<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <HTML> <HEAD> <TITLE>SeekableDevice</TITLE> <LINK REL="stylesheet" HREF="../../../../boost.css"> <LINK REL="stylesheet" HREF="../theme/iostreams.css"> </HEAD> <BODY> <!-- Begin Banner --> <H1 CLASS="title">SeekableDevice</H1> <HR CLASS="banner"> <!-- End Banner --> <H2>Definition</H2> <P> 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>. </P> <H2>Description</H2> <P> 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: <OL> <LI STYLE="list-style-type:lower-roman"> by defining member functions <CODE>read</CODE>, <CODE>write</CODE> and <CODE>seek</CODE>; or </LI> <LI STYLE="list-style-type:lower-roman"> 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> </LI> </OL> </P> <P>A SeekableDevice has mode <A HREF="../guide/modes.html#seekable"><CODE>seekable</CODE></A>.</P> <H2>Note</H2> <P>To be usable with streams and stream buffers, SeekableDevices must model <A HREF="blocking.html">Blocking</A>. <H2>Example</H2> <P>A model of SeekableDevice can be defined as follows:</P> <PRE CLASS="broken_ie"><SPAN CLASS="keyword">struct</SPAN> SeekableDevice { <SPAN CLASS="keyword">typedef</SPAN> <SPAN CLASS="keyword">char</SPAN> char_type; <SPAN CLASS="keyword">typedef</SPAN> seekable_device_tag category; std::streamsize read(<SPAN CLASS="keyword">char</SPAN>* s, std::streamsize n) { <SPAN CLASS="comment">// Read up to n characters from the input // sequence into the buffer s, returning // the number of characters read, or -1 // to indicate end-of-sequence.</SPAN> } <SPAN CLASS="keyword">void</SPAN> write(<SPAN CLASS="keyword">const</SPAN> <SPAN CLASS="keyword">char</SPAN>* s, std::streamsize n) { <SPAN CLASS="comment">// Write up to n characters from the buffer // s to the output sequence, returning the // number of characters written</SPAN> } std::streampos seek(stream_offset off, std::ios_base::seekdir way) { <SPAN CLASS="comment">// Advances the read/write head by off characters, // returning the new position, where the offset is // calculated from: // - the start of the sequence if way == ios_base::beg // - the current position if way == ios_base::cur // - the end of the sequence if way == ios_base::end</SPAN> } };</PRE> <P> 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>. </P> <H2>Refinement of</H2> <P><A HREF="source.html">Source</A>, <A HREF="sink.html">Sink</A>.</P> <H2>Associated Types</H2> <P>Same as <A HREF="device.html#types">Device</A>, with the following additional requirements:</P> <TABLE CELLPADDING="5" BORDER="1"> <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> </TABLE> <H2>Notation</H2> <TABLE CELLPADDING="2"> <TR><TD><CODE>D</CODE></TD><TD>- A type which is a model of SeekableDevice</TD></TR> <TR><TD><CODE>Ch</CODE></TD><TD>- The character type of <CODE>D</CODE></TD></TR> <TR><TD><CODE>dev</CODE></TD><TD>- Object of type <CODE>D</CODE></TD></TR> <TR><TD><CODE>s1</CODE></TD><TD>- Object of type <CODE>Ch*</CODE></SPAN></TD></TR> <TR><TD><CODE>s2</CODE></TD><TD>- Object of type <CODE>const Ch*</CODE></SPAN></TD></TR> <TR><TD><CODE>n</CODE></TD><TD>- Object of type <CODE>std::streamsize</CODE></TD></TR> <TR><TD><CODE>off</CODE></TD><TD>- Object of type <A HREF="../functions/positioning.html#synopsis"><CODE>stream_offset</CODE></A></TD></TR> <TR><TD><CODE>way</CODE></TD><TD>- Object of type <CODE>std::ios_base::seekdir</CODE></TD></TR> <TR><TD><CODE>io</CODE></TD><TD>- Alias for namespace <CODE>boost::iostreams</CODE></TD></TR> </TABLE> <H2>Valid Expressions / Semantics</H2> <P>Same as <A HREF="device.html#types">Device</A>, with the following additional requirements:</P> <TABLE CELLPADDING="5" BORDER="1"> <TR><TH>Expression</TH><TH>Expression Type</TH><TH>Category Precondition</TH><TH>Semantics</TH></TR> <TR> <TD><PRE CLASS="plain_code"><CODE><A HREF="../functions/read.html">io::read</A>(dev, s1, n)</CODE></PRE></TD> <TD><CODE>std::streamsize</CODE></TD> <TD ROWSPAN="3">Not convertible to <A HREF="direct.html"><CODE>direct_tag</CODE></A></TD> <TD> 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 </TD> </TR> <TR> <TD><PRE CLASS="plain_code"><CODE><A HREF="../functions/write.html">io::write</A>(dev, s2, n)</CODE></PRE></TD> <TD><CODE>std::streamsize</CODE></TD> <TD> 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 </TD> </TR> <TR> <TD><PRE CLASS="plain_code"><CODE><A HREF="../functions/seek.html">io::seek</A>(dev, off, way)</CODE></PRE></TD> <TD><CODE>std::streampos</CODE></TD> <TD>Advances the read/write head by <CODE>off</CODE> characters, returning the new position, where the offset is calculated from: <UL STYLE="margin:0,0,0,auto"> <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> <LI STYLE="list-style-type:disc;list-style-image:none">the current position if <CODE>way</CODE> is <CODE>ios_base::cur</CODE> <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> </UL> </TD> </TR> <TR> <TD><PRE CLASS="plain_code"><CODE>dev.input_sequence()</CODE></PRE></TD> <TD><PRE CLASS="plain_code"><CODE>std::pair<Ch*,Ch*></CODE></PRE></TD> <TD ROWSPAN="2">Convertible to <A HREF="direct.html"><CODE>direct_tag</CODE></A></TD> <TD>Returns a pair of pointers delimiting the sequence controlled by <CODE>dev</CODE></TD> </TR> <TR> <TD><PRE CLASS="plain_code"><CODE>dev.output_sequence()</CODE></PRE></TD> <TD><PRE CLASS="plain_code"><CODE>std::pair<Ch*,Ch*></CODE></PRE></TD> <TD>Returns a pair of pointers delimiting the sequence controlled by <CODE>dev</CODE></TD> </TR> </TABLE> <H2>Exceptions</H2> <P> 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. </P> <P> 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. </P> <H2>Models</H2> <UL> <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>. </UL> <!-- Begin Footnotes --> <P> <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>. </P> <!-- End Footnotes --> <!-- Begin Footer --> <HR> <P CLASS="copyright">Revised 02 Feb 2008</P> <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> <P CLASS="copyright"> 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>) </P> <!-- End Footer --> </BODY>