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4 | <TITLE>SeekableDevice</TITLE> | |
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8 | <BODY> | |
9 | ||
10 | <!-- Begin Banner --> | |
11 | ||
12 | <H1 CLASS="title">SeekableDevice</H1> | |
13 | <HR CLASS="banner"> | |
14 | ||
15 | <!-- End Banner --> | |
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<seekable></CODE></A> or <A HREF="../classes/device.html#synopsis"><CODE>wdevice<seekable></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<Ch*,Ch*></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<Ch*,Ch*></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 | ||
170 | <!-- End Footnotes --> | |
171 | ||
172 | <!-- Begin Footer --> | |
173 | ||
174 | <HR> | |
175 | ||
176 | <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> | |
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>) | |
179 | </P> | |
180 | ||
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