<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <HTML> <HEAD> <TITLE>Null Devices</TITLE> <LINK REL="stylesheet" HREF="../../../../boost.css"> <LINK REL="stylesheet" HREF="../theme/iostreams.css"> </HEAD> <BODY> <!-- Begin Banner --> <H1 CLASS="title">Null Devices</H1> <HR CLASS="banner"> <!-- End Banner --> <DL class="page-index"> <DT><A href="#overview">Overview</A></DT> <DT><A href="#acknowledgments">Acknowledgments</A></DT> <DT><A href="#headers">Headers</A></DT> <DT><A href="#reference">Reference</A> <DL> <DT><A href="#null_source">Class template <CODE>basic_null_source</CODE></A></DT> <DT><A href="#null_sink">Class template <CODE>basic_null_sink</CODE></A></DT> <DT><A href="#null_device">Class template <CODE>basic_null_device</CODE></A></DT> </DL> </DT> </DL> <HR> <A NAME="overview"></A> <H2>Overview</H2> <P> The class templates <CODE>basic_null_source</CODE>, <CODE>basic_null_sink</CODE> and <CODE>basic_null_device</CODE> are models of Device whose member functions all have trivial implementations. A <CODE>basic_null_source</CODE> is a <A HREF="../concepts/source.html">Source</A> whose member function <CODE>read</CODE> returns <CODE>-1</CODE>, indicating end-of-sequence. A <CODE>basic_null_sink</CODE> is a <A HREF="../concepts/sink.html">Sink</A> whose member function <CODE>write</CODE> consumes and ignores the characters passed to it. A <CODE>basic_null_device</CODE> combines — depending on its template parameter <A HREF="#null_device_params"><CODE>Mode</CODE></A> — the trivial <CODE>read</CODE> and <CODE>write</CODE> functions just described with a member <CODE>seek</CODE> that always returns an invalid stream position. </P> <P> In addition, each of the above templates is <A HREF='../concepts/closable.html'>Closable</A>, with a no-op implementation of <CODE>close</CODE>. </P> <A NAME="acknowledgments"></A> <H2>Acknowledgments</H2> <P> The templates described here were inspired by <A HREF="http://www.boost.org/people/daryle_walker.html" TARGET="_top">Daryle Walker's</A> <CODE>basic_nullbuf</CODE> template. <I>See</I> <A HREF="http://cvs.sourceforge.net/viewcvs.py/boost-sandbox/boost-sandbox/libs/io/doc/null_stream.html" TARGET="_top">Disconnected Streams</A>. </P> <A NAME="headers"></A> <H2>Headers</H2> <DL class="page-index"> <DT><A CLASS="header" HREF="../../../../boost/iostreams/device/null.hpp"><CODE><boost/iostreams/device/null.hpp></CODE></A></DT> </DL> <A NAME="reference"></A> <H2>Reference</H2> <A NAME="null_source"></A> <H3>Class template <CODE>basic_null_source</CODE></H3> <H4>Description</H4> <P>Model of <A HREF="../concepts/source.html">Source</A> whose member function <CODE>read</CODE> returns <CODE>-1</CODE>, indicating end-of-sequence.</P> <H4>Synopsis</H4> <PRE CLASS="broken_ie"><SPAN CLASS="keyword">namespace</SPAN> boost { <SPAN CLASS="keyword">namespace</SPAN> iostreams { <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> <A CLASS="documented" HREF="#null_source_params">Ch</A>> <SPAN CLASS="keyword">class</SPAN> <A CLASS="documented" HREF="#null_source_params">basic_null_source</A> { <SPAN CLASS="keyword">public</SPAN>: <SPAN CLASS="keyword">typedef</SPAN> Ch char_type; <SPAN CLASS="keyword">typedef</SPAN> <SPAN CLASS='omitted'>[implementation-defined]</SPAN> category; <A CLASS="documented" HREF="#null_source_ctor">basic_null_source</A>(); std::streamsize <A CLASS="documented" HREF="#null_source_read">read</A>(char_type* s, std::streamsize n); <SPAN CLASS='keyword'>void</SPAN> <A CLASS='documented' HREF='#null_source_close'>close</A>(); }; <SPAN CLASS='keyword'>typedef</SPAN> basic_null_source<<SPAN CLASS='keyword'>char</SPAN>> <SPAN CLASS='defined'>null_source</SPAN>; <SPAN CLASS='keyword'>typedef</SPAN> basic_null_source<<SPAN CLASS='keyword'>wchar_t</SPAN>> <SPAN CLASS='defined'>wnull_source</SPAN>; } } <SPAN CLASS='comment'>// End namespace boost::io</SPAN></PRE> <A NAME="null_source_params"></A> <H4>Template parameters</H4> <TABLE STYLE="margin-left:2em"> <TR> <TD><P><I>Ch</I></TD><TD VALIGN="top">-</TD> <TD><P>The <A HREF='../guide/traits.html#char_type'>character type</A> type.</TD> </TR> </TABLE> <A NAME="null_source_ctor"></A> <H4><CODE>basic_null_source::basic_null_source</CODE></H4> <PRE CLASS="broken_ie"> basic_null_source();</PRE> <P>Constructs a <CODE>basic_null_source</CODE>.</P> <A NAME="null_source_read"></A> <H4><CODE>basic_null_source::read</CODE></H4> <PRE CLASS="broken_ie"> std::streamsize read(char_type* s, std::streamsize n);</PRE> <P>Returns <CODE>-1</CODE>.</P> <A NAME="null_source_close"></A> <H4><CODE>basic_null_source::close</CODE></H4> <PRE CLASS="broken_ie"> <SPAN CLASS='keyword'>void</SPAN> close();</PRE> <P>No-op.</P> <A NAME="null_sink"></A> <H3>Class template <CODE>basic_null_sink</CODE></H3> <H4>Description</H4> <P>Model of <A HREF="../concepts/sink.html">Sink</A> whose member function <CODE>write</CODE> consumes and ignores the contents of the character buffer passed to it. <H4>Synopsis</H4> <PRE CLASS="broken_ie"><SPAN CLASS="keyword">namespace</SPAN> boost { <SPAN CLASS="keyword">namespace</SPAN> iostreams { <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> <A CLASS="documented" HREF="#null_sink_params">Ch</A>> <SPAN CLASS="keyword">class</SPAN> <A CLASS="documented" HREF="#null_sink_params">basic_null_sink</A> { <SPAN CLASS="keyword">public</SPAN>: <SPAN CLASS="keyword">typedef</SPAN> Ch char_type; <SPAN CLASS="keyword">typedef</SPAN> <SPAN CLASS='omitted'>[implementation-defined]</SPAN> category; <A CLASS="documented" HREF="#null_sink_ctor">basic_null_sink</A>(); std::streamsize <A CLASS="documented" HREF="#null_sink_write">write</A>(<SPAN CLASS='keyword'>const</SPAN> char_type* s, std::streamsize n); <SPAN CLASS='keyword'>void</SPAN> <A CLASS='documented' HREF='#null_sink_close'>close</A>(); }; <SPAN CLASS='keyword'>typedef</SPAN> basic_null_sink<<SPAN CLASS='keyword'>char</SPAN>> <SPAN CLASS='defined'>null_sink</SPAN>; <SPAN CLASS='keyword'>typedef</SPAN> basic_null_sink<<SPAN CLASS='keyword'>wchar_t</SPAN>> <SPAN CLASS='defined'>wnull_sink</SPAN>; } } <SPAN CLASS='comment'>// End namespace boost::io</SPAN></PRE> <A NAME="null_sink_params"></A> <H4>Template parameters</H4> <TABLE STYLE="margin-left:2em"> <TR> <TD><P><I>Ch</I></TD><TD VALIGN="top">-</TD> <TD><P>The <A HREF='../guide/traits.html#char_type'>character type</A> type.</TD> </TR> </TABLE> <A NAME="null_sink_ctor"></A> <H4><CODE>basic_null_sink::basic_null_sink</CODE></H4> <PRE CLASS="broken_ie"> basic_null_sink();</PRE> <P>Constructs a <CODE>basic_null_sink</CODE>.</P> <A NAME="null_sink_write"></A> <H4><CODE>basic_null_sink::write</CODE></H4> <PRE CLASS="broken_ie"> std::streamsize write(<SPAN CLASS='keyword'>const</SPAN> char_type* s, std::streamsize n);</PRE> <P>Returns <CODE>n</CODE>.</P> <A NAME="null_sink_close"></A> <H4><CODE>basic_null_sink::close</CODE></H4> <PRE CLASS="broken_ie"> <SPAN CLASS='keyword'>void</SPAN> close();</PRE> <P>No-op.</P> <A NAME="null_device"></A> <H3>Class template <CODE>basic_null_device</CODE></H3> <H4>Description</H4> <P>Model of <A HREF="../concepts/seekable_device.html">Device</A> whose mode is specified as a template parameter, and whose member functions have trivial implementations, as follows:</P> <UL> <LI><CODE>read</CODE> returns <CODE>-1</CODE>, indicating end-of-sequence <LI><CODE>write</CODE> consumes but ignores the entire contents of the character buffer passed to it <LI><CODE>seek</CODE> returns an invalid stream position </UL> <H4>Synopsis</H4> <PRE CLASS="broken_ie"><SPAN CLASS="keyword">namespace</SPAN> boost { <SPAN CLASS="keyword">namespace</SPAN> iostreams { <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> <A CLASS="documented" HREF="#null_device_params">Ch</A>, <SPAN CLASS="keyword">typename</SPAN> <A CLASS="documented" HREF="#null_device_params">Mode</A>> <SPAN CLASS="keyword">class</SPAN> <A CLASS="documented" HREF="#null_device_params">basic_null_device</A> { <SPAN CLASS="keyword">public</SPAN>: <SPAN CLASS="keyword">typedef</SPAN> Ch char_type; <SPAN CLASS="keyword">typedef</SPAN> <SPAN CLASS='omitted'>[implementation-defined]</SPAN> category; <A CLASS="documented" HREF="#null_device_ctor">basic_null_device</A>(); std::streamsize <A CLASS="documented" HREF="#null_device_read">read</A>(char_type* s, std::streamsize n); std::streamsize <A CLASS="documented" HREF="#null_device_write">write</A>(<SPAN CLASS='keyword'>const</SPAN> char_type* s, std::streamsize n); <A CLASS='documented' HREF='../functions/positioning.html#synopsis'>stream_offset</A> <A CLASS='documented' HREF='#null_device_seek'>seek</A>( <A CLASS='documented' HREF='../functions/positioning.html#synopsis'>stream_offset</A> off, std::ios_base::seekdir way, std::ios_base::openmode which = std::ios_base::in | std::ios_base::out ); <SPAN CLASS='keyword'>void</SPAN> <A CLASS='documented' HREF='#null_device_close'>close</A>(); }; } } <SPAN CLASS='comment'>// End namespace boost::io</SPAN></PRE> <A NAME="null_device_params"></A> <H4>Template parameters</H4> <TABLE STYLE="margin-left:2em"> <TR> <TD><P><I>Ch</I></TD><TD VALIGN="top">-</TD> <TD><P>The <A HREF='../guide/traits.html#char_type'>character type</A>.</TD> </TR> <TR> <TD><P><I>Mode</I></TD><TD VALIGN="top">-</TD> <TD><P>The <A HREF='../guide/modes.html'>mode</A>.</TD> </TR> </TABLE> <A NAME="null_device_ctor"></A> <H4><CODE>basic_null_device::basic_null_device</CODE></H4> <PRE CLASS="broken_ie"> basic_null_device();</PRE> <P>Constructs a <CODE>basic_null_device</CODE>.</P> <A NAME="null_device_read"></A> <H4><CODE>basic_null_device::read</CODE></H4> <PRE CLASS="broken_ie"> std::streamsize read(char_type* s, std::streamsize n);</PRE> <P>Returns <CODE>-1</CODE>. Enabled if <CODE>Mode</CODE> refines <A HREF='../guide/modes.html#mode_tags'><CODE>input</CODE></A>.</P> <A NAME="null_device_write"></A> <H4><CODE>basic_null_device::write</CODE></H4> <PRE CLASS="broken_ie"> std::streamsize write(<SPAN CLASS='keyword'>const</SPAN> char_type* s, std::streamsize n);</PRE> <P>Returns <CODE>n</CODE>. Enabled if <CODE>Mode</CODE> refines <A HREF='../guide/modes.html#mode_tags'><CODE>output</CODE></A>.</P> <A NAME="null_device_seek"></A> <H4><CODE>basic_null_device::seek</CODE></H4> <PRE CLASS="broken_ie"> <A CLASS='documented' HREF='../functions/positioning.html#synopsis'>stream_offset</A> seek( <A CLASS='documented' HREF='../functions/positioning.html#synopsis'>stream_offset</A> off, std::ios_base::seekdir way, std::ios_base::openmode which = std::ios_base::in | std::ios_base::out );</PRE> <P>Returns an invalid stream position. Enabled if <CODE>Mode</CODE> permits random access.</P> <A NAME="null_device_close"></A> <H4><CODE>basic_null_device::close</CODE></H4> <PRE CLASS="broken_ie"> <SPAN CLASS='keyword'>void</SPAN> close();</PRE> <P>No-op.</P> <!-- 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>