1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 2<HTML> 3<HEAD> 4 <TITLE>Function Template seek</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">Function Template <CODE>seek</CODE></H1> 13 <HR CLASS="banner"> 14 15<!-- End Banner --> 16 17<DL class="page-index"> 18 <DT><A href="#overview">Overview</A></DT> 19 <DT><A href="#headers">Headers</A></DT> 20 <DT><A href="#reference">Reference</A></DT> 21</DL> 22 23<A NAME="overview"></A> 24<H2>Overview</H2> 25 26<P> 27 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/device.html">Device</A>.</P> 28</P> 29 30<A NAME="headers"></A> 31<H2>Headers</H2> 32 33<DL> 34 <DT><A CLASS="header" HREF="../../../../boost/iostreams/operations.hpp"><CODE><boost/iostreams/operations.hpp></CODE></A></DT> 35 <DT><A CLASS="header" HREF="../../../../boost/iostreams/seek.hpp"><CODE><boost/iostreams/seek.hpp></CODE></A></DT> 36</DL> 37 38<A NAME="reference"></A> 39<H2>Reference</H2> 40 41<A NAME="description"></A> 42<H4>Description</H4> 43 44Repositions one or both heads within the sequence controlled by a given instance of the template parameter <CODE>T</CODE>, returning the resulting stream position. 45 46<A NAME="synopsis"></A> 47<H4>Synopsis</H4> 48 49<PRE CLASS="broken_ie"><SPAN CLASS="keyword">namespace</SPAN> boost { <SPAN CLASS="keyword">namespace</SPAN> iostreams { 50 51<SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> <A CLASS="documented" HREF="#template_params">T</A>> 52std::streampos 53<A CLASS="documented" HREF="#semantics">seek</A>( T&, <A CLASS="documented" HREF="positioning.html#synopsis">stream_offset</A> <A CLASS="documented" HREF="#function_params">off</A>, std::ios_base::seekdir <A CLASS="documented" HREF="#function_params">way</A>, 54 std::ios_base::openmode <A CLASS="documented" HREF="#function_params">which</A> = 55 std::ios_base::in | std::ios_base::out ); 56 57} } <SPAN CLASS="comment">// End namespace boost::io</SPAN></PRE> 58 59<A NAME="template_params"></A> 60<H4>Template Parameters</H4> 61 62<TABLE STYLE="margin-left:2em" BORDER=0 CELLPADDING=2> 63<TR> 64 <TR> 65 <TD VALIGN="top"><I>T</I></TD><TD WIDTH="2em" VALIGN="top">-</TD> 66 <TD>A model of one of the <A HREF="../guide/concepts.html#device_concepts">Device</A> concepts which allows random access. 67 </TR> 68</TABLE> 69 70<A NAME="function_params"></A> 71<H4>Function Parameters</H4> 72 73<TABLE STYLE="margin-left:2em" BORDER=0 CELLPADDING=2> 74<TR> 75 <TR> 76 <TD VALIGN="top"><I>off</I></TD><TD WIDTH="2em" VALIGN="top">-</TD> 77 <TD>A <A HREF="positioning.html#synopsis"><CODE>stream_offset</CODE></A> 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>. 78 </TR> 79 <TR> 80 <TD VALIGN="top"><I>way</I></TD><TD WIDTH="2em" VALIGN="top">-</TD> 81 <TD><P>Determines the initial position to which the offset <I>off</I> is applied, as follows: 82 <UL> 83 <LI CLASS="square">if <I>way</I> is <CODE>std::ios_base::beg</CODE>, the initial position is the begining of the stream 84 <LI CLASS="square">if <I>way</I> is <CODE>std::ios_base::cur</CODE>, the initial position is the current position 85 <LI CLASS="square">if <I>way</I> is <CODE>std::ios_base::end</CODE>, the initial position is the end of the stream 86 </UL> 87 </P> 88 </TR> 89 <TR> 90 <TD VALIGN="top"><I>which</I></TD><TD WIDTH="2em" VALIGN="top">-</TD> 91 <TD>Determines whether the reading head, the writing head or both are repositioned. 92 </TR> 93</TABLE> 94 95<A NAME="semantics"></A> 96<H4>Semantics</H4> 97 98<P>The semantics of <CODE>seek</CODE> depends on the <A HREF="../guide/traits.html#category">category</A> of <CODE>T</CODE> as follows:</P> 99 100<TABLE STYLE="margin-left:2em" BORDER=1 CELLPADDING=4> 101 <TR><TH><CODE>category_of<T>::type</CODE></TH><TH>semantics</TH></TR> 102 <TR> 103 <TD VALIGN="top">convertible to <CODE>istream_tag</CODE> or <CODE>ostream_tag</CODE></TD> 104 <TD>returns: 105 <UL STYLE='margin-top:0'> 106 <LI><CODE>t.rdbuf()->pubseekpos(<A HREF="positioning.html#offset_to_position">offset_to_position</A>(off))</CODE>, if <CODE>way</CODE> is <CODE>std::ios_base::beg</CODE> and <CODE>off</CODE> is outside the range of <CODE>std::streamoff</CODE>, and 107 <LI><CODE>t.rdbuf()->pubseekoff(off, way)</CODE>, otherwise 108 <UL> 109 </TR> 110 <TR> 111 <TD VALIGN="top">convertible to <CODE>streambuf_tag</CODE></TD> 112 <TD>returns: 113 <UL STYLE='margin-top:0'> 114 <LI><CODE>t.pubseekpos(<A HREF="positioning.html#offset_to_position">offset_to_position</A>(off))</CODE>, if <CODE>way</CODE> is <CODE>std::ios_base::beg</CODE> and <CODE>off</CODE> is outside the range of <CODE>std::streamoff</CODE>, and 115 <LI><CODE>t.pubseekoff(off, way)</CODE>, otherwise 116 <UL> 117 </TR> 118 <TR> 119 <TD VALIGN="top">convertible to <CODE>input_seekable</CODE> but not to <CODE>output_seekable</CODE></TD> 120 <TD>returns <CODE>t.seek(off, way)</CODE></TD> 121 </TR> 122 <TR> 123 <TD VALIGN="top">convertible to <CODE>output_seekable</CODE> but not to <CODE>input_seekable</CODE></TD> 124 <TD>returns <CODE>t.seek(off, way)</CODE></TD> 125 </TR> 126 <TR> 127 <TD VALIGN="top">convertible to <CODE>dual_seekable</CODE> or to <CODE>bidirectional_seekable</CODE></TD> 128 <TD>returns <CODE>t.seek(off, way, which)</CODE></TD> 129 </TR> 130 <TR> 131 <TD VALIGN="top">convertible to <CODE>seekable</CODE></TD> 132 <TD>returns <CODE>t.seek(off, way)</CODE></TD> 133 </TR> 134 <TR> 135 <TD VALIGN="top"><I>otherwise</I></TD> 136 <TD>compile-time error</TD> 137 </TR> 138</TABLE> 139 140<P>In short: 141<UL> 142 <LI CLASS="square">If <CODE>T</CODE> is a standard stream or stream buffer type, delegates to <CODE>std::basic_streambuf::pubseekoff</CODE> or <CODE>std::basic_streambuf::pubseekpos</CODE>, as appropriate. 143 <LI CLASS="square">Otherwise, if the <A HREF="../guide/modes.html">mode</A> of <CODE>T</CODE> allows only one reading head, delegates to a member function <CODE>seek</CODE> which takes a <CODE>stream_offset</CODE> and a <CODE>seekdir</CODE> but no <CODE>openmode</CODE>. 144 <LI CLASS="square">Otherwise, if the <A HREF="../guide/modes.html">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>stream_offset</CODE>, a <CODE>seekdir</CODE> and an <CODE>openmode</CODE>. 145 <LI CLASS="square">If the <A HREF="../guide/modes.html">mode</A> of T does not permit random access, attempting to invoke <CODE>seek</CODE> is a compile-time error. 146</UL> 147 148<!-- Begin Footer --> 149 150<HR> 151 152<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> 153<P CLASS="copyright"> 154 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>) 155</P> 156 157<!-- End Footer --> 158 159</BODY>