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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>&lt;boost/iostreams/operations.hpp&gt;</CODE></A></DT>
35  <DT><A CLASS="header" HREF="../../../../boost/iostreams/seek.hpp"><CODE>&lt;boost/iostreams/seek.hpp&gt;</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>&lt;<SPAN CLASS="keyword">typename</SPAN> <A CLASS="documented" HREF="#template_params">T</A>&gt;
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::beg</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&lt;T&gt;::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<P CLASS="copyright">Revised
152<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
15320 May, 2004
154<!--webbot bot="Timestamp" endspan i-checksum="38504" -->
155</P>
156
157<P CLASS="copyright">&copy; Copyright <A HREF="http://www.kangaroologic.com" TARGET="_top">Jonathan Turkanis</A>, 2004</P>
158<P CLASS="copyright"> 
159    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>)
160</P>
161
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163
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