1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 2<HTML> 3<HEAD> 4 <TITLE>SeekableFilter</TITLE> 5 <LINK REL="stylesheet" HREF="../../../../boost.css"> 6 <LINK REL="stylesheet" HREF="../theme/iostreams.css"> 7</HEAD> 8<BODY> 9 10<!-- Begin Banner --> 11 12 <H1 CLASS="title">SeekableFilter</H1> 13 <HR CLASS="banner"> 14 15<!-- End Banner --> 16 17<H2>Definition</H2> 18 19<P> 20 A SeekableFilter is a <A HREF="filter.html">Filter</A> whose <A HREF="../guide/modes.html">mode</A> refines <A HREF="../guide/modes.html#seekable">seekable</A>. 21</P> 22 23<H2>Description</H2> 24 25<P> 26 A SeekableFilter operates on the character sequence controlled by a <A HREF="seekable_device.html">SeekableDevice</A>, providing access to a filtered sequence having the same character type. It may expose the filtered sequence in two ways: 27 <OL> 28 <LI STYLE="list-style-type:lower-roman"> 29 by defining member function <CODE>get</CODE>, <CODE>put</CODE> and <CODE>seek</CODE>. 30 </LI> 31 <LI STYLE="list-style-type:lower-roman"> 32 by defining member functions <CODE>read</CODE>, <CODE>write</CODE> and <CODE>seek</CODE>. 33 </LI> 34 </OL> 35 The second alternative is provided for enhanced performance. SeekableFilters implementing this alternative are referred to as <I>Multi-Character</I>. (<I>See</I> <A HREF="multi_character.html">Multi-Character Filter</A>.) 36</P> 37 38<H2>Refinement of</H2> 39 40<P><A HREF="input_filter.html">InputFilter</A>, <A HREF="output_filter.html">OutputFilter</A>.</P> 41 42<H2>Associated Types</H2> 43 44<TABLE CELLPADDING="5" BORDER="1"> 45 <TR><TD>Character type</TD><TD>The type of the characters in the filtered sequences</TD></TR> 46 <TR> 47 <TD>Category</TD> 48 <TD> 49 A type convertible to <A HREF="../guide/traits.html#category_tags"><CODE>filter_tag</CODE></A> and to <A HREF="../guide/modes.html#seekable"><CODE>seekable</CODE></A> 50 </TD> 51 </TR> 52 <TR> 53 <TD>Mode</TD> 54 <TD> 55 The unique <I>most-derived</I> <A HREF="../guide/modes.html#mode_tags">mode tag</A> to which Category is convertible 56 </TD> 57 </TR> 58</TABLE> 59 60<H2>Notation</H2> 61 62<TABLE CELLPADDING="2"> 63 <TR><TD><CODE>F</CODE></TD><TD>- A type which is a model of SeekableFilter</TD></TR> 64 <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 convertible to the mode of <CODE>F</CODE></TD></TR> 65 <TR><TD><CODE>Ch</CODE></TD><TD>- The character type of <CODE>F</CODE></TD></TR> 66 <TR><TD><CODE>Tr</CODE></A></TD><TD>- <A HREF="../classes/char_traits.html"><CODE>boost::iostreams::char_traits<Ch></CODE></A></TD></TR> 67 <TR><TD><CODE>f</CODE></TD><TD>- Object of type <CODE>F</CODE></TD></TR> 68 <TR><TD><CODE>d</CODE></TD><TD>- Object of type <CODE>D</CODE></TD></TR> 69 <TR><TD><CODE>c</CODE></TD><TD>- Object of type <CODE>Ch</CODE></TD></TR> 70 <TR><TD><CODE>s1</CODE></TD><TD>- Object of type <CODE>Ch*</CODE></TD></TR> 71 <TR><TD><CODE>s2</CODE></TD><TD>- Object of type <CODE>const Ch*</CODE></TD></TR> 72 <TR><TD><CODE>n</CODE></TD><TD>- Object of type <CODE>std::streamsize</CODE></TD></TR> 73 <TR><TD><CODE>off</CODE></TD><TD>- Object of type <A HREF="../functions/positioning.html#synopsis"><CODE>stream_offset</CODE></A></TD></TR> 74 <TR><TD><CODE>way</CODE></TD><TD>- Object of type <CODE>std::ios_base::seekdir</CODE></TD></TR> 75 <TR><TD><CODE>which</CODE></TD><TD>- Object of type <CODE>std::ios_base::openmode</CODE></TD></TR> 76 <TR><TD><CODE>io</CODE></TD><TD>- Alias for namespace <CODE>boost::iostreams</CODE></TD></TR> 77</TABLE> 78 79<H2>Valid Expressions / Semantics</H2> 80 81<TABLE CELLPADDING="5" BORDER="1"> 82 <TR><TH>Expression</TH><TH>Expression Type</TH><TH>Category Precondition</TH><TH>Semantics</TH></TR> 83 <TR> 84 <TD> 85 <PRE CLASS="plain_code"><CODE>typename <A HREF="../guide/traits.html#char_type_of_ref">char_type_of</A><F>::type</CODE></PRE> 86 </TD> 87 <TD><CODE>typename</CODE> of the character type</TD> 88 <TD ALIGN="center">-</TD><TD ALIGN="center">-</TD> 89 </TR> 90 <TR> 91 <TD> 92 <PRE CLASS="plain_code"><CODE>typename <A HREF="../guide/traits.html#category_ref">category_of</A><F>::type</CODE></PRE> 93 </TD> 94 <TD><CODE>typename</CODE> of the category</TD> 95 <TD ALIGN="center">-</TD><TD ALIGN="center">-</TD> 96 </TR> 97 <TR> 98 <TD><PRE CLASS="plain_code"><CODE>f.get(d)</CODE></PRE></TD> 99 <TD><CODE>Tr::int_type</CODE></TD> 100 <TD ROWSPAN="2"> 101 Not convertible to <A HREF="../guide/traits.html#category_tags"><CODE>multi_char_tag</CODE></A> 102 </TD> 103 <TD> 104 Returns the next character in the sequence controlled by <CODE>f</CODE>, or <CODE>Tr::eof()</CODE> if the end of the sequence has been reached. The sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/read.html"><CODE>io::read</CODE></A> and <A HREF="../functions/putback.html"><CODE>io::putback</CODE></A>. 105 106 <!-- reads up to <CODE>n</CODE> characters from the sequence controlled by <CODE>dev</CODE> into <CODE>s</CODE>, returning the number of characters read; returning a value less than n indicates end-of-sequence --> 107 </TD> 108 </TR> 109 <TR> 110 <TD><PRE CLASS="plain_code"><CODE>f.put(d, c)</CODE></PRE></TD> 111 <TD><CODE>bool</CODE></TD> 112 <TD> 113 Writes the character <CODE>c</CODE> to the sequence controlled by <CODE>f</CODE>. The sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/write.html"><CODE>io::write</CODE></A>. 114 </TD> 115 </TR> 116 <TR> 117 <TD><PRE CLASS="plain_code"><CODE>f.read(d, s1, n)</CODE></PRE></TD> 118 <TD><PRE CLASS="plain_code"><CODE>std::streamsize</CODE></PRE></TD> 119 <TD ROWSPAN="2"> 120 Convertible to <A HREF="../guide/traits.html#category_tags"><CODE>multi_char_tag</CODE></A> 121 </TD> 122 <TD> 123 Reads up to <CODE></CODE> characters from the sequence controlled by <CODE>f</CODE> into the buffer <CODE>s1</CODE>, returning the number of characters read. Returning a value less than <CODE>n</CODE> indicates end-of-sequence. The sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/read.html"><CODE>io::read</CODE></A> and <A HREF="../functions/putback.html"><CODE>io::putback</CODE></A>. 124 </TD> 125 </TR> 126 <TR> 127 <TD><PRE CLASS="plain_code"><CODE>f.write(d, s2, n)</CODE></PRE></TD> 128 <TD><PRE CLASS="plain_code"><CODE>std::streamsize</CODE></PRE></TD> 129 <TD> 130 Writes n characters from the buffer <CODE>s2</CODE> to the sequence controlled by <CODE>f</CODE>. The sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/write.html"><CODE>io::write</CODE></A>. 131 </TD> 132 </TR> 133 <TR> 134 <TD><PRE CLASS="plain_code"><CODE>f.seek(d, off, way)</CODE></PRE></TD> 135 <TD><CODE>std::streampos</CODE></TD> 136 <TD ALIGN="center">-</TD> 137 <TD> 138 <P CLASS="concept"> 139 Advances the read/write head by <CODE>off</CODE> characters, returning the new position, where the offset is calculated from: 140 </P> 141 <UL STYLE="margin:0,0,0,auto"> 142 <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> 143 <LI STYLE="list-style-type:disc;list-style-image:none">the current position if <CODE>way</CODE> is <CODE>ios_base::cur</CODE> 144 <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> 145 </UL> 146 <P CLASS="concept"> 147 The sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/read.html"><CODE>io::seek</CODE></A> and using <A HREF="../functions/putback.html"><CODE>io::write</CODE></A> if the <A HREF="../guide/modes.html">mode</A> of <CODE>F</CODE> is convertible to <A HREF="../guide/modes.html#output"><CODE>output</CODE></A>. 148 </P> 149 </TD> 150 </TR> 151</TABLE> 152 153<H2>Exceptions</H2> 154 155<P> 156 Errors which occur during the execution of <CODE>get</CODE>, <CODE>put</CODE>, <CODE>read</CODE>, <CODE>write</CODE> or <CODE>seek</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. 157</P> 158 159<P> 160 After an exception is thrown, a SeekableFilter 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. 161</P> 162 163<H2>Models</H2> 164 165<!-- Begin Footer --> 166 167<HR> 168 169<P CLASS="copyright">© Copyright 2008 <a href="http://www.coderage.com/" target="_top">CodeRage, LLC</a><br/>© Copyright 2004-2007 <a href="https://www.boost.org/users/people/jonathan_turkanis.html" target="_top">Jonathan Turkanis</a></P> 170<P CLASS="copyright"> 171 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>) 172</P> 173 174<!-- End Footer --> 175 176</BODY>