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<H2>3.4 Filtering Streams and Stream Buffers</H2>

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    The class templates <A href="../classes/filtering_streambuf.html"><CODE>filtering_streambuf</CODE></A> and <A href="../classes/filtering_stream.html"><CODE>filtering_stream</CODE></A> are the fundamental components provided by the Iostreams library for filtering data. Each <CODE>filtering_streambuf</CODE> or <CODE>filtering_stream</CODE> contains a chain of zero or more <A HREF="../concepts/filter.html">Filters</A> followed by an optional <A HREF="../concepts/device.html">Device</A>, accessed with an interface similar to that of <CODE>std::stack</CODE>. These chains are represented by of the class template <A HREF="../classes/chain.html"><CODE>chain</CODE></A>. If the chain ends with a Device it is called <SPAN CLASS="term">complete</SPAN> and may be used to perform i/o. 
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    The <A HREF="modes.html">mode</A> of each <CODE>stream_buffer</CODE> in the chain must refine the mode of the <CODE>filtering_streambuf</CODE> or <CODE>filtering_stream</CODE>. For example, one cannot use a Filter or Device having mode <A HREF="modes.html#output">output</A> with a <CODE>filtering_streambuf</CODE> having mode <A HREF="modes.html#input">input</A>.
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    If a <CODE>filtering_streambuf</CODE> or <CODE>filtering_stream</CODE> has mode <A HREF="modes.html#input">input</A>, data flows from the chain's end to its beginning &#8212; starting at a <A href="../concepts/source.html">Source</A> and passing through zero or more <A href="../concepts/input_filter.html">InputFilters</A>. If it has mode <A HREF="modes.html#output">output</A>, data flows in the opposite direction &#8212; from the beginning of the chain, through zero or more <A href="../concepts/output_filter.html">OutputFilters</A>, towards <A href="../concepts/source.html">Sink</A> at the end of the chain. If a <CODE>filtering_streambuf</CODE> or <CODE>filtering_stream</CODE> performs both input and output, data flows in both directions.

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