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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&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>
146
147<!-- End Footnotes -->
148
149<!-- Begin Footer -->
150
151<HR>
152<P CLASS="copyright">Revised
153<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
15420 May, 2004
155<!--webbot bot="Timestamp" endspan i-checksum="38504" -->
156</P>
157
158<P CLASS="copyright">&copy; Copyright <A HREF="http://www.kangaroologic.com" TARGET="_top">Jonathan Turkanis</A>, 2004</P>
159<P CLASS="copyright"> 
160    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>)
161</P>
162
163<!-- End Footer -->
164
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