1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 2<HTML> 3<HEAD> 4 <TITLE>SeekableDevice</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">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="https://www.boost.org/users/people/jonathan_turkanis.html" 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> 180 181<!-- End Footer --> 182 183</BODY> 184