<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <HTML> <HEAD> <TITLE>BidirectionalDevice</TITLE> <LINK REL="stylesheet" HREF="../../../../boost.css"> <LINK REL="stylesheet" HREF="../theme/iostreams.css"> </HEAD> <BODY> <!-- Begin Banner --> <H1 CLASS="title">BidirectionalDevice</H1> <HR CLASS="banner"> <!-- End Banner --> <H2>Definition</H2> <P> 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>. </P> <H2>Description</H2> <P> An BidirectionalDevice provides read-access to a sequence of characters of a given type and write-access 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> <OL> <LI STYLE="list-style-type:lower-roman"> 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>; </LI> <LI STYLE="list-style-type:lower-roman"> 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 </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 two sequences in their entirety. </LI> </OL> </P> <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> <H2>Example</H2> <P>A model of BidirectionalDevice can be defined as follows:</P> <PRE CLASS="broken_ie"><SPAN CLASS="keyword">struct</SPAN> BidirectionalDevice { <SPAN CLASS="keyword">typedef</SPAN> <SPAN CLASS="keyword">char</SPAN> char_type; <SPAN CLASS="keyword">typedef</SPAN> bidirectional_device_tag category; std::streamsize read(<SPAN CLASS="keyword">char</SPAN>* s, std::streamsize n) { <SPAN CLASS="comment">// Reads up to n characters from the input // sequence into the buffer s, returning the number // of characters read. Returning a value less than n // indicates end-of-sequence.</SPAN> } std::streamsize 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> } };</PRE> <P> 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>. </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">device_tag</A> and to <A HREF="../guide/modes.html#mode_tags"><CODE>bidirectional</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 BidirectionalDevice</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> </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">boost::iostreams::read</A>(dev, s1, n)</CODE></PRE></TD> <TD><CODE>std::streamsize</CODE></TD> <TD ROWSPAN="2">Not convertible to <A HREF="direct.html"><CODE>direct_tag</CODE></A></TD> <TD> 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 </TD> </TR> <TR> <TD><PRE CLASS="plain_code"><CODE><A HREF="../functions/write.html">boost::iostreams::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>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 input 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 output 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>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. </P> <P> 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. </P> <H2>Models</H2> <HR> <!-- Begin Footnotes --> <P> <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. </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>