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     <H1 CLASS="title">InputFilter</H1>
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 <DL CLASS="page-index" STYLE="margin-top:0">
     <DT><A HREF="#definition">Definition</A>
     <DT><A HREF="#description">Description</A>
     <DT><A HREF="#examples">Examples</A>
     <DT><A HREF="#detailed_specification">Detailed Specification</A>
 </DL>

 <A NAME="definition"></A>
 <H2>Definition</H2>

 <P>
     An InputFilter is a <A HREF="filter.html">Filter</A> whose <A HREF="../guide/modes.html">mode</A> refines <A HREF="../guide/modes.html#input">input</A>.
 </P>

 <A NAME="description"></A>
 <H2>Description</H2>

 <P>
     An InputFilter operates on the character sequence controlled by a <A HREF="source.html">Source</A>, providing access to a filtered sequence having the same character type. It may expose the filtered sequence in two ways:
     <OL>
         <LI STYLE="list-style-type:lower-roman">
             by defining a member function <CODE>get</CODE>
         </LI>
         <LI STYLE="list-style-type:lower-roman">
             by defining a member function <CODE>read</CODE>
     </OL>
     The second alternative is provided for enhanced performance. InputFilters implementing this alternative are referred to as <I>Multi-Character</I>. (<I>See</I> <A HREF="multi_character.html">Multi-Character Filter</A>.)
 </P>

 <A NAME="examples"></A>
 <H2>Examples</H2>

 <H4>I. Ordinary InputFilter</H4>

 <P>
     The following example shows an InputFilter which removes all non-alphabetic characters from a sequence.
 </P>

 <PRE CLASS="broken_ie">    <SPAN CLASS="preprocessor">#include</SPAN> <SPAN CLASS="literal">&lt;ctype.h&gt;</SPAN>                 <SPAN CLASS="comment">// isalpha</SPAN>
     <SPAN CLASS="preprocessor">#include</SPAN> <SPAN CLASS="literal">&lt;cstdio.h&gt;</SPAN>                <SPAN CLASS="comment">// EOF</SPAN>
     <SPAN CLASS="preprocessor">#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/categories.hpp"><SPAN CLASS="literal">&lt;boost/iostreams/categories.hpp&gt;</SPAN></A> <SPAN CLASS="comment">// input_filter_tag</SPAN>
     <SPAN CLASS="preprocessor">#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/operations.hpp"><SPAN CLASS="literal">&lt;boost/iostreams/operations.hpp&gt;</SPAN></A> <SPAN CLASS="comment">// get, WOULD_BLOCK</SPAN>

     <SPAN CLASS="keyword">using</SPAN> <SPAN CLASS="keyword">namespace</SPAN> std;
     <SPAN CLASS="keyword">using</SPAN> <SPAN CLASS="keyword">namespace</SPAN> boost::iostreams;

     <SPAN CLASS="keyword">struct</SPAN> alphabetic_input_filter {
         <SPAN CLASS="keyword">typedef</SPAN> <SPAN CLASS="keyword">char</SPAN>              char_type;
         <SPAN CLASS="keyword">typedef</SPAN> input_filter_tag  category;

         <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Source&gt;
         <SPAN CLASS="keyword">int</SPAN> get(Source& src)
         {
             <SPAN CLASS="keyword">int</SPAN> c;
             <SPAN CLASS="keyword">while</SPAN> ( (c = boost::iostreams::get(src)) != EOF &amp;&amp;
                      c != WOULD_BLOCK &amp;&amp;
                     !isalpha((<SPAN CLASS="keyword">unsigned</SPAN> <SPAN CLASS="keyword">char</SPAN>) c) )
                 ;
             <SPAN CLASS="keyword">return</SPAN> c;
         }
     };</PRE>

 <P>
     Here <CODE>char_type</CODE> is the <A HREF="../guide/traits.html#char_type">character type</A> of the Filter, <CODE>input_filter_tag</CODE> is a <A HREF="../guide/traits.html#category_tags">category tag</A> identifying the Filter as a model of InputFilter, and the function <A HREF="../functions/get.html"><CODE>boost::iostreams::get</CODE></A> reads a character from an arbitrary Source.<A CLASS="footnote_ref" NAME="note_1_ref" HREF="#note_1"><SUP>[1]</SUP></A> The constant <A HREF="../classes/char_traits.html#WOULD_BLOCK"><CODE>WOULD_BLOCK</CODE></A>, defined in the header <A HREF="../../../../boost/iostreams/char_traits.hpp"><CODE>&lt;boost/iostreams/char_traits.hpp&gt;</CODE></A>, is used to indicate that input is temporarily unavilable.
 </P>

 <P>
     The Iostreams library defines two convenience classes, <A HREF="../classes/filter.html#synopsis"><CODE>input_filter</CODE></A> and <A HREF="../classes/filter.html#synopsis"><CODE>input_wfilter</CODE></A>, which provide member <CODE>typedef</CODE>s <CODE>char_type</CODE> and <CODE>category</CODE> as well as default implementations of several member functions. When defining a new model of InputFilter, it is often sufficient to derive from <CODE>input_filter</CODE> or <CODE>input_wfilter</CODE> and to define a member function <CODE>get</CODE>.
 </P>

 <H4>II. Multi-Character InputFilter</H4>

 <P>
     The following example shows a Multi-Character InputFilter which performs the same filtering operation as the Filter in Example I.
 </P>

 <PRE CLASS="broken_ie">    <SPAN CLASS="preprocessor">#include</SPAN> <SPAN CLASS="literal">&lt;ctype.h&gt;</SPAN>                 <SPAN CLASS="comment">// isalpha</SPAN>
     <SPAN CLASS="preprocessor">#include</SPAN> <SPAN CLASS="literal">&lt;cstdio.h&gt;</SPAN>                <SPAN CLASS="comment">// EOF</SPAN>
     <SPAN CLASS="preprocessor">#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/categories.hpp"><SPAN CLASS="literal">&lt;boost/iostreams/categories.hpp&gt;</SPAN></A> <SPAN CLASS="comment">// input_filter_tag</SPAN>
     <SPAN CLASS="preprocessor">#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/concepts.hpp"><SPAN CLASS="literal">&lt;boost/iostreams/operations.hpp&gt;</SPAN></A> <SPAN CLASS="comment">// get</SPAN>

     <SPAN CLASS="keyword">using</SPAN> <SPAN CLASS="keyword">namespace</SPAN> std;
     <SPAN CLASS="keyword">using</SPAN> <SPAN CLASS="keyword">namespace</SPAN> boost::io;

     <SPAN CLASS="keyword">struct</SPAN> alphabetic_input_filter {
         <SPAN CLASS="keyword">typedef</SPAN> <SPAN CLASS="keyword">char</SPAN>                       char_type;
         <SPAN CLASS="keyword">typedef</SPAN> multichar_input_filter_tag  category;

         <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Source&gt;
         streamsize read(Source& src, <SPAN CLASS="keyword">char</SPAN>* s, streamsize n)
             {
                 <SPAN CLASS="keyword">int</SPAN>   c;
                 <SPAN CLASS="keyword">char</SPAN>* first = s;
                 <SPAN CLASS="keyword">char</SPAN>* last  = s + n;
                 <SPAN CLASS="keyword">while</SPAN> ( first != last &amp;&amp;
                         (c = boost::iostreams::get(src)) != EOF &amp;&amp;
                          c != WOULD_BLOCK &amp;&amp;
                         isalpha((<SPAN CLASS="keyword">unsigned</SPAN> <SPAN CLASS="keyword">char</SPAN>) c) )
                 {
                     *first++ = c;
                 }
                 streamsize result = <SPAN CLASS="keyword">static_cast</SPAN>&lt;streamsize&gt;(first - s);
                 <SPAN CLASS="keyword">return</SPAN> result == <SPAN CLASS='numeric_literal'>0</SPAN> &amp;&amp; c != WOULD_BLOCK ?
                     <SPAN CLASS='numeric_literal'>-1</SPAN> :
                     result;
             }
     };</PRE>

 <P>
     Here <CODE>multichar_input_filter_tag</CODE> is a <A HREF="../guide/traits.html#category">category tag</A> identifying the Filter as a Multi-Character InputFilter.
 </P>
 <P>
     The Iostreams library defines two convenience classes, <A HREF="../classes/filter.html#synopsis"><CODE>multichar_input_filter</CODE></A> and <A HREF="../classes/filter.html#synopsis"><CODE>multichar_input_wfilter</CODE></A>, which provide the member <CODE>typedef</CODE>s <CODE>char_type</CODE> and <CODE>category</CODE> as well as default implementations of several member functions. When defining a new Multi-Character InputFilter, it is often sufficient to derive from <CODE>multichar_input_filter</CODE> or <CODE>multichar_input_wfilter</CODE> and to define a member function <CODE>read</CODE>.
 </P>

 <A NAME="detailed_specification"></A>
 <H2>Refinement of</H2>

 <P><A HREF="filter.html">Filter</A>.</P>

 <H2>Associated Types</H2>

 <TABLE CELLPADDING="5" BORDER="1">
     <TR><TD>Character type</TD><TD>The type of the characters in the filtered sequences</TD></TR>
     <TR>
         <TD>Category</TD>
         <TD>
             A type convertible to <A HREF="../guide/traits.html#category_tags"><CODE>filter_tag</CODE></A> and to <A HREF="../guide/modes.html#input"><CODE>input</CODE></A>
         </TD>
     </TR>
     <TR>
         <TD>Mode</TD>
         <TD>
             The unique <I>most-derived</I> <A HREF="../guide/modes.html#mode_tags">mode tag</A> to which Category is convertible
         </TD>
     </TR>
 </TABLE>

 <H2>Notation</H2>

 <TABLE CELLPADDING="2">
     <TR><TD><CODE>F</CODE></TD><TD>- A type which is a model of InputFilter</TD></TR>
     <TR><TD><CODE>D</CODE></TD><TD>- A type which is a model of <A HREF="device.html">Device</A>, with the same character type as <CODE>F</CODE> and with mode refining the mode of <CODE>F</CODE></TD></TR>
     <TR><TD><CODE>Ch</CODE></TD><TD>- The character type of <CODE>F</CODE></TD></TR>
     <TR><TD><CODE>Tr</CODE></A></TD><TD>- <A HREF="../classes/char_traits.html"><CODE>boost::iostreams::char_traits&lt;Ch&gt;</CODE></A></TD></TR>
     <TR><TD><CODE>f</CODE></TD><TD>- Object of type <CODE>F</CODE></TD></TR>
     <TR><TD><CODE>d</CODE></TD><TD>- Object of type <CODE>D</CODE></TD></TR>
     <TR><TD><CODE>s</CODE></TD><TD>- Object of type <CODE>Ch*</CODE></TD></TR>
     <TR><TD><CODE>n</CODE></TD><TD>- Object of type <CODE>std::streamsize</CODE></TD></TR>
     <TR><TD><CODE>io</CODE></TD><TD>- Alias for namespace <CODE>boost::iostreams</CODE></TD></TR>
 </TABLE>

 <A NAME="semantics"></A>
 <H2>Valid Expressions / Semantics</H2>

 <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>typename <A HREF="../guide/traits.html#char_type_of_ref">char_type_of</A>&lt;F&gt;::type</CODE></PRE>
         </TD>
         <TD><CODE>typename</CODE> of the character type</TD>
         <TD ALIGN="center">-</TD><TD ALIGN="center">-</TD>
     </TR>
     <TR>
         <TD>
             <PRE CLASS="plain_code"><CODE>typename <A HREF="../guide/traits.html#category_ref">category_of</A>&lt;F&gt;::type</CODE></PRE>
         </TD>
         <TD><CODE>typename</CODE> of the category</TD>
         <TD ALIGN="center">-</TD><TD ALIGN="center">-</TD>
     </TR>
     <TR>
         <TD><PRE CLASS="plain_code"><CODE>f.get(d)</CODE></PRE></TD>
         <TD><CODE>Tr::int_type</CODE></TD>
         <TD>
             Convertible to <A HREF="../guide/modes.html#mode_tags"><CODE>input</CODE></A> but not to <A HREF="../guide/traits.html#category_tags"><CODE>multichar_tag</CODE></A>
         </TD>
         <TD>
             Returns the next character in the input sequence controlled by <CODE>f</CODE>, <CODE>Tr::eof()</CODE> if the end of the sequence has been reached or <CODE>Tr::would_block()</CODE> if input is temporarily unavilable because a call to <CODE>d</CODE> has produced fewer characters than requested. The input sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/get.html"><CODE>io::get</CODE></A>, <A HREF="../functions/read.html"><CODE>io::read</CODE></A> and <A HREF="../functions/putback.html"><CODE>io::putback</CODE></A>.
         </TD>
     </TR>
     <TR>
         <TD><PRE CLASS="plain_code"><CODE>f.read(d, s, n)</CODE></PRE></TD>
         <TD><PRE CLASS="plain_code"><CODE>std::streamsize</CODE></PRE></TD>
         <TD>
             Convertible to <A HREF="../guide/modes.html#mode_tags"><CODE>input</CODE></A> and to <A HREF="../guide/traits.html#category_tags"><CODE>multichar_tag</CODE></A>
         </TD>
         <TD>
             Reads up to <CODE>n</CODE> characters from the input sequence controlled by <CODE>f</CODE> into the buffer <CODE>s</CODE>, returning the number of characters read or <CODE>-1</CODE> to indicate end-of-sequence. A value less than <CODE>n</CODE> may be returned only at end-of-sequence or if input is temporarily unavilable because a call to <CODE>d</CODE> has produced fewer characters than requested. The input sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/get.html"><CODE>io::get</CODE></A>, <A HREF="../functions/read.html"><CODE>io::read</CODE></A> and <A HREF="../functions/putback.html"><CODE>io::putback</CODE></A>.
         </TD>
     </TR>
 </TABLE>

 <H2>Exceptions</H2>

 <P>
     Errors which occur during the execution of <CODE>get</CODE> and <CODE>read</CODE> are indicated by throwing exceptions. Reaching the end of the sequence is not an error.
 </P>

 <P>
     After an exception is thrown, an InputFilter must be in a consistent state; further i/o operations may throw exceptions but must have well-defined behaviour. Furthermore, unless it is <A HREF="closable.html">Closable</A>, it must be ready to begin processing a new character sequence.
 </P>

 <H2>Models</H2>

 <UL>
     <LI>The <A HREF="../guide/text_processing.html">Text Processing Filters</A>.
     <LI>The compression and decompression filters.
 </UL>

 <H2>Acknowledgments</H2>

 <P>
     The concept InputFilter was inspired by the <I>extractors</I> of <A CLASS="footnote_ref" HREF="../bibliography.html#kanze">[Kanze]</A>.
 </P>

 <!-- Begin Footnotes -->

 <HR>

 <P>
     <A CLASS="footnote_ref" NAME="note_1" HREF="#note_1_ref"><SUP>[1]</SUP></A>Technically, <CODE>boost::iostreams::get</CODE> requires that a Source be <A HREF="../concepts/direct.html"><I>indirect</I></A>.
 </P>

 <!-- End Footnotes -->

 <!-- Begin Footer -->

 <HR>
 <P CLASS="copyright">Revised 02 Feb 2008</P>

 <P CLASS="copyright">&copy; Copyright 2008 <a href="http://www.coderage.com/" target="_top">CodeRage, LLC</a><br/>&copy; 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 -->

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	<HTML>
	<HEAD>
	<TITLE>InputFilter</TITLE>
	<LINK REL="stylesheet" HREF="../../../../boost.css">
	<LINK REL="stylesheet" HREF="../theme/iostreams.css">
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	<BODY>

	<!-- Begin Banner -->

	<H1 CLASS="title">InputFilter</H1>
	<HR CLASS="banner">

	<!-- End Banner -->

	<DL CLASS="page-index" STYLE="margin-top:0">
	<DT><A HREF="#definition">Definition</A>
	<DT><A HREF="#description">Description</A>
	<DT><A HREF="#examples">Examples</A>
	<DT><A HREF="#detailed_specification">Detailed Specification</A>
	</DL>

	<A NAME="definition"></A>
	<H2>Definition</H2>

	<P>
	An InputFilter is a <A HREF="filter.html">Filter</A> whose <A HREF="../guide/modes.html">mode</A> refines <A HREF="../guide/modes.html#input">input</A>.
	</P>

	<A NAME="description"></A>
	<H2>Description</H2>

	<P>
	An InputFilter operates on the character sequence controlled by a <A HREF="source.html">Source</A>, providing access to a filtered sequence having the same character type. It may expose the filtered sequence in two ways:
	<OL>
	<LI STYLE="list-style-type:lower-roman">
	by defining a member function <CODE>get</CODE>
	</LI>
	<LI STYLE="list-style-type:lower-roman">
	by defining a member function <CODE>read</CODE>
	</OL>
	The second alternative is provided for enhanced performance. InputFilters implementing this alternative are referred to as <I>Multi-Character</I>. (<I>See</I> <A HREF="multi_character.html">Multi-Character Filter</A>.)
	</P>

	<A NAME="examples"></A>
	<H2>Examples</H2>

	<H4>I. Ordinary InputFilter</H4>

	<P>
	The following example shows an InputFilter which removes all non-alphabetic characters from a sequence.
	</P>

	<PRE CLASS="broken_ie"> <SPAN CLASS="preprocessor">#include</SPAN> <SPAN CLASS="literal"><ctype.h></SPAN> <SPAN CLASS="comment">// isalpha</SPAN>
	<SPAN CLASS="preprocessor">#include</SPAN> <SPAN CLASS="literal"><cstdio.h></SPAN> <SPAN CLASS="comment">// EOF</SPAN>
	<SPAN CLASS="preprocessor">#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/categories.hpp"><SPAN CLASS="literal"><boost/iostreams/categories.hpp></SPAN></A> <SPAN CLASS="comment">// input_filter_tag</SPAN>
	<SPAN CLASS="preprocessor">#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/operations.hpp"><SPAN CLASS="literal"><boost/iostreams/operations.hpp></SPAN></A> <SPAN CLASS="comment">// get, WOULD_BLOCK</SPAN>

	<SPAN CLASS="keyword">using</SPAN> <SPAN CLASS="keyword">namespace</SPAN> std;
	<SPAN CLASS="keyword">using</SPAN> <SPAN CLASS="keyword">namespace</SPAN> boost::iostreams;

	<SPAN CLASS="keyword">struct</SPAN> alphabetic_input_filter {
	<SPAN CLASS="keyword">typedef</SPAN> <SPAN CLASS="keyword">char</SPAN> char_type;
	<SPAN CLASS="keyword">typedef</SPAN> input_filter_tag category;

	<SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Source>
	<SPAN CLASS="keyword">int</SPAN> get(Source& src)
	{
	<SPAN CLASS="keyword">int</SPAN> c;
	<SPAN CLASS="keyword">while</SPAN> ( (c = boost::iostreams::get(src)) != EOF &&
	c != WOULD_BLOCK &&
	!isalpha((<SPAN CLASS="keyword">unsigned</SPAN> <SPAN CLASS="keyword">char</SPAN>) c) )
	;
	<SPAN CLASS="keyword">return</SPAN> c;
	}
	};</PRE>

	<P>
	Here <CODE>char_type</CODE> is the <A HREF="../guide/traits.html#char_type">character type</A> of the Filter, <CODE>input_filter_tag</CODE> is a <A HREF="../guide/traits.html#category_tags">category tag</A> identifying the Filter as a model of InputFilter, and the function <A HREF="../functions/get.html"><CODE>boost::iostreams::get</CODE></A> reads a character from an arbitrary Source.<A CLASS="footnote_ref" NAME="note_1_ref" HREF="#note_1"><SUP>[1]</SUP></A> The constant <A HREF="../classes/char_traits.html#WOULD_BLOCK"><CODE>WOULD_BLOCK</CODE></A>, defined in the header <A HREF="../../../../boost/iostreams/char_traits.hpp"><CODE><boost/iostreams/char_traits.hpp></CODE></A>, is used to indicate that input is temporarily unavilable.
	</P>

	<P>
	The Iostreams library defines two convenience classes, <A HREF="../classes/filter.html#synopsis"><CODE>input_filter</CODE></A> and <A HREF="../classes/filter.html#synopsis"><CODE>input_wfilter</CODE></A>, which provide member <CODE>typedef</CODE>s <CODE>char_type</CODE> and <CODE>category</CODE> as well as default implementations of several member functions. When defining a new model of InputFilter, it is often sufficient to derive from <CODE>input_filter</CODE> or <CODE>input_wfilter</CODE> and to define a member function <CODE>get</CODE>.
	</P>

	<H4>II. Multi-Character InputFilter</H4>

	<P>
	The following example shows a Multi-Character InputFilter which performs the same filtering operation as the Filter in Example I.
	</P>

	<PRE CLASS="broken_ie"> <SPAN CLASS="preprocessor">#include</SPAN> <SPAN CLASS="literal"><ctype.h></SPAN> <SPAN CLASS="comment">// isalpha</SPAN>
	<SPAN CLASS="preprocessor">#include</SPAN> <SPAN CLASS="literal"><cstdio.h></SPAN> <SPAN CLASS="comment">// EOF</SPAN>
	<SPAN CLASS="preprocessor">#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/categories.hpp"><SPAN CLASS="literal"><boost/iostreams/categories.hpp></SPAN></A> <SPAN CLASS="comment">// input_filter_tag</SPAN>
	<SPAN CLASS="preprocessor">#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/concepts.hpp"><SPAN CLASS="literal"><boost/iostreams/operations.hpp></SPAN></A> <SPAN CLASS="comment">// get</SPAN>

	<SPAN CLASS="keyword">using</SPAN> <SPAN CLASS="keyword">namespace</SPAN> std;
	<SPAN CLASS="keyword">using</SPAN> <SPAN CLASS="keyword">namespace</SPAN> boost::io;

	<SPAN CLASS="keyword">struct</SPAN> alphabetic_input_filter {
	<SPAN CLASS="keyword">typedef</SPAN> <SPAN CLASS="keyword">char</SPAN> char_type;
	<SPAN CLASS="keyword">typedef</SPAN> multichar_input_filter_tag category;

	<SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Source>
	streamsize read(Source& src, <SPAN CLASS="keyword">char</SPAN>* s, streamsize n)
	{
	<SPAN CLASS="keyword">int</SPAN> c;
	<SPAN CLASS="keyword">char</SPAN>* first = s;
	<SPAN CLASS="keyword">char</SPAN>* last = s + n;
	<SPAN CLASS="keyword">while</SPAN> ( first != last &&
	(c = boost::iostreams::get(src)) != EOF &&
	c != WOULD_BLOCK &&
	isalpha((<SPAN CLASS="keyword">unsigned</SPAN> <SPAN CLASS="keyword">char</SPAN>) c) )
	{
	*first++ = c;
	}
	streamsize result = <SPAN CLASS="keyword">static_cast</SPAN><streamsize>(first - s);
	<SPAN CLASS="keyword">return</SPAN> result == <SPAN CLASS='numeric_literal'>0</SPAN> && c != WOULD_BLOCK ?
	<SPAN CLASS='numeric_literal'>-1</SPAN> :
	result;
	}
	};</PRE>

	<P>
	Here <CODE>multichar_input_filter_tag</CODE> is a <A HREF="../guide/traits.html#category">category tag</A> identifying the Filter as a Multi-Character InputFilter.
	</P>
	<P>
	The Iostreams library defines two convenience classes, <A HREF="../classes/filter.html#synopsis"><CODE>multichar_input_filter</CODE></A> and <A HREF="../classes/filter.html#synopsis"><CODE>multichar_input_wfilter</CODE></A>, which provide the member <CODE>typedef</CODE>s <CODE>char_type</CODE> and <CODE>category</CODE> as well as default implementations of several member functions. When defining a new Multi-Character InputFilter, it is often sufficient to derive from <CODE>multichar_input_filter</CODE> or <CODE>multichar_input_wfilter</CODE> and to define a member function <CODE>read</CODE>.
	</P>

	<A NAME="detailed_specification"></A>
	<H2>Refinement of</H2>

	<P><A HREF="filter.html">Filter</A>.</P>

	<H2>Associated Types</H2>

	<TABLE CELLPADDING="5" BORDER="1">
	<TR><TD>Character type</TD><TD>The type of the characters in the filtered sequences</TD></TR>
	<TR>
	<TD>Category</TD>
	<TD>
	A type convertible to <A HREF="../guide/traits.html#category_tags"><CODE>filter_tag</CODE></A> and to <A HREF="../guide/modes.html#input"><CODE>input</CODE></A>
	</TD>
	</TR>
	<TR>
	<TD>Mode</TD>
	<TD>
	The unique <I>most-derived</I> <A HREF="../guide/modes.html#mode_tags">mode tag</A> to which Category is convertible
	</TD>
	</TR>
	</TABLE>

	<H2>Notation</H2>

	<TABLE CELLPADDING="2">
	<TR><TD><CODE>F</CODE></TD><TD>- A type which is a model of InputFilter</TD></TR>
	<TR><TD><CODE>D</CODE></TD><TD>- A type which is a model of <A HREF="device.html">Device</A>, with the same character type as <CODE>F</CODE> and with mode refining the mode of <CODE>F</CODE></TD></TR>
	<TR><TD><CODE>Ch</CODE></TD><TD>- The character type of <CODE>F</CODE></TD></TR>
	<TR><TD><CODE>Tr</CODE></A></TD><TD>- <A HREF="../classes/char_traits.html"><CODE>boost::iostreams::char_traits<Ch></CODE></A></TD></TR>
	<TR><TD><CODE>f</CODE></TD><TD>- Object of type <CODE>F</CODE></TD></TR>
	<TR><TD><CODE>d</CODE></TD><TD>- Object of type <CODE>D</CODE></TD></TR>
	<TR><TD><CODE>s</CODE></TD><TD>- Object of type <CODE>Ch*</CODE></TD></TR>
	<TR><TD><CODE>n</CODE></TD><TD>- Object of type <CODE>std::streamsize</CODE></TD></TR>
	<TR><TD><CODE>io</CODE></TD><TD>- Alias for namespace <CODE>boost::iostreams</CODE></TD></TR>
	</TABLE>

	<A NAME="semantics"></A>
	<H2>Valid Expressions / Semantics</H2>

	<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>typename <A HREF="../guide/traits.html#char_type_of_ref">char_type_of</A><F>::type</CODE></PRE>
	</TD>
	<TD><CODE>typename</CODE> of the character type</TD>
	<TD ALIGN="center">-</TD><TD ALIGN="center">-</TD>
	</TR>
	<TR>
	<TD>
	<PRE CLASS="plain_code"><CODE>typename <A HREF="../guide/traits.html#category_ref">category_of</A><F>::type</CODE></PRE>
	</TD>
	<TD><CODE>typename</CODE> of the category</TD>
	<TD ALIGN="center">-</TD><TD ALIGN="center">-</TD>
	</TR>
	<TR>
	<TD><PRE CLASS="plain_code"><CODE>f.get(d)</CODE></PRE></TD>
	<TD><CODE>Tr::int_type</CODE></TD>
	<TD>
	Convertible to <A HREF="../guide/modes.html#mode_tags"><CODE>input</CODE></A> but not to <A HREF="../guide/traits.html#category_tags"><CODE>multichar_tag</CODE></A>
	</TD>
	<TD>
	Returns the next character in the input sequence controlled by <CODE>f</CODE>, <CODE>Tr::eof()</CODE> if the end of the sequence has been reached or <CODE>Tr::would_block()</CODE> if input is temporarily unavilable because a call to <CODE>d</CODE> has produced fewer characters than requested. The input sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/get.html"><CODE>io::get</CODE></A>, <A HREF="../functions/read.html"><CODE>io::read</CODE></A> and <A HREF="../functions/putback.html"><CODE>io::putback</CODE></A>.
	</TD>
	</TR>
	<TR>
	<TD><PRE CLASS="plain_code"><CODE>f.read(d, s, n)</CODE></PRE></TD>
	<TD><PRE CLASS="plain_code"><CODE>std::streamsize</CODE></PRE></TD>
	<TD>
	Convertible to <A HREF="../guide/modes.html#mode_tags"><CODE>input</CODE></A> and to <A HREF="../guide/traits.html#category_tags"><CODE>multichar_tag</CODE></A>
	</TD>
	<TD>
	Reads up to <CODE>n</CODE> characters from the input sequence controlled by <CODE>f</CODE> into the buffer <CODE>s</CODE>, returning the number of characters read or <CODE>-1</CODE> to indicate end-of-sequence. A value less than <CODE>n</CODE> may be returned only at end-of-sequence or if input is temporarily unavilable because a call to <CODE>d</CODE> has produced fewer characters than requested. The input sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/get.html"><CODE>io::get</CODE></A>, <A HREF="../functions/read.html"><CODE>io::read</CODE></A> and <A HREF="../functions/putback.html"><CODE>io::putback</CODE></A>.
	</TD>
	</TR>
	</TABLE>

	<H2>Exceptions</H2>

	<P>
	Errors which occur during the execution of <CODE>get</CODE> and <CODE>read</CODE> are indicated by throwing exceptions. Reaching the end of the sequence is not an error.
	</P>

	<P>
	After an exception is thrown, an InputFilter must be in a consistent state; further i/o operations may throw exceptions but must have well-defined behaviour. Furthermore, unless it is <A HREF="closable.html">Closable</A>, it must be ready to begin processing a new character sequence.
	</P>

	<H2>Models</H2>

	<UL>
	<LI>The <A HREF="../guide/text_processing.html">Text Processing Filters</A>.
	<LI>The compression and decompression filters.
	</UL>

	<H2>Acknowledgments</H2>

	<P>
	The concept InputFilter was inspired by the <I>extractors</I> of <A CLASS="footnote_ref" HREF="../bibliography.html#kanze">[Kanze]</A>.
	</P>

	<!-- Begin Footnotes -->

	<HR>

	<P>
	<A CLASS="footnote_ref" NAME="note_1" HREF="#note_1_ref"><SUP>[1]</SUP></A>Technically, <CODE>boost::iostreams::get</CODE> requires that a Source be <A HREF="../concepts/direct.html"><I>indirect</I></A>.
	</P>

	<!-- End Footnotes -->

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	<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>

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