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

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  12     <H1 CLASS="title">SeekableDevice</H1>
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  16
  17 <H2>Definition</H2>
  18
  19 <P>
  20     A SeekableDevice is a <A HREF="device.html">Device</A> whose <A HREF="../guide/modes.html">mode</A> refines <A HREF="../guide/modes.html#seekable">seekable</A>.
  21 </P>
  22
  23 <H2>Description</H2>
  24
  25 <P>
  26     A SeekableDevice provides read- write- and random-access to a sequence of characters of a given type, with a single repositionable read/write head. A SeekableDevice may expose this sequence in two ways:
  27     <OL>
  28         <LI STYLE="list-style-type:lower-roman">
  29             by defining member functions <CODE>read</CODE>, <CODE>write</CODE> and <CODE>seek</CODE>; or
  30         </LI>
  31         <LI STYLE="list-style-type:lower-roman">
  32             by defining member functions <CODE>input_sequence</CODE> and <CODE>output_sequence</CODE>, returning pairs of pointers delimiting the sequences in its entirety. The return values of these two functions must be the same.<A CLASS="footnote_ref" NAME="note_1_ref" HREF="#note_1"><SUP>[1]</SUP></A>
  33         </LI>
  34     </OL>
  35 </P>
  36 <P>A SeekableDevice has mode <A HREF="../guide/modes.html#seekable"><CODE>seekable</CODE></A>.</P>
  37
  38 <H2>Note</H2>
  39
  40 <P>To be usable with streams and stream buffers, SeekableDevices must model <A HREF="blocking.html">Blocking</A>.
  41
  42 <H2>Example</H2>
  43
  44 <P>A model of SeekableDevice can be defined as follows:</P>
  45
  46 <PRE CLASS="broken_ie"><SPAN CLASS="keyword">struct</SPAN> SeekableDevice {
  47     <SPAN CLASS="keyword">typedef</SPAN> <SPAN CLASS="keyword">char</SPAN>                 char_type;
  48     <SPAN CLASS="keyword">typedef</SPAN> seekable_device_tag  category;
  49     std::streamsize read(<SPAN CLASS="keyword">char</SPAN>* s, std::streamsize n)
  50         {
  51             <SPAN CLASS="comment">// Read up to n characters from the input
  52             // sequence into the buffer s, returning
  53             // the number of characters read, or -1
  54             // to indicate end-of-sequence.</SPAN>
  55         }
  56     <SPAN CLASS="keyword">void</SPAN> write(<SPAN CLASS="keyword">const</SPAN> <SPAN CLASS="keyword">char</SPAN>* s, std::streamsize n)
  57         {
  58             <SPAN CLASS="comment">// Write up to n characters from the buffer
  59             // s to the output sequence, returning the
  60             // number of characters written</SPAN>
  61         }
  62     std::streampos seek(stream_offset off, std::ios_base::seekdir way)
  63         {
  64             <SPAN CLASS="comment">// Advances the read/write head by off characters,
  65             // returning the new position, where the offset is
  66             // calculated from:
  67             //  - the start of the sequence if way == ios_base::beg
  68             //  - the current position if way == ios_base::cur
  69             //  - the end of the sequence if way == ios_base::end</SPAN>
  70         }
  71 };</PRE>
  72
  73 <P>
  74     Here <CODE>seekable_device_tag</CODE> is a <A HREF="../guide/traits.html#category_tags">category tag</A> identifying the containing type as a model of SeekableDevice. When defining a new SeekableDevice, it suffices to use the tag <CODE>seekable_device_tag</CODE>. One can also derive from the helper classes <A HREF="../classes/device.html"><CODE>device&lt;seekable&gt;</CODE></A> or <A HREF="../classes/device.html#synopsis"><CODE>wdevice&lt;seekable&gt;</CODE></A>.
  75 </P>
  76
  77 <H2>Refinement of</H2>
  78
  79 <P><A HREF="source.html">Source</A>, <A HREF="sink.html">Sink</A>.</P>
  80
  81 <H2>Associated Types</H2>
  82
  83 <P>Same as <A HREF="device.html#types">Device</A>, with the following additional requirements:</P>
  84
  85 <TABLE CELLPADDING="5" BORDER="1">
  86     <TR><TD>Category</TD><TD>A type convertible to <A HREF="../guide/traits.html#category_tags"><CODE>device_tag</CODE></A> and to <A HREF="../guide/modes.html#mode_tags"><CODE>seekable</CODE></A></TD></TR>
  87 </TABLE>
  88
  89 <H2>Notation</H2>
  90
  91 <TABLE CELLPADDING="2">
  92     <TR><TD><CODE>D</CODE></TD><TD>- A type which is a model of SeekableDevice</TD></TR>
  93     <TR><TD><CODE>Ch</CODE></TD><TD>- The character type of <CODE>D</CODE></TD></TR>
  94     <TR><TD><CODE>dev</CODE></TD><TD>- Object of type <CODE>D</CODE></TD></TR>
  95     <TR><TD><CODE>s1</CODE></TD><TD>- Object of type <CODE>Ch*</CODE></SPAN></TD></TR>
  96     <TR><TD><CODE>s2</CODE></TD><TD>- Object of type <CODE>const Ch*</CODE></SPAN></TD></TR>
  97     <TR><TD><CODE>n</CODE></TD><TD>- Object of type <CODE>std::streamsize</CODE></TD></TR>
  98     <TR><TD><CODE>off</CODE></TD><TD>- Object of type <A HREF="../functions/positioning.html#synopsis"><CODE>stream_offset</CODE></A></TD></TR>
  99     <TR><TD><CODE>way</CODE></TD><TD>- Object of type <CODE>std::ios_base::seekdir</CODE></TD></TR>
 100     <TR><TD><CODE>io</CODE></TD><TD>- Alias for namespace <CODE>boost::iostreams</CODE></TD></TR>
 101 </TABLE>
 102
 103 <H2>Valid Expressions / Semantics</H2>
 104
 105 <P>Same as <A HREF="device.html#types">Device</A>, with the following additional requirements:</P>
 106
 107 <TABLE CELLPADDING="5" BORDER="1">
 108     <TR><TH>Expression</TH><TH>Expression Type</TH><TH>Category Precondition</TH><TH>Semantics</TH></TR>
 109     <TR>
 110         <TD><PRE CLASS="plain_code"><CODE><A HREF="../functions/read.html">io::read</A>(dev, s1, n)</CODE></PRE></TD>
 111         <TD><CODE>std::streamsize</CODE></TD>
 112         <TD ROWSPAN="3">Not convertible to <A HREF="direct.html"><CODE>direct_tag</CODE></A></TD>
 113         <TD>
 114             Reads up to <CODE>n</CODE> characters from the sequence controlled by <CODE>dev</CODE> into <CODE>s1</CODE>, returning the number of characters read, or <CODE>-1</CODE> to indicate end-of-sequence
 115         </TD>
 116     </TR>
 117     <TR>
 118         <TD><PRE CLASS="plain_code"><CODE><A HREF="../functions/write.html">io::write</A>(dev, s2, n)</CODE></PRE></TD>
 119         <TD><CODE>std::streamsize</CODE></TD>
 120         <TD>
 121             Writes up to <CODE>n</CODE> characters from the sequence beginning at <CODE>s2</CODE> to the sequence controlled by <CODE>dev</CODE>, returning the number of characters written
 122         </TD>
 123     </TR>
 124     <TR>
 125         <TD><PRE CLASS="plain_code"><CODE><A HREF="../functions/seek.html">io::seek</A>(dev, off, way)</CODE></PRE></TD>
 126         <TD><CODE>std::streampos</CODE></TD>
 127         <TD>Advances the read/write head by <CODE>off</CODE> characters, returning the new position, where the offset is calculated from:
 128             <UL STYLE="margin:0,0,0,auto">
 129                 <LI STYLE="list-style-type:disc;list-style-image:none">the start of the sequence if <CODE>way</CODE> is <CODE>ios_base::beg</CODE>
 130                 <LI STYLE="list-style-type:disc;list-style-image:none">the current position if <CODE>way</CODE> is <CODE>ios_base::cur</CODE>
 131                 <LI STYLE="list-style-type:disc;list-style-image:none">the end of the sequence if <CODE>way</CODE> is <CODE>ios_base::end</CODE>
 132             </UL>
 133         </TD>
 134     </TR>
 135     <TR>
 136         <TD><PRE CLASS="plain_code"><CODE>dev.input_sequence()</CODE></PRE></TD>
 137         <TD><PRE CLASS="plain_code"><CODE>std::pair&lt;Ch*,Ch*&gt;</CODE></PRE></TD>
 138         <TD ROWSPAN="2">Convertible to <A HREF="direct.html"><CODE>direct_tag</CODE></A></TD>
 139         <TD>Returns a pair of pointers delimiting the sequence controlled by <CODE>dev</CODE></TD>
 140     </TR>
 141     <TR>
 142         <TD><PRE CLASS="plain_code"><CODE>dev.output_sequence()</CODE></PRE></TD>
 143         <TD><PRE CLASS="plain_code"><CODE>std::pair&lt;Ch*,Ch*&gt;</CODE></PRE></TD>
 144         <TD>Returns a pair of pointers delimiting the sequence controlled by <CODE>dev</CODE></TD>
 145     </TR>
 146 </TABLE>
 147
 148 <H2>Exceptions</H2>
 149
 150 <P>
 151     Errors which occur during the execution of member functions <CODE>read</CODE>, <CODE>write</CODE>, <CODE>seek</CODE>, <CODE>input_sequence</CODE> or <CODE>output_sequence</CODE> are indicated by throwing exceptions. Reaching the end of the sequence is not an error, but attempting to write past the end of the sequence is.
 152 </P>
 153
 154 <P>
 155     After an exception is thrown, a SeekableDevice must be in a consistent state; further i/o operations may throw exceptions but must have well-defined behaviour.
 156 </P>
 157
 158 <H2>Models</H2>
 159
 160 <UL>
 161     <LI><A HREF="../classes/array.html#array"><CODE>array</CODE></A>, <A HREF="../classes/file.html#file"><CODE>file</CODE></A>, <A HREF="../classes/file_descriptor.html#file_descriptor"><CODE>file_descriptor</CODE></A>, <A HREF="../classes/mapped_file.html#mapped_file"><CODE>mapped_file</CODE></A>.
 162 </UL>
 163
 164 <!-- Begin Footnotes -->
 165
 166 <P>
 167     <A CLASS="footnote_ref" NAME="note_1" HREF="#note_1_ref"><SUP>[1]</SUP></A>A more elegant specification would require a single member function <CODE>sequence</CODE>; the present version was selected to simplify the implementation of <A HREF="../guide/generic_streams.html#stream_buffer"><CODE>stream_buffer</CODE></A>.
 168 </P>
 169
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 176 <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>
 177 <P CLASS="copyright">
 178     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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