category tag</A> identifying the Filter as a multichar OutputFilter.
 110 </P>
 111 <P>
 112     The Iostreams library defines two convenience classes, <A HREF="../classes/filter.html#synopsis"><CODE>multichar_output_filter</CODE></A> and <A HREF="../classes/filter.html#synopsis"><CODE>multichar_output_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 multichar OutputFilter, it is often sufficient to derive from <CODE>multichar_output_filter</CODE> or <CODE>multichar_output_wfilter</CODE> and to define a member function <CODE>write</CODE>.
 113 </P>
 114
 115 <A NAME="detailed_specification"></A>
 116 <H2>Refinement of</H2>
 117
 118 <P><A HREF="filter.html">Filter</A>.</P>
 119
 120 <H2>Associated Types</H2>
 121
 122 <TABLE CELLPADDING="5" BORDER="1">
 123     <TR><TD>Character type</TD><TD>The type of the characters in the filtered sequences</TD></TR>
 124     <TR>
 125         <TD>Category</TD>
 126         <TD>
 127             A type convertible to <A HREF="../guide/traits.html#category_tags"><CODE>filter_tag</CODE></A> and to <A HREF="../guide/modes.html#output"><CODE>output</CODE></A>
 128         </TD>
 129     </TR>
 130     <TR>
 131         <TD>Mode</TD>
 132         <TD>
 133             The unique <I>most-derived</I> <A HREF="../guide/modes.html#mode_tags">mode tag</A> to which Category is convertible
 134         </TD>
 135     </TR>
 136 </TABLE>
 137
 138 <H2>Notation</H2>
 139
 140 <TABLE CELLPADDING="2">
 141     <TR><TD><CODE>F</CODE></TD><TD>- A type which is a model of OutputFilter</TD></TR>
 142     <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>
 143     <TR><TD><CODE>Ch</CODE></TD><TD>- The character type of <CODE>F</CODE></TD></TR>
 144     <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>
 145     <TR><TD><CODE>f</CODE></TD><TD>- Object of type <CODE>F</CODE></TD></TR>
 146     <TR><TD><CODE>d</CODE></TD><TD>- Object of type <CODE>D</CODE></TD></TR>
 147     <TR><TD><CODE>c</CODE></TD><TD>- Object of type <CODE>Ch</CODE></TD></TR>
 148     <TR><TD><CODE>s</CODE></TD><TD>- Object of type <CODE>const Ch*</CODE></TD></TR>
 149     <TR><TD><CODE>n</CODE></TD><TD>- Object of type <CODE>std::streamsize</CODE></TD></TR>
 150     <TR><TD><CODE>io</CODE></TD><TD>- Alias for namespace <CODE>boost::iostreams</CODE></TD></TR>
 151 </TABLE>
 152
 153 <A NAME="semantics"></A>
 154 <H2>Valid Expressions / Semantics</H2>
 155
 156 <TABLE CELLPADDING="5" BORDER="1">
 157     <TR><TH>Expression</TH><TH>Expression Type</TH><TH>Category Precondition</TH><TH>Semantics</TH></TR>
 158     <TR>
 159         <TD>
 160             <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>
 161         </TD>
 162         <TD><CODE>typename</CODE> of the character type</TD>
 163         <TD ALIGN="center">-</TD><TD ALIGN="center">-</TD>
 164     </TR>
 165     <TR>
 166         <TD>
 167             <PRE CLASS="plain_code"><CODE>typename <A HREF="../guide/traits.html#category_ref">category_of</A>&lt;F&gt;::type</CODE></PRE>
 168         </TD>
 169         <TD><CODE>typename</CODE> of the category</TD>
 170         <TD ALIGN="center">-</TD><TD ALIGN="center">-</TD>
 171     </TR>
 172     <TR>
 173         <TD><PRE CLASS="plain_code"><CODE>f.put(d, c)</CODE></PRE></TD>
 174         <TD><CODE>bool</CODE></TD>
 175         <TD>
 176             Convertible to <A HREF="../guide/modes.html#mode_tags"><CODE>output</CODE></A> but not to <A HREF="../guide/traits.html#category_tags"><CODE>multichar_tag</CODE></A>
 177         </TD>
 178         <TD>
 179             Attempts to writes the character <CODE>c</CODE> to the output sequence controlled by <CODE>f</CODE>, returning <CODE>false</CODE> if <CODE>c</CODE> cannot be consumed because a call to <CODE>d</CODE> has consumed fewer characters than requested. The output sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/put.html"><CODE>io::put</CODE></A> and <A HREF="../functions/write.html"><CODE>io::write</CODE></A>.
 180         </TD>
 181     </TR>
 182     <TR>
 183         <TD><PRE CLASS="plain_code"><CODE>f.write(d, s, n)</CODE></PRE></TD>
 184         <TD><PRE CLASS="plain_code"><CODE>std::streamsize</CODE></PRE></TD>
 185         <TD>
 186             Convertible to <A HREF="../guide/modes.html#mode_tags"><CODE>output</CODE></A> and to <A HREF="../guide/traits.html#category_tags"><CODE>multichar_tag</CODE></A>
 187         </TD>
 188         <TD>
 189             Writes up to <CODE>n</CODE> characters from the buffer <CODE>s</CODE> to the output sequence controlled by <CODE>d</CODE>, returning the number of characters written. A value less than <CODE>n</CODE> may be returned only if a call to <CODE>d</CODE> has consumed fewer characters than requested. The output sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/put.html"><CODE>io::put</CODE></A> and <A HREF="../functions/write.html"><CODE>io::write</CODE></A>.
 190         </TD>
 191     </TR>
 192 </TABLE>
 193
 194 <H2>Exceptions</H2>
 195
 196 <P>
 197     Errors which occur during the execution of <CODE>put</CODE> or <CODE>write</CODE> are indicated by throwing exceptions. Attempting to write past the end of the sequence is always an error.
 198 </P>
 199
 200 <P>
 201     After an exception is thrown, an OutputFilter 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.
 202 </P>
 203
 204 <H2>Models</H2>
 205
 206 <H2>Acknowledgments</H2>
 207
 208 <P>
 209     The concept OutputFilter was inspired by the <I>inserters</I> of <A CLASS="footnote_ref" HREF="../bibliography.html#kanze">[Kanze]</A>.
 210 </P>
 211
 212 <!-- Begin Footnotes -->
 213
 214 <HR>
 215
 216 <P>
 217     <A CLASS="footnote_ref" NAME="note_1" HREF="#note_1_ref"><SUP>[1]</SUP></A>Technically, <CODE>boost::iostreams::put</CODE> requires that a Sink be <A HREF="../concepts/direct.html"><I>indirect</I></A>.
 218 </P>
 219
 220 <!-- End Footnotes -->
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 224
 225 <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>
 226 <P CLASS="copyright">
 227     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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