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  26 <div class="chapter">
  27 <div class="titlepage"><div>
  28 <div><h2 class="title">
  29 <a name="chrono"></a>Chapter&#160;8.&#160;Boost.Chrono 2.0.5</h2></div>
  30 <div><div class="author"><h3 class="author">
  31 <span class="firstname">Howard</span> <span class="surname">Hinnant</span>
  32 </h3></div></div>
  33 <div><div class="author"><h3 class="author">
  34 <span class="firstname">Beman</span> <span class="surname">Dawes</span>
  35 </h3></div></div>
  36 <div><div class="author"><h3 class="author">
  37 <span class="firstname">Vicente J.</span> <span class="surname">Botet Escriba</span>
  38 </h3></div></div>
  39 <div><p class="copyright">Copyright &#169; 2008 Howard Hinnant</p></div>
  40 <div><p class="copyright">Copyright &#169; 2006, 2008 Beman Dawes</p></div>
  41 <div><p class="copyright">Copyright &#169; 2009-2013 Vicente J. Botet Escriba</p></div>
  42 <div><div class="legalnotice">
  43 <a name="chrono.legal"></a><p>
  44         Distributed under the Boost Software License, Version 1.0. (See accompanying
  45         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>)
  46       </p>
  47 </div></div>
  48 </div></div>
  49 <div class="toc">
  50 <p><b>Table of Contents</b></p>
  51 <dl class="toc">
  52 <dt><span class="section"><a href="chrono.html#chrono.overview">Overview</a></span></dt>
  53 <dd><dl>
  54 <dt><span class="section"><a href="chrono.html#chrono.overview.motivation">Motivation</a></span></dt>
  55 <dt><span class="section"><a href="chrono.html#chrono.overview.description">Description</a></span></dt>
  56 </dl></dd>
  57 <dt><span class="section"><a href="chrono/users_guide.html">User's Guide</a></span></dt>
  58 <dd><dl>
  59 <dt><span class="section"><a href="chrono/users_guide.html#chrono.users_guide.getting_started">Getting Started</a></span></dt>
  60 <dt><span class="section"><a href="chrono/users_guide.html#chrono.users_guide.tutorial">Tutorial</a></span></dt>
  61 <dt><span class="section"><a href="chrono/users_guide.html#chrono.users_guide.examples">Examples</a></span></dt>
  62 <dt><span class="section"><a href="chrono/users_guide.html#chrono.users_guide.ext_references">External Resources</a></span></dt>
  63 </dl></dd>
  64 <dt><span class="section"><a href="chrono/reference.html">Reference </a></span></dt>
  65 <dd><dl>
  66 <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>
  67 <dt><span class="section"><a href="chrono/reference.html#chrono.reference.cpp0x">Included on the C++11 Recommendation</a></span></dt>
  68 <dt><span class="section"><a href="chrono/reference.html#chrono.reference.io_v1">Chrono I/O V1</a></span></dt>
  69 <dt><span class="section"><a href="chrono/reference.html#chrono.reference.io">Chrono I/O V2</a></span></dt>
  70 <dt><span class="section"><a href="chrono/reference.html#chrono.reference.round">Chrono Rounding Utilities</a></span></dt>
  71 <dt><span class="section"><a href="chrono/reference.html#chrono.reference.other_clocks">Other Clocks</a></span></dt>
  72 </dl></dd>
  73 <dt><span class="section"><a href="chrono/appendices.html">Appendices</a></span></dt>
  74 <dd><dl>
  75 <dt><span class="section"><a href="chrono/appendices.html#chrono.appendices.history">Appendix: History</a></span></dt>
  76 <dt><span class="section"><a href="chrono/appendices.html#chrono.appendices.rationale">Appendix: Rationale</a></span></dt>
  77 <dt><span class="section"><a href="chrono/appendices.html#chrono.appendices.implementation">Appendix: Implementation
  78       Notes</a></span></dt>
  79 <dt><span class="section"><a href="chrono/appendices.html#chrono.appendices.faq">Appendix: FAQ</a></span></dt>
  80 <dt><span class="section"><a href="chrono/appendices.html#chrono.appendices.acknowledgements">Appendix: Acknowledgements</a></span></dt>
  81 <dt><span class="section"><a href="chrono/appendices.html#chrono.appendices.todo">Appendix: Future plans</a></span></dt>
  82 </dl></dd>
  83 </dl>
  84 </div>
  85 <p>
  86     ]
  87   </p>
  88 <p>
  89     ]
  90   </p>
  91 <div class="section">
  92 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  93 <a name="chrono.overview"></a><a class="link" href="chrono.html#chrono.overview" title="Overview">Overview</a>
  94 </h2></div></div></div>
  95 <div class="toc"><dl class="toc">
  96 <dt><span class="section"><a href="chrono.html#chrono.overview.motivation">Motivation</a></span></dt>
  97 <dt><span class="section"><a href="chrono.html#chrono.overview.description">Description</a></span></dt>
  98 </dl></div>
  99 <div class="blockquote"><blockquote class="blockquote"><p>
 100         <span class="quote">&#8220;<span class="quote">What is time, then? If nobody asks me, I know; if I have to explain
 101         it to someone who has asked me, I do not know."</span>&#8221;</span>
 102       </p></blockquote></div>
 103 <div class="blockquote"><blockquote class="blockquote"><p>
 104         <span class="bold"><strong><span class="emphasis"><em>-- Augustine </em></span></strong></span>
 105       </p></blockquote></div>
 106 <h4>
 107 <a name="chrono.overview.h0"></a>
 108       <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
 109       to Use This Documentation</a>
 110     </h4>
 111 <p>
 112       This documentation makes use of the following naming and formatting conventions.
 113     </p>
 114 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 115 <li class="listitem">
 116           Code is in <code class="computeroutput"><span class="identifier">fixed</span> <span class="identifier">width</span>
 117           <span class="identifier">font</span></code> and is syntax-highlighted.
 118         </li>
 119 <li class="listitem">
 120           Replaceable text that you will need to supply is in <em class="replaceable"><code>italics</code></em>.
 121         </li>
 122 <li class="listitem">
 123           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>.
 124         </li>
 125 <li class="listitem">
 126           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>;
 127           that is, it is in code font and its name is followed by <code class="computeroutput"><span class="special">&lt;&gt;</span></code>
 128           to indicate that it is a class template.
 129         </li>
 130 <li class="listitem">
 131           If a name refers to a function-like macro, it is specified like this:
 132           <code class="computeroutput"><span class="identifier">MACRO</span><span class="special">()</span></code>;
 133           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
 134           macro. Object-like macros appear without the trailing <code class="computeroutput"><span class="special">()</span></code>.
 135         </li>
 136 <li class="listitem">
 137           Names that refer to <span class="emphasis"><em>concepts</em></span> in the generic programming
 138           sense are specified in CamelCase.
 139         </li>
 140 </ul></div>
 141 <div class="note"><table border="0" summary="Note">
 142 <tr>
 143 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../doc/src/images/note.png"></td>
 144 <th align="left">Note</th>
 145 </tr>
 146 <tr><td align="left" valign="top"><p>
 147         In addition, notes such as this one specify non-essential information that
 148         provides additional background or rationale.
 149       </p></td></tr>
 150 </table></div>
 151 <p>
 152       Finally, you can mentally add the following to any code fragments in this document:
 153     </p>
 154 <pre class="programlisting"><span class="comment">// Include all of Chrono files</span>
 155 <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>
 156 </pre>
 157 <div class="section">
 158 <div class="titlepage"><div><div><h3 class="title">
 159 <a name="chrono.overview.motivation"></a><a class="link" href="chrono.html#chrono.overview.motivation" title="Motivation">Motivation</a>
 160 </h3></div></div></div>
 161 <h5>
 162 <a name="chrono.overview.motivation.h0"></a>
 163         <span class="phrase"><a name="chrono.overview.motivation.time"></a></span><a class="link" href="chrono.html#chrono.overview.motivation.time">Time</a>
 164       </h5>
 165 <p>
 166         We all deal with time every day of our lives. We've intuitively known it
 167         since birth. Thus we are all very familiar with it and believe it to be a
 168         simple matter. The modeling of time in computer programs should be similarly
 169         simple. The unfortunate truth is that this perceived simplicity is only skin
 170         deep. Fortunately, we do not need a terribly complicated solution to meet
 171         the bulk of our needs. However, overly simplistic solutions can be dangerous
 172         and inefficient, and won't adapt as the computer industry evolves.
 173       </p>
 174 <p>
 175         <span class="bold"><strong>Boost.Chrono</strong></span> implements the new time facilities
 176         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
 177         provides background and motivation for key design decisions and is the source
 178         of a good deal of information in this documentation.
 179       </p>
 180 <p>
 181         In addition to the clocks provided by the standard proposal, <span class="bold"><strong>Boost.Chrono</strong></span>
 182         provides specific process and thread clocks.
 183       </p>
 184 <h5>
 185 <a name="chrono.overview.motivation.h1"></a>
 186         <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
 187         clock versus system and user time</a>
 188       </h5>
 189 <p>
 190         To make the timing facilities of Boost.Chrono more generally useful, the
 191         library provides a number of clocks that are thin wrappers around the operating
 192         system's process time API, thereby allowing the extraction of wall clock
 193         time, user CPU time, and system CPU time of the process. Wall clock time
 194         is the sum of CPU time and system CPU time. (On POSIX-like systems, this
 195         relies on <code class="computeroutput"><span class="identifier">times</span><span class="special">()</span></code>.
 196         On Windows, it relies on <code class="computeroutput"><span class="identifier">GetProcessTimes</span><span class="special">()</span></code>.)
 197       </p>
 198 </div>
 199 <div class="section">
 200 <div class="titlepage"><div><div><h3 class="title">
 201 <a name="chrono.overview.description"></a><a class="link" href="chrono.html#chrono.overview.description" title="Description">Description</a>
 202 </h3></div></div></div>
 203 <p>
 204         The <span class="bold"><strong>Boost.Chrono</strong></span> library provides:
 205       </p>
 206 <h5>
 207 <a name="chrono.overview.description.h0"></a>
 208         <span class="phrase"><a name="chrono.overview.description.standard"></a></span><a class="link" href="chrono.html#chrono.overview.description.standard">Standard</a>
 209       </h5>
 210 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 211 <li class="listitem">
 212             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
 213             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
 214             with a fixed number of clock ticks per unit. All of these units of time
 215             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.
 216           </li>
 217 <li class="listitem">
 218             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
 219             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
 220             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
 221             a <span class="emphasis"><em>clock</em></span>.
 222           </li>
 223 <li class="listitem">
 224             Several clocks, some of which may not be available on a particular platform:
 225             <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
 226             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
 227             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>.
 228           </li>
 229 </ul></div>
 230 <h5>
 231 <a name="chrono.overview.description.h1"></a>
 232         <span class="phrase"><a name="chrono.overview.description.other_clocks"></a></span><a class="link" href="chrono.html#chrono.overview.description.other_clocks">Other
 233         clocks</a>
 234       </h5>
 235 <p>
 236         To make the timing facilities more generally useful, <span class="bold"><strong>Boost.Chrono</strong></span>
 237         provides a number of clocks that are thin wrappers around the operating system's
 238         time APIs, thereby allowing the extraction of wall clock time, user CPU time,
 239         system CPU time spent by the process,
 240       </p>
 241 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 242 <li class="listitem">
 243             <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
 244             wall clock CPU time spent by the current process.
 245           </li>
 246 <li class="listitem">
 247             <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
 248             user-CPU time spent by the current process.
 249           </li>
 250 <li class="listitem">
 251             <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
 252             system-CPU time spent by the current process.
 253           </li>
 254 <li class="listitem">
 255             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
 256             real, user-CPU, and system-CPU process times together.
 257           </li>
 258 <li class="listitem">
 259             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
 260             giving the time spent by the current thread (when supported by a platform).
 261           </li>
 262 </ul></div>
 263 <p>
 264         Lastly, <span class="bold"><strong>Boost.Chrono</strong></span> includes <a href="http://www.boost.org/libs/typeof" target="_top">typeof</a>
 265         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
 266         auto with C++03 compilers.
 267       </p>
 268 <h5>
 269 <a name="chrono.overview.description.h2"></a>
 270         <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>
 271       </h5>
 272 <p>
 273         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
 274         these types much more convenient. In following the "you only pay for
 275         what you use" philosophy, this extra functionality is located in a header
 276         separate from &lt;boost/chrono/chrono.hpp&gt;, namely &lt;boost/chrono/chrono_io.hpp&gt;.
 277       </p>
 278 <p>
 279         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>
 280         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>.
 281         This textural representation uses <a href="http://en.wikipedia.org/wiki/SI_prefix#List_of_SI_prefixes" target="_top">SI
 282         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
 283         "milliseconds", or a hypothetical meter class to print out "millimeter".
 284         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
 285         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
 286         these facets so that the chrono i/o could be localizable. However Boost.Chrono
 287         doesn't provides a complete locale solution.
 288       </p>
 289 <p>
 290         <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
 291         UTC timepoints, strongly guided by ISO 9899:1999, Programming languages -
 292         C, ISO 9945:2003, Information Technology - Portable Operating System Interface
 293         (POSIX) and ISO 8601:2004, Data elements and interchange formats - Information
 294         interchange - Representation of dates and times.
 295       </p>
 296 <h5>
 297 <a name="chrono.overview.description.h3"></a>
 298         <span class="phrase"><a name="chrono.overview.description.rounding_utilities"></a></span><a class="link" href="chrono.html#chrono.overview.description.rounding_utilities">Rounding
 299         utilities</a>
 300       </h5>
 301 <p>
 302         A few simple rounding utility functions for working with durations.
 303       </p>
 304 <h5>
 305 <a name="chrono.overview.description.h4"></a>
 306         <span class="phrase"><a name="chrono.overview.description.caveat_emptor"></a></span><a class="link" href="chrono.html#chrono.overview.description.caveat_emptor">Caveat
 307         Emptor</a>
 308       </h5>
 309 <p>
 310         The underlying clocks provided by operating systems are subject to many seemingly
 311         arbitrary policies and implementation irregularities. That's a polite way
 312         of saying they tend to be flakey, and each operating system or even each
 313         clock has its own cruel and unusual forms of flakiness. Don't bet the farm
 314         on their accuracy, unless you have become deeply familiar with exactly what
 315         the specific operating system is guaranteeing, which is often very little.
 316       </p>
 317 </div>
 318 </div>
 319 </div>
 320 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 321 <td align="left"><p><small>Last revised: April 17, 2017 at 02:30:28 GMT</small></p></td>
 322 <td align="right"><div class="copyright-footer"></div></td>
 323 </tr></table>
 324 <hr>
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