1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 2<HTML> 3<HEAD> 4 <TITLE>BidirectionalDevice</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">BidirectionalDevice</H1> 13 <HR CLASS="banner"> 14 15<!-- End Banner --> 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<bidirectional></CODE></A> or <A HREF="../classes/device.html#synopsis"><CODE>wdevice<bidirectional></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<Ch*,Ch*></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<Ch*,Ch*></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> 146 147<!-- End Footnotes --> 148 149<!-- Begin Footer --> 150 151<HR> 152 153<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> 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>) 156</P> 157 158<!-- End Footer --> 159 160</BODY>