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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_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">basic_waitable_timer</code> template uses wait traits to allow
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programs to customize <code class="computeroutput">wait</code> and <code class="computeroutput">async_wait</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">e</code> to be <code class="computeroutput">Clock::max()</code>
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(meaning never reached) or <code class="computeroutput">Clock::min()</code> (meaning always in the
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past). As a result, computing the duration until timer expiry as <code class="computeroutput">e
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- Clock::now()</code> may 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">Clock</code> meeting the <code class="computeroutput">Clock</code> requirements
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(C++Std [time.clock.req]), a type <code class="computeroutput">X</code> meets the <code class="computeroutput">WaitTraits</code>
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requirements 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">t</code> denotes a (possibly const) value of type
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<code class="computeroutput">Clock::time_point</code>; and <code class="computeroutput">d</code> denotes a (possibly const)
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value of type <code class="computeroutput">Clock::duration</code>.
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</p>
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<div class="table">
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<a name="boost_asio.reference.WaitTraits.t0"></a><p class="title"><b>Table 33. 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">X::to_wait_duration(d)</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">Clock::duration</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">Clock::duration</code> value to be used in a <code class="computeroutput">wait</code>
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or <code class="computeroutput">async_wait</code> operation. [<span class="emphasis"><em>Note:</em></span>
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The return value is typically representative of the duration <code class="computeroutput">d</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">X::to_wait_duration(t)</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">Clock::duration</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">Clock::duration</code> value to be used in a <code class="computeroutput">wait</code>
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or <code class="computeroutput">async_wait</code> operation. [<span class="emphasis"><em>Note:</em></span>
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The return value is typically representative of the duration from
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<code class="computeroutput">Clock::now()</code> until the time point <code class="computeroutput">t</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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</tbody>
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</table></div>
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<br class="table-break">
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2021 Christopher
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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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