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137 lines
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<title>Wait traits requirements</title>
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<table cellpadding="2" width="100%"><tr><td valign="top"><img alt="asio C++ library" width="250" height="60" src="../../asio.png"></td></tr></table>
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<a accesskey="p" href="WaitToken.html"><img src="../../prev.png" alt="Prev"></a><a accesskey="u" href="../reference.html"><img src="../../up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../home.png" alt="Home"></a><a accesskey="n" href="WriteHandler.html"><img src="../../next.png" alt="Next"></a>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="asio.reference.WaitTraits"></a><a class="link" href="WaitTraits.html" title="Wait traits requirements">Wait traits requirements</a>
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</h3></div></div></div>
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<p>
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The <code class="computeroutput"><span class="identifier">basic_waitable_timer</span></code>
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template uses wait traits to allow programs to customize <code class="computeroutput"><span class="identifier">wait</span></code>
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and <code class="computeroutput"><span class="identifier">async_wait</span></code> behavior.
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[<span class="emphasis"><em>Note:</em></span> Possible uses of wait traits include:<br> — To
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enable timers based on non-realtime clocks.<br> — Determining how quickly
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wallclock-based timers respond to system time changes.<br> — Correcting for
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errors or rounding timeouts to boundaries.<br> — Preventing duration overflow.
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That is, a program may set a timer's expiry <code class="computeroutput"><span class="identifier">e</span></code>
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to be <code class="computeroutput"><span class="identifier">Clock</span><span class="special">::</span><span class="identifier">max</span><span class="special">()</span></code> (meaning
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never reached) or <code class="computeroutput"><span class="identifier">Clock</span><span class="special">::</span><span class="identifier">min</span><span class="special">()</span></code> (meaning
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always in the past). As a result, computing the duration until timer expiry
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as <code class="computeroutput"><span class="identifier">e</span> <span class="special">-</span>
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<span class="identifier">Clock</span><span class="special">::</span><span class="identifier">now</span><span class="special">()</span></code> may
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cause overflow. —<span class="emphasis"><em>end note</em></span>]
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</p>
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<p>
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For a type <code class="computeroutput"><span class="identifier">Clock</span></code> meeting
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the <code class="computeroutput"><span class="identifier">Clock</span></code> requirements (C++Std
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[time.clock.req]), a type <code class="computeroutput"><span class="identifier">X</span></code>
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meets the <code class="computeroutput"><span class="identifier">WaitTraits</span></code> requirements
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if it satisfies the requirements listed below.
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</p>
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<p>
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In the table below, <code class="computeroutput"><span class="identifier">t</span></code> denotes
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a (possibly const) value of type <code class="computeroutput"><span class="identifier">Clock</span><span class="special">::</span><span class="identifier">time_point</span></code>;
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and <code class="computeroutput"><span class="identifier">d</span></code> denotes a (possibly
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const) value of type <code class="computeroutput"><span class="identifier">Clock</span><span class="special">::</span><span class="identifier">duration</span></code>.
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</p>
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<div class="table">
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<a name="asio.reference.WaitTraits.t0"></a><p class="title"><b>Table 35. WaitTraits requirements</b></p>
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<div class="table-contents"><table class="table" summary="WaitTraits requirements">
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<colgroup>
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<col>
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<col>
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<col>
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</colgroup>
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<thead><tr>
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<th>
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<p>
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expression
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</p>
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</th>
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<th>
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<p>
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return type
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</p>
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</th>
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<th>
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<p>
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assertion/note<br> pre/post-condition
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</p>
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</th>
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</tr></thead>
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<tbody>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">X</span><span class="special">::</span><span class="identifier">to_wait_duration</span><span class="special">(</span><span class="identifier">d</span><span class="special">)</span></code>
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</p>
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</td>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">Clock</span><span class="special">::</span><span class="identifier">duration</span></code>
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</p>
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</td>
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<td>
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<p>
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Returns a <code class="computeroutput"><span class="identifier">Clock</span><span class="special">::</span><span class="identifier">duration</span></code>
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value to be used in a <code class="computeroutput"><span class="identifier">wait</span></code>
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or <code class="computeroutput"><span class="identifier">async_wait</span></code> operation.
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[<span class="emphasis"><em>Note:</em></span> The return value is typically representative
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of the duration <code class="computeroutput"><span class="identifier">d</span></code>.
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—<span class="emphasis"><em>end note</em></span>]
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">X</span><span class="special">::</span><span class="identifier">to_wait_duration</span><span class="special">(</span><span class="identifier">t</span><span class="special">)</span></code>
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</p>
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</td>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">Clock</span><span class="special">::</span><span class="identifier">duration</span></code>
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</p>
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</td>
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<td>
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<p>
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Returns a <code class="computeroutput"><span class="identifier">Clock</span><span class="special">::</span><span class="identifier">duration</span></code>
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value to be used in a <code class="computeroutput"><span class="identifier">wait</span></code>
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or <code class="computeroutput"><span class="identifier">async_wait</span></code> operation.
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[<span class="emphasis"><em>Note:</em></span> The return value is typically representative
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of the duration from <code class="computeroutput"><span class="identifier">Clock</span><span class="special">::</span><span class="identifier">now</span><span class="special">()</span></code> until the time point <code class="computeroutput"><span class="identifier">t</span></code>. —<span class="emphasis"><em>end note</em></span>]
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</p>
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</td>
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</tr>
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</tbody>
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</table></div>
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</div>
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<br class="table-break">
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</div>
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<div class="copyright-footer">Copyright © 2003-2023 Christopher M. Kohlhoff<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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</p>
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</div>
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<hr>
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<a accesskey="p" href="WaitToken.html"><img src="../../prev.png" alt="Prev"></a><a accesskey="u" href="../reference.html"><img src="../../up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../home.png" alt="Home"></a><a accesskey="n" href="WriteHandler.html"><img src="../../next.png" alt="Next"></a>
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