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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<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="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>) | |
156 | </P> | |
157 | ||
158 | <!-- End Footer --> | |
159 | ||
160 | </BODY> |