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157 lines
6.9 KiB
HTML
Executable File
157 lines
6.9 KiB
HTML
Executable File
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML>
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<HEAD>
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<TITLE>Function Template seek</TITLE>
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<LINK REL='stylesheet' HREF='../../../../boost.css'>
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<LINK REL='stylesheet' HREF='../theme/iostreams.css'>
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</HEAD>
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<BODY>
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<!-- Begin Banner -->
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<H1 CLASS='title'>Function Template <CODE>seek</CODE></H1>
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<HR CLASS='banner'>
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<!-- End Banner -->
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<DL class='page-index'>
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<DT><A href='#overview'>Overview</A></DT>
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<DT><A href='#headers'>Headers</A></DT>
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<DT><A href='#reference'>Reference</A></DT>
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</DL>
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<A NAME='overview'>
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<H2>Overview</H2>
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<P>
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The function template <CODE>seek</CODE> is used by the Iostreams Library to perform random access withing a sequence controlled by a <A HREF='../concepts.html#device_concepts.html'>Device</A> or by a standard stream or stream buffer. <CODE>seek</CODE> is designed to be overloaded or specialized for <A HREF='../concepts.html#device_concepts.html'>Device</A> types external to the Iostreams Library. The default implementation, however, should be suitable for most purposes.</P>
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</P>
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<P>
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<I>Note:</I> Unlike <CODE>read</CODE> and <CODE>write</CODE>, <CODE>seek</CODE> has no overload for Filter types. Filters which perform random access are required to have a member function <CODE>seek</CODE>.
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</P>
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<A NAME='headers'>
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<H2>Headers</H2>
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<DL>
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<DT><A CLASS='header' HREF='../../../../boost/iostreams/operations.hpp'><CODE><boost/iostreams/operations.hpp></CODE></A></DT>
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</DL>
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<A NAME='reference'>
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<H2>Reference</H2>
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<A NAME='description'>
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<H4>Description</H4>
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Repositions one or both heads within the sequence controlled by a given instance of the template parameter <CODE>T</CODE>, returning the resulting position.
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<A NAME='synopsis'>
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<H4>Synopsis</H4>
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<PRE CLASS='broken_ie'><SPAN CLASS="keyword">namespace</SPAN> boost { <SPAN CLASS="keyword">namespace</SPAN> iostreams {
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<SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> <A CLASS="documented" HREF="#template_params">T</A>>
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std::streamoff <A CLASS="documented" HREF="#semantics">seek</A>( T&, std::streamoff <A CLASS="documented" HREF="#function_params">off</A>, std::ios_base::seekdir <A CLASS="documented" HREF="#function_params">way</A>,
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std::ios_base::openmode <A CLASS="documented" HREF="#function_params">which</A> =
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std::ios_base::in | std::ios_base::out );
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} } <SPAN CLASS='comment'>// End namespace boost::io</SPAN></PRE>
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<A NAME='template_params'></A>
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<H4>Template Parameters</H4>
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<TABLE STYLE='margin-left:2em' BORDER=0 CELLPADDING=2>
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<TR>
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<TR>
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<TD VALIGN='top'><I>T</I></TD><TD WIDTH='2em' VALIGN='top'>-</TD>
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<TD>A model of one of the <A HREF='../concepts.html#device_concepts'>Device</A> concepts which allows random access, or a standard stream or stream buffer type.
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</TR>
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</TABLE>
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<A NAME='function_params'></A>
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<H4>Function Parameters</H4>
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<TABLE STYLE='margin-left:2em' BORDER=0 CELLPADDING=2>
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<TR>
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<TR>
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<TD VALIGN='top'><I>off</I></TD><TD WIDTH='2em' VALIGN='top'>-</TD>
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<TD>A <CODE>std::streamoff</CODE> value indicating the number of characters by which the appropriate reading or writing heads should be advanced. The initial position is determined by the parameter <I>way</I>.
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</TR>
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<TR>
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<TD VALIGN='top'><I>way</I></TD><TD WIDTH='2em' VALIGN='top'>-</TD>
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<TD><P>Determines the initial position to which the offset <I>off</I> is applied, as follows:
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<UL>
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<LI CLASS='square'>if <I>way</I> is <CODE>std::ios_base::beg</CODE>, the initial position is the begining of the stream
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<LI CLASS='square'>if <I>way</I> is <CODE>std::ios_base::cur</CODE>, the initial position is the current position
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<LI CLASS='square'>if <I>way</I> is <CODE>std::ios_base::beg</CODE>, the initial position is the end of the stream
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</UL>
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</P>
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</TR>
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<TR>
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<TD VALIGN='top'><I>which</I></TD><TD WIDTH='2em' VALIGN='top'>-</TD>
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<TD>Determines whether the reading head, the writing head or both are repositioned.
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</TR>
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</TABLE>
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<A NAME='semantics'></A>
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<H4>Semantics</H4>
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<P>The semantics of <CODE>seek</CODE> depends on the <A HREF="../traits.html#category">category</A> of <CODE>T</CODE> as follows:</P>
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<TABLE STYLE='margin-left:2em' BORDER=1 CELLPADDING=4>
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<TR><TH><CODE>io_category<T>::type</CODE></TH><TH>semantics</TH></TR>
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<TR>
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<TD VALIGN='top'>convertible to <CODE>istream_tag</CODE> or <CODE>ostream_tag</CODE></TD>
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<TD>returns <CODE>t.rdbuf()->pubseekoff(off, way)</CODE></TD>
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</TR>
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<TR>
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<TD VALIGN='top'>convertible to <CODE>streambuf_tag</CODE></TD>
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<TD>returns <CODE>t.pubseekoff(off, way)</CODE></TD>
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</TR>
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<TR>
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<TD VALIGN='top'>convertible to <CODE>input_seekable</CODE> but not to <CODE>output_seekable</CODE></TD>
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<TD>returns <CODE>t.seek(off, way)</CODE></TD>
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</TR>
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<TR>
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<TD VALIGN='top'>convertible to <CODE>output_seekable</CODE> but not to <CODE>input_seekable</CODE></TD>
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<TD>returns <CODE>t.seek(off, way)</CODE></TD>
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</TR>
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<TR>
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<TD VALIGN='top'>convertible to <CODE>dual_seekable</CODE> or to <CODE>bidirectional_seekable</CODE></TD>
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<TD>returns <CODE>t.seek(off, way, which)</CODE></TD>
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</TR>
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<TR>
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<TD VALIGN='top'>convertible to <CODE>seekable</CODE></TD>
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<TD>returns <CODE>t.seek(off, way)</CODE></TD>
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</TR>
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<TR>
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<TD VALIGN='top'><I>otherwise</I></TD>
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<TD>compile-time error</TD>
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</TR>
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</TABLE>
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<P>In short:
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<UL>
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<LI CLASS='square'>If <CODE>T</CODE> is a standard stream or stream buffer type, delegates to <CODE>std::basic_streambuf::pubseekoff</CODE>.
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<LI CLASS='square'>Otherwise, if the <A HREF='../modes.html'>i/o mode</A> of <CODE>T</CODE> allows only one reading head, delegates to a member function <CODE>seek</CODE> which takes a <CODE>streamoff</CODE> and a <CODE>seekdir</CODE> but no <CODE>openmode</CODE>.
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<LI CLASS='square'>Otherwise, if the <A HREF='../modes.html'>i/o mode</A> of <CODE>T</CODE> allows random access with two reading heads, delegates to a member function <CODE>seek</CODE> which takes a <CODE>streamoff</CODE>, a <CODE>seekdir</CODE> and an <CODE>openmode</CODE>.
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<LI CLASS='square'>If the <A HREF='../modes.html'>i/o mode</A> of T does not permit random access, attempting to invoke <CODE>seek</CODE> is a compile-time error.
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</UL>
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<!-- Begin Footer -->
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<HR>
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<P CLASS='copyright'>Revised
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<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
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20 May, 2004
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<!--webbot bot="Timestamp" endspan i-checksum="38504" -->
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</P>
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<P CLASS='copyright'>© Copyright Jonathan Turkanis, 2004</P>
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<P CLASS='copyright'>
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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>)
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</P>
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