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<div class="chapter">
<div class="titlepage"><div>
<div><h2 class="title">
<a name="chrono"></a>Chapter&#160;7.&#160;Boost.Chrono 2.0.5</h2></div>
<div><div class="author"><h3 class="author">
<span class="firstname">Howard</span> <span class="surname">Hinnant</span>
</h3></div></div>
<div><div class="author"><h3 class="author">
<span class="firstname">Beman</span> <span class="surname">Dawes</span>
</h3></div></div>
<div><div class="author"><h3 class="author">
<span class="firstname">Vicente J.</span> <span class="surname">Botet Escriba</span>
</h3></div></div>
<div><p class="copyright">Copyright &#169; 2008 Howard Hinnant</p></div>
<div><p class="copyright">Copyright &#169; 2006, 2008 Beman Dawes</p></div>
<div><p class="copyright">Copyright &#169; 2009-2013 Vicente J. Botet Escriba</p></div>
<div><div class="legalnotice">
<a name="chrono.legal"></a><p>
        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" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div></div>
</div></div>
<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="section"><a href="chrono.html#chrono.overview">Overview</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="chrono.html#chrono.overview.motivation">Motivation</a></span></dt>
<dt><span class="section"><a href="chrono.html#chrono.overview.description">Description</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="chrono/users_guide.html">User's Guide</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="chrono/users_guide.html#chrono.users_guide.getting_started">Getting Started</a></span></dt>
<dt><span class="section"><a href="chrono/users_guide.html#chrono.users_guide.tutorial">Tutorial</a></span></dt>
<dt><span class="section"><a href="chrono/users_guide.html#chrono.users_guide.examples">Examples</a></span></dt>
<dt><span class="section"><a href="chrono/users_guide.html#chrono.users_guide.ext_references">External Resources</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="chrono/reference.html">Reference </a></span></dt>
<dd><dl>
<dt><span class="section"><a href="chrono/reference.html#chrono.reference.chrono_include_hpp">Header <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">chrono</span><span class="special">/</span><span class="identifier">include</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code></a></span></dt>
<dt><span class="section"><a href="chrono/reference.html#chrono.reference.cpp0x">Included on the C++11 Recommendation</a></span></dt>
<dt><span class="section"><a href="chrono/reference.html#chrono.reference.io_v1">Chrono I/O V1</a></span></dt>
<dt><span class="section"><a href="chrono/reference.html#chrono.reference.io">Chrono I/O V2</a></span></dt>
<dt><span class="section"><a href="chrono/reference.html#chrono.reference.round">Chrono Rounding Utilities</a></span></dt>
<dt><span class="section"><a href="chrono/reference.html#chrono.reference.other_clocks">Other Clocks</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="chrono/appendices.html">Appendices</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="chrono/appendices.html#chrono.appendices.history">Appendix: History</a></span></dt>
<dt><span class="section"><a href="chrono/appendices.html#chrono.appendices.rationale">Appendix: Rationale</a></span></dt>
<dt><span class="section"><a href="chrono/appendices.html#chrono.appendices.implementation">Appendix: Implementation
      Notes</a></span></dt>
<dt><span class="section"><a href="chrono/appendices.html#chrono.appendices.faq">Appendix: FAQ</a></span></dt>
<dt><span class="section"><a href="chrono/appendices.html#chrono.appendices.acknowledgements">Appendix: Acknowledgements</a></span></dt>
<dt><span class="section"><a href="chrono/appendices.html#chrono.appendices.todo">Appendix: Future plans</a></span></dt>
</dl></dd>
</dl>
</div>
<p>
    ]
  </p>
<p>
    ]
  </p>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="chrono.overview"></a><a class="link" href="chrono.html#chrono.overview" title="Overview">Overview</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="chrono.html#chrono.overview.motivation">Motivation</a></span></dt>
<dt><span class="section"><a href="chrono.html#chrono.overview.description">Description</a></span></dt>
</dl></div>
<div class="blockquote"><blockquote class="blockquote"><p>
        <span class="quote">&#8220;<span class="quote">What is time, then? If nobody asks me, I know; if I have to explain
        it to someone who has asked me, I do not know."</span>&#8221;</span>
      </p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
        <span class="bold"><strong><span class="emphasis"><em>-- Augustine </em></span></strong></span>
      </p></blockquote></div>
<h4>
<a name="chrono.overview.h0"></a>
      <span class="phrase"><a name="chrono.overview.how_to_use_this_documentation"></a></span><a class="link" href="chrono.html#chrono.overview.how_to_use_this_documentation">How
      to Use This Documentation</a>
    </h4>
<p>
      This documentation makes use of the following naming and formatting conventions.
    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
          Code is in <code class="computeroutput"><span class="identifier">fixed</span> <span class="identifier">width</span>
          <span class="identifier">font</span></code> and is syntax-highlighted.
        </li>
<li class="listitem">
          Replaceable text that you will need to supply is in <em class="replaceable"><code>italics</code></em>.
        </li>
<li class="listitem">
          Free functions are rendered in the code font followed by <code class="computeroutput"><span class="special">()</span></code>, as in <code class="computeroutput"><span class="identifier">free_function</span><span class="special">()</span></code>.
        </li>
<li class="listitem">
          If a name refers to a class template, it is specified like this: <code class="computeroutput"><span class="identifier">class_template</span><span class="special">&lt;&gt;</span></code>;
          that is, it is in code font and its name is followed by <code class="computeroutput"><span class="special">&lt;&gt;</span></code>
          to indicate that it is a class template.
        </li>
<li class="listitem">
          If a name refers to a function-like macro, it is specified like this:
          <code class="computeroutput"><span class="identifier">MACRO</span><span class="special">()</span></code>;
          that is, it is uppercase in code font and its name is followed by <code class="computeroutput"><span class="special">()</span></code> to indicate that it is a function-like
          macro. Object-like macros appear without the trailing <code class="computeroutput"><span class="special">()</span></code>.
        </li>
<li class="listitem">
          Names that refer to <span class="emphasis"><em>concepts</em></span> in the generic programming
          sense are specified in CamelCase.
        </li>
</ul></div>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
        In addition, notes such as this one specify non-essential information that
        provides additional background or rationale.
      </p></td></tr>
</table></div>
<p>
      Finally, you can mentally add the following to any code fragments in this document:
    </p>
<pre class="programlisting"><span class="comment">// Include all of Chrono files</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">chrono</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="chrono.overview.motivation"></a><a class="link" href="chrono.html#chrono.overview.motivation" title="Motivation">Motivation</a>
</h3></div></div></div>
<h5>
<a name="chrono.overview.motivation.h0"></a>
        <span class="phrase"><a name="chrono.overview.motivation.time"></a></span><a class="link" href="chrono.html#chrono.overview.motivation.time">Time</a>
      </h5>
<p>
        We all deal with time every day of our lives. We've intuitively known it
        since birth. Thus we are all very familiar with it and believe it to be a
        simple matter. The modeling of time in computer programs should be similarly
        simple. The unfortunate truth is that this perceived simplicity is only skin
        deep. Fortunately, we do not need a terribly complicated solution to meet
        the bulk of our needs. However, overly simplistic solutions can be dangerous
        and inefficient, and won't adapt as the computer industry evolves.
      </p>
<p>
        <span class="bold"><strong>Boost.Chrono</strong></span> implements the new time facilities
        in C++11, as proposed in <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2661.htm" target="_top"><span class="bold"><strong>N2661 - A Foundation to Sleep On</strong></span></a>. That document
        provides background and motivation for key design decisions and is the source
        of a good deal of information in this documentation.
      </p>
<p>
        In addition to the clocks provided by the standard proposal, <span class="bold"><strong>Boost.Chrono</strong></span>
        provides specific process and thread clocks.
      </p>
<h5>
<a name="chrono.overview.motivation.h1"></a>
        <span class="phrase"><a name="chrono.overview.motivation.wall_clock_versus_system_and_user_time"></a></span><a class="link" href="chrono.html#chrono.overview.motivation.wall_clock_versus_system_and_user_time">Wall
        clock versus system and user time</a>
      </h5>
<p>
        To make the timing facilities of Boost.Chrono more generally useful, the
        library provides a number of clocks that are thin wrappers around the operating
        system's process time API, thereby allowing the extraction of wall clock
        time, user CPU time, and system CPU time of the process. Wall clock time
        is the sum of CPU time and system CPU time. (On POSIX-like systems, this
        relies on <code class="computeroutput"><span class="identifier">times</span><span class="special">()</span></code>.
        On Windows, it relies on <code class="computeroutput"><span class="identifier">GetProcessTimes</span><span class="special">()</span></code>.)
      </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="chrono.overview.description"></a><a class="link" href="chrono.html#chrono.overview.description" title="Description">Description</a>
</h3></div></div></div>
<p>
        The <span class="bold"><strong>Boost.Chrono</strong></span> library provides:
      </p>
<h5>
<a name="chrono.overview.description.h0"></a>
        <span class="phrase"><a name="chrono.overview.description.standard"></a></span><a class="link" href="chrono.html#chrono.overview.description.standard">Standard</a>
      </h5>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            A means to represent time durations: managed by the generic <a class="link" href="chrono/reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration&lt;&gt;"><code class="computeroutput"><span class="identifier">duration</span></code></a> class . Examples of
            time durations include days, <a class="link" href="chrono/reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput"><span class="identifier">minutes</span></code></a>, <a class="link" href="chrono/reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput"><span class="identifier">seconds</span></code></a> and <a class="link" href="chrono/reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput"><span class="identifier">nanoseconds</span></code></a>, which can be represented
            with a fixed number of clock ticks per unit. All of these units of time
            duration are united with a generic interface by the <a class="link" href="chrono/reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration&lt;&gt;"><code class="computeroutput"><span class="identifier">duration</span></code></a> facility.
          </li>
<li class="listitem">
            A type for representing points in time: <a class="link" href="chrono/reference.html#chrono.reference.cpp0x.time_point_hpp.time_point" title="Class template time_point&lt;&gt;"><code class="computeroutput"><span class="identifier">time_point</span></code></a>. A <a class="link" href="chrono/reference.html#chrono.reference.cpp0x.time_point_hpp.time_point" title="Class template time_point&lt;&gt;"><code class="computeroutput"><span class="identifier">time_point</span></code></a> represents an epoch
            plus or minus a <a class="link" href="chrono/reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration&lt;&gt;"><code class="computeroutput"><span class="identifier">duration</span></code></a>. The library leaves
            epochs unspecified. A <a class="link" href="chrono/reference.html#chrono.reference.cpp0x.time_point_hpp.time_point" title="Class template time_point&lt;&gt;"><code class="computeroutput"><span class="identifier">time_point</span></code></a> is associated with
            a <span class="emphasis"><em>clock</em></span>.
          </li>
<li class="listitem">
            Several clocks, some of which may not be available on a particular platform:
            <a class="link" href="chrono/reference.html#chrono.reference.cpp0x.system_clocks_hpp.system_clock" title="Class system_clock"><code class="computeroutput"><span class="identifier">system_clock</span></code></a>, <a class="link" href="chrono/reference.html#chrono.reference.cpp0x.system_clocks_hpp.steady_clock" title="Class steady_clock"><code class="computeroutput"><span class="identifier">steady_clock</span></code></a> and <a class="link" href="chrono/reference.html#chrono.reference.cpp0x.system_clocks_hpp.high_resolution_clock" title="Class high_resolution_clock"><code class="computeroutput"><span class="identifier">high_resolution_clock</span></code></a>. A clock
            is a pairing of a <a class="link" href="chrono/reference.html#chrono.reference.cpp0x.time_point_hpp.time_point" title="Class template time_point&lt;&gt;"><code class="computeroutput"><span class="identifier">time_point</span></code></a> and <a class="link" href="chrono/reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration&lt;&gt;"><code class="computeroutput"><span class="identifier">duration</span></code></a>, and a function which
            returns a <a class="link" href="chrono/reference.html#chrono.reference.cpp0x.time_point_hpp.time_point" title="Class template time_point&lt;&gt;"><code class="computeroutput"><span class="identifier">time_point</span></code></a> representing <span class="emphasis"><em>now</em></span>.
          </li>
</ul></div>
<h5>
<a name="chrono.overview.description.h1"></a>
        <span class="phrase"><a name="chrono.overview.description.other_clocks"></a></span><a class="link" href="chrono.html#chrono.overview.description.other_clocks">Other
        clocks</a>
      </h5>
<p>
        To make the timing facilities more generally useful, <span class="bold"><strong>Boost.Chrono</strong></span>
        provides a number of clocks that are thin wrappers around the operating system's
        time APIs, thereby allowing the extraction of wall clock time, user CPU time,
        system CPU time spent by the process,
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            <a class="link" href="chrono/reference.html#chrono.reference.other_clocks.process_cpu_clocks_hpp.process_real_cpu_clock" title="Class process_real_cpu_clock"><code class="computeroutput"><span class="identifier">process_real_cpu_clock</span></code></a>, captures
            wall clock CPU time spent by the current process.
          </li>
<li class="listitem">
            <a class="link" href="chrono/reference.html#chrono.reference.other_clocks.process_cpu_clocks_hpp.process_user_cpu_clock" title="Class process_user_cpu_clock"><code class="computeroutput"><span class="identifier">process_user_cpu_clock</span></code></a>, captures
            user-CPU time spent by the current process.
          </li>
<li class="listitem">
            <a class="link" href="chrono/reference.html#chrono.reference.other_clocks.process_cpu_clocks_hpp.process_system_cpu_clock" title="Class process_system_cpu_clock"><code class="computeroutput"><span class="identifier">process_system_cpu_clock</span></code></a>, captures
            system-CPU time spent by the current process.
          </li>
<li class="listitem">
            A tuple-like class <a class="link" href="chrono/reference.html#chrono.reference.other_clocks.process_cpu_clocks_hpp.process_cpu_clock" title="Class process_cpu_clock"><code class="computeroutput"><span class="identifier">process_cpu_clock</span></code></a>, that captures
            real, user-CPU, and system-CPU process times together.
          </li>
<li class="listitem">
            A <a class="link" href="chrono/reference.html#chrono.reference.other_clocks.thread_clock_hpp.thread_clock" title="Class thread_clock"><code class="computeroutput"><span class="identifier">thread_clock</span></code></a> thread steady clock
            giving the time spent by the current thread (when supported by a platform).
          </li>
</ul></div>
<p>
        Lastly, <span class="bold"><strong>Boost.Chrono</strong></span> includes <a href="http://www.boost.org/libs/typeof" target="_top">typeof</a>
        registration for <a class="link" href="chrono/reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration&lt;&gt;"><code class="computeroutput"><span class="identifier">duration</span></code></a> and <a class="link" href="chrono/reference.html#chrono.reference.cpp0x.time_point_hpp.time_point" title="Class template time_point&lt;&gt;"><code class="computeroutput"><span class="identifier">time_point</span></code></a> to permit using emulated
        auto with C++03 compilers.
      </p>
<h5>
<a name="chrono.overview.description.h2"></a>
        <span class="phrase"><a name="chrono.overview.description.i_o"></a></span><a class="link" href="chrono.html#chrono.overview.description.i_o">I/O</a>
      </h5>
<p>
        It provides I/O for <a class="link" href="chrono/reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration&lt;&gt;"><code class="computeroutput"><span class="identifier">duration</span></code></a> and <a class="link" href="chrono/reference.html#chrono.reference.cpp0x.time_point_hpp.time_point" title="Class template time_point&lt;&gt;"><code class="computeroutput"><span class="identifier">time_point</span></code></a>. This I/O makes use of
        these types much more convenient. In following the "you only pay for
        what you use" philosophy, this extra functionality is located in a header
        separate from &lt;boost/chrono/chrono.hpp&gt;, namely &lt;boost/chrono/chrono_io.hpp&gt;.
      </p>
<p>
        It builds on <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">ratio</span><span class="special">/</span><span class="identifier">ratio_io</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
        to provide readable and flexible formatting and parsing for types in <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">chrono</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>.
        This textural representation uses <a href="http://en.wikipedia.org/wiki/SI_prefix#List_of_SI_prefixes" target="_top">SI
        prefixes</a> whenever possible. This makes it easy for <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">milliseconds</span></code> to be represented by the text
        "milliseconds", or a hypothetical meter class to print out "millimeter".
        The <a class="link" href="chrono/reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration&lt;&gt;"><code class="computeroutput"><span class="identifier">duration</span></code></a> and the <a class="link" href="chrono/reference.html#chrono.reference.cpp0x.time_point_hpp.time_point" title="Class template time_point&lt;&gt;"><code class="computeroutput"><span class="identifier">time_point</span></code></a> i/o can be customized
        through the new facets: <a class="link" href="chrono/reference.html#chrono.reference.io.duration_units_hpp.duration_units" title="Template Class duration_units"><code class="computeroutput"><span class="identifier">duration_units</span></code></a> and <a class="link" href="chrono/reference.html#chrono.reference.io.time_point_units_hpp.time_point_units" title="Template Class time_point_units"><code class="computeroutput"><span class="identifier">time_point_units</span></code></a>. The user can specialize
        these facets so that the chrono i/o could be localizable. However Boost.Chrono
        doesn't provides a complete locale solution.
      </p>
<p>
        <code class="computeroutput"><span class="identifier">system_clock</span><span class="special">::</span><span class="identifier">time_point</span></code> I/O is proposed in terms of
        UTC timepoints, strongly guided by ISO 9899:1999, Programming languages -
        C, ISO 9945:2003, Information Technology - Portable Operating System Interface
        (POSIX) and ISO 8601:2004, Data elements and interchange formats - Information
        interchange - Representation of dates and times.
      </p>
<h5>
<a name="chrono.overview.description.h3"></a>
        <span class="phrase"><a name="chrono.overview.description.rounding_utilities"></a></span><a class="link" href="chrono.html#chrono.overview.description.rounding_utilities">Rounding
        utilities</a>
      </h5>
<p>
        A few simple rounding utility functions for working with durations.
      </p>
<h5>
<a name="chrono.overview.description.h4"></a>
        <span class="phrase"><a name="chrono.overview.description.caveat_emptor"></a></span><a class="link" href="chrono.html#chrono.overview.description.caveat_emptor">Caveat
        Emptor</a>
      </h5>
<p>
        The underlying clocks provided by operating systems are subject to many seemingly
        arbitrary policies and implementation irregularities. That's a polite way
        of saying they tend to be flakey, and each operating system or even each
        clock has its own cruel and unusual forms of flakiness. Don't bet the farm
        on their accuracy, unless you have become deeply familiar with exactly what
        the specific operating system is guaranteeing, which is often very little.
      </p>
</div>
</div>
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<td align="left"><p><small>Last revised: December 14, 2017 at 00:08:22 GMT</small></p></td>
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