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

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  12     <H1 CLASS="title">BidirectionalDevice</H1>
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  16
  17 <H2>Definition</H2>
  18
  19 <P>
  20     An BidirectionalDevice is a <A HREF="device.html">Device</A> whose <A HREF="../guide/modes.html">mode</A> refines <A HREF="../guide/modes.html#bidirectional">bidirectional</A>.
  21 </P>
  22
  23 <H2>Description</H2>
  24
  25 <P>
  26     An BidirectionalDevice provides read-access to a sequence of characters of a given type and write-access
  27     to a separate sequence of characters of the same type. An BidirectionalDevice may expose these sequences in three ways:<A CLASS="footnote_ref" NAME="note_1_ref" HREF="#note_1"><SUP>[1]</SUP></A>
  28     <OL>
  29         <LI STYLE="list-style-type:lower-roman">
  30             by defining member functions <CODE>read</CODE> and <CODE>write</CODE> , invoked indirectly by the Iostreams Library through the functions <A HREF="../functions/read.html"><CODE>boost::iostreams::read</CODE></A> and <A HREF="../functions/write.html"><CODE>boost::iostreams::write</CODE></A>;
  31         </LI>
  32         <LI STYLE="list-style-type:lower-roman">
  33             by overloading or specializing <A HREF="../functions/read.html"><CODE>boost::iostreams::read</CODE></A> and <A HREF="../functions/write.html"><CODE>boost::iostreams::write</CODE></A>; or
  34         </LI>
  35         <LI STYLE="list-style-type:lower-roman">
  36             by defining member functions <CODE>input_sequence</CODE> and <CODE>output_sequence</CODE> returning pairs of pointers delimiting the two sequences in their entirety.
  37         </LI>
  38     </OL>
  39 </P>
  40
  41 <P>The i/o mode of a BidirectionalDevice is <A HREF="../guide/modes.html#bidirectional">bidirectional</A> or <A HREF="../guide/modes.html#bidirectional_seekable">bidirectional-seekable</A>.</P>
  42
  43 <H2>Example</H2>
  44
  45 <P>A model of BidirectionalDevice can be defined as follows:</P>
  46
  47 <PRE CLASS="broken_ie"><SPAN CLASS="keyword">struct</SPAN> BidirectionalDevice {
  48     <SPAN CLASS="keyword">typedef</SPAN> <SPAN CLASS="keyword">char</SPAN>                char_type;
  49     <SPAN CLASS="keyword">typedef</SPAN> bidirectional_device_tag    category;
  50     std::streamsize read(<SPAN CLASS="keyword">char</SPAN>* s, std::streamsize n)
  51         {
  52             <SPAN CLASS="comment">// Reads up to n characters from the input
  53             // sequence into the buffer s, returning the number
  54             // of characters read. Returning a value less than n
  55             // indicates end-of-sequence.</SPAN>
  56         }
  57     std::streamsize write(<SPAN CLASS="keyword">const</SPAN> <SPAN CLASS="keyword">char</SPAN>* s, std::streamsize n)
  58         {
  59             <SPAN CLASS="comment">// Write up to n characters from the buffer
  60             // s to the output sequence, returning the
  61             // number of characters written</SPAN>
  62         }
  63 };</PRE>
  64
  65 <P>
  66     Here <CODE>category</CODE> is a tag <CODE>struct</CODE> identifying the containing type as a model of BidirectionalDevice. When defining a new BidirectionalDevice, it suffices to use the tag <CODE>bidirectional_device_tag</CODE>. One can also derive from the helper classes <A HREF="../classes/device.html"><CODE>device&lt;bidirectional&gt;</CODE></A> or <A HREF="../classes/device.html#synopsis"><CODE>wdevice&lt;bidirectional&gt;</CODE></A>.
  67 </P>
  68
  69 <H2>Refinement of</H2>
  70
  71 <P><A HREF="source.html">Source</A>, <A HREF="sink.html">Sink</A>.</P>
  72
  73 <H2>Associated Types</H2>
  74
  75 <P>Same as <A HREF="device.html#types">Device</A>, with the following additional requirements:</P>
  76
  77 <TABLE CELLPADDING="5" BORDER="1">
  78     <TR><TD>Category</TD><TD>A type convertible to <A HREF="../guide/traits.html#category_tags">device_tag</A> and to <A HREF="../guide/modes.html#mode_tags"><CODE>bidirectional</CODE></A></TD></TR>
  79 </TABLE>
  80
  81 <H2>Notation</H2>
  82
  83 <TABLE CELLPADDING="2">
  84     <TR><TD><CODE>D</CODE></TD><TD>- A type which is a model of BidirectionalDevice</TD></TR>
  85     <TR><TD><CODE>Ch</CODE></TD><TD>- The character type of <CODE>D</CODE></TD></TR>
  86     <TR><TD><CODE>dev</CODE></TD><TD>- Object of type <CODE>D</CODE></TD></TR>
  87     <TR><TD><CODE>s1</CODE></TD><TD>- Object of type <CODE>Ch*</CODE></SPAN></TD></TR>
  88     <TR><TD><CODE>s2</CODE></TD><TD>- Object of type <CODE>const Ch*</CODE></SPAN></TD></TR>
  89     <TR><TD><CODE>n</CODE></TD><TD>- Object of type <CODE>std::streamsize</CODE></TD></TR>
  90 </TABLE>
  91
  92 <H2>Valid Expressions / Semantics</H2>
  93
  94 <P>Same as <A HREF="device.html#types">Device</A>, with the following additional requirements:</P>
  95
  96 <TABLE CELLPADDING="5" BORDER="1">
  97     <TR><TH>Expression</TH><TH>Expression Type</TH><TH>Category Precondition</TH><TH>Semantics</TH></TR>
  98     <TR>
  99         <TD><PRE CLASS="plain_code"><CODE><A HREF="../functions/read.html">boost::iostreams::read</A>(dev, s1, n)</CODE></PRE></TD>
 100         <TD><CODE>std::streamsize</CODE></TD>
 101         <TD ROWSPAN="2">Not convertible to <A HREF="direct.html"><CODE>direct_tag</CODE></A></TD>
 102         <TD>
 103             Reads up to <CODE>n</CODE> characters from the input sequence controlled by <CODE>dev</CODE> into <CODE>s1</CODE>, returning the number of characters read; returning a value less than n indicates end-of-sequence
 104         </TD>
 105     </TR>
 106     <TR>
 107         <TD><PRE CLASS="plain_code"><CODE><A HREF="../functions/write.html">boost::iostreams::write</A>(dev, s2, n)</CODE></PRE></TD>
 108         <TD><CODE>std::streamsize</CODE></TD>
 109         <TD>
 110             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
 111         </TD>
 112     </TR>
 113     <TR>
 114         <TD><PRE CLASS="plain_code"><CODE>dev.input_sequence()</CODE></PRE></TD>
 115         <TD><PRE CLASS="plain_code"><CODE>std::pair&lt;Ch*,Ch*&gt;</CODE></PRE></TD>
 116         <TD ROWSPAN="2">Convertible to <A HREF="direct.html"><CODE>direct_tag</CODE></A></TD>
 117         <TD>Returns a pair of pointers delimiting the input sequence controlled by <CODE>dev</CODE></TD>
 118     </TR>
 119     <TR>
 120         <TD><PRE CLASS="plain_code"><CODE>dev.output_sequence()</CODE></PRE></TD>
 121         <TD><PRE CLASS="plain_code"><CODE>std::pair&lt;Ch*,Ch*&gt;</CODE></PRE></TD>
 122         <TD>Returns a pair of pointers delimiting the output sequence controlled by <CODE>dev</CODE></TD>
 123     </TR>
 124 </TABLE>
 125
 126 <H2>Exceptions</H2>
 127
 128 <P>
 129     Errors which occur during the execution of member functions <CODE>read</CODE>, <CODE>write</CODE>, <CODE>input_sequence</CODE> or <CODE>output_sequence</CODE> are indicated by throwing exceptions. Reaching the end of the input sequence is not an error, but attempting to write past the end of the output sequence is.
 130 </P>
 131
 132 <P>
 133     After an exception is thrown, an BidirectionalDevice must be in a consistent state; further i/o operations may throw exceptions but must have well-defined behaviour.
 134 </P>
 135
 136
 137 <H2>Models</H2>
 138
 139 <HR>
 140
 141 <!-- Begin Footnotes -->
 142
 143 <P>
 144     <A CLASS="footnote_ref" NAME="note_1" HREF="#note_1_ref"><SUP>[1]</SUP></A>Strictly speaking, (i) and (ii) can be varied independently for input and output, so there are actually five possibilities.
 145 </P>
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 153 <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>
 154 <P CLASS="copyright">
 155     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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