boost-react-native-bundle

<html> <head> <meta http-equiv="Content-Language" content="en-us"> <meta name="GENERATOR" content="Microsoft FrontPage 5.0"> <meta name="ProgId" content="FrontPage.Editor.Document"> <meta http-equiv="Content-Type" content="text/html; charset=windows-1252"> <title>CPU Timers</title> <style type="text/css"> ins {background-color:#A0FFA0} del {background-color:#FFA0A0} body { font-family: sans-serif; max-width : 8.5in; margin: 1em; } </style> </head> <body> <table border="0" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111" width="750"> <tr> <td width="300"> <a href="../../../index.htm"> <img src="../../../boost.png" alt="boost.png (6897 bytes)" align="middle" width="300" height="86" border="0"></a></td> <td align="middle" width="430"> Timer Library CPU Timers</td> </tr> </table> <table border="0" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111" bgcolor="#D7EEFF" width="100%"> <tr> <td><a href="index.html">Timer Home</a>    <a href="cpu_timers.html">CPU timers</a>    <a href="original_timer.html">Original timers</a>    </td> </tr> </table> <h2><a name="Introduction">Introduction</a></h2> <table border="1" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111" align="right"> <tr> <td width="100%" bgcolor="#D7EEFF" align="center"> Contents</td> </tr> <tr> <td width="100%" bgcolor="#E8F5FF"> <a href="#Introduction">Introduction</a> <a href="#Example">Using the timers</a>   <a href="#using-auto_cpu_timer">Using <code>auto_cpu_timer</code></a>   <a href="#using-cpu_timer">Using <code>cpu_timer</code></a> <a href="#Timer-accuracy">Timer accuracy</a>   <a href="#Resolution">Resolution</a>   <a href="#Other-concerns">Other concerns</a>   <a href="#Recommendations">Recommendations</a> <a href="#Reference">Reference</a> <code> <a href="#Synopsis"><boost/timer/timer.hpp></a></code><a href="#Synopsis"> synopsis</a>   <a href="#Default-format">Default format</a>   <a href="#nanosecond_type">Typedef <code>nanosecond_type</code></a>   <a href="#Namespace-scope-functions">Namespace scope functions</a>       <a href="#format"><code>format()</code></a>   <a href="#Class-cpu_timer">Class <code>cpu_timer</code></a>   <code>  <a href="#cpu_timer-constructors">cpu_timer</a></code><a href="#cpu_timer-constructors"> constructors, destructor</a>      <code><a href="#cpu_timer-observers">cpu_timer</a></code><a href="#cpu_timer-observers"> observers</a>   <code>  <a href="#cpu_timer-actions">cpu_timer</a></code><a href="#cpu_timer-actions"> actions</a>   <a href="#Class-auto_cpu_timer">Class <code>auto_cpu_timer</code></a>    <code> <a href="#auto_cpu_timer-constructors">auto_cpu_timer</a></code><a href="#auto_cpu_timer-constructors"> constructors</a>    <code> <a href="#auto_cpu_timer-destructor">auto_cpu_timer</a></code><a href="#auto_cpu_timer-destructor"> destructor</a>      <a href="#auto_cpu_timer-observers"><code> auto_cpu_timer</code> observers</a>    <code> <a href="#auto_cpu_timer-actions">auto_cpu_timer</a></code><a href="#auto_cpu_timer-actions"> actions</a> <a href="#History">History</a> <a href="#Acknowledgements">Acknowledgements</a></tr> </table> Knowing how long a program takes to execute is useful in both test and production environments. It may also be helpful if such timing information is broken down into wall clock time, CPU time spent by the user, and CPU time spent by the operating system servicing user requests. Class <code><a href="#Class-cpu_timer">cpu_timer</a></code> measures wall clock time, user CPU process time, and system CPU process time. Class <code> <a href="#Class-auto_cpu_timer">auto_cpu_timer</a></code> is a refinement of <code>cpu_timer</code> that automatically reports the elapsed times when an <code> auto_cpu_timer</code> object is destroyed. <h2><a name="Setup">Setup</a></h2> Boost.Timer is implemented as a separately compiled library, so you must install binaries in a location that can be found by your linker. If you followed the <a href="http://www.boost.org/doc/libs/release/more/getting_started/index.html"> Boost Getting Started</a> instructions, that's already done for you. <h2><a name="Example">Using the timers</a></h2> <h3>Using <code><a name="using-auto_cpu_timer">auto_cpu_timer</a></code></h3> The simplest and most common use is to add the two lines highlighted below to a scope you want to time. See <code> <a href="../example/auto_cpu_example.cpp">auto_cpu_timer_example.cpp</a></code> for the source code. <blockquote> <pre>#include <boost/timer/timer.hpp> #include <cmath> int main() { boost::timer::auto_cpu_timer t; for (long i = 0; i < 100000000; ++i) std::sqrt(123.456L); // burn some time return 0; }</pre> </blockquote> When the <code>auto_cpu_timer</code> object is created, it starts timing. When it is destroyed at the end of the scope, its destructor stops the timer and displays timing information on the default output stream, <code>std::cout</code>. The output of this program will look something like this: <code>    5.713010s wall, 5.709637s user + 0.000000s system = 5.709637s CPU (99.9%)</code> In other words, this program ran in <code>5.713010</code> seconds as would be measured by a clock on the wall, the operating system charged it for <code>5.709637</code> seconds of user CPU time and 0 seconds of system CPU time, the total of these two was <code>5.709637</code>, and that represented <code>99.9</code> percent of the wall clock time. The output stream, number of decimal places reported, and reporting format can be controlled by <code>auto_cpu_timer</code> constructor arguments. Here is what the output from the above program would look like for several different sets of constructor arguments: <table border="1" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111"> <tr> <td>Construction</td> <td>Output</td> </tr> <tr> <td><code>t</code></td> <td><code>5.713010s wall, 5.709637s user + 0.000000s system = 5.709637s CPU (99.9%)</code></td> </tr> <tr> <td><code>t(std::cerr, 2)</code></td> <td><code>5.71s wall, 5.70s user + 0.00s system = 5.70s CPU (99.9%)</code></td> </tr> <tr> <td><code>t(1)</code></td> <td><code>5.7s wall, 5.7s user + 0.0s system = 5.7s CPU (99.9%)</code></td> </tr> <tr> <td><code>t(3, "%w seconds\n")</code></td> <td><code>5.713 seconds  </code></td> </tr> <tr> <td><code>t("%t sec CPU, %w sec real") </code></td> <td><code>5.709637 sec CPU, 5.713010 sec real</code></td> </tr> </table> The processing of the format string is described <a href="#format">here</a>. <h3> Using <code><a name="using-cpu_timer">cpu_timer</a></code></h3> The following code creates a checkpoint every 20 CPU seconds: <blockquote> <pre>using boost::timer::cpu_timer; using boost::timer::cpu_times; using boost::timer::nanosecond_type; ... nanosecond_type const twenty_seconds(20 * 1000000000LL); nanosecond_type last(0); cpu_timer timer; while (more_transactions) { process_a_transaction(); cpu_times const elapsed_times(timer.elapsed()); nanosecond_type const elapsed(elapsed_times.system + elapsed_times.user); if (elapsed >= twenty_seconds) { ... create a checkpoint ... last = elapsed; } }</pre> </blockquote> <h2><a name="Timer-accuracy">Timer accuracy</a></h2> How accurate are these timers? <h3><a name="Resolution">Resolution</a></h3> The resolution of a clock, and thus timers built on that clock, is the minimum period time that can be measured. The program <code> <a href="../test/cpu_timer_info.cpp">cpu_timer_info.cpp</a></code> measures the resolution of <code>cpu_timer</code>. <table border="1" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111"> <tr> <td rowspan="2" bgcolor="#D7EEFF">O/S</td> <td rowspan="2" bgcolor="#D7EEFF">Processor</td> <td colspan="2" align="center" bgcolor="#D7EEFF">Wall-clock</td> <td colspan="2" align="center" bgcolor="#D7EEFF">CPU</td> </tr> <tr> <td bgcolor="#D7EEFF">Resolution</td> <td bgcolor="#D7EEFF">Comments</td> <td align="center" bgcolor="#D7EEFF">User Resolution</td> <td align="center" bgcolor="#D7EEFF">System Resolution</td> </tr> <tr> <td>Mac OS X Lion</td> <td>Intel circa 2007</td> <td align="right">2100ns 2200ns</td> <td>Some variation within a range.</td> <td>10000000ns</td> <td>10000000ns</td> </tr> <tr> <td>Ubuntu Linux 11.4</td> <td>Intel circa 2005</td> <td align="right">516ns</td> <td>Very little variation, typically less than 5ns </td> <td>10000000ns</td> <td>10000000ns</td> </tr> <tr> <td>Windows 7</td> <td>Intel Core i7 860 @ 2.9 GHz</td> <td align="right">366ns</td> <td>Some variation, usually in multiples of 366ns</td> <td>15600100ns</td> <td>15600100ns</td> </tr> <tr> <td>Windows 7</td> <td>Intel Mobile T7200 @ 2.0 GHz</td> <td align="right">2050ns</td> <td>Much variation. Resolution degrades when processor slows, probably due to known chipset errata. </td> <td>15600100ns</td> <td>15600100ns</td> </tr> <tr> <td>Windows XP</td> <td>Intel Atom N2800 @ 1.0 GHz</td> <td align="right">1437ns</td> <td>Some variation.</td> <td>15625000ns</td> <td>15625000ns</td> </tr> </table> <h3><a name="Other-concerns">Other concerns</a></h3> Wall-clock timings are subject to many outside influences, such as the impact of other processes. <blockquote> <code>cpu_timer</code> and <code>auto_cpu_timer</code> obtain Wall-clock timings from Boost.Chrono's <code>high_resolution_clock</code>. On Intel compatible CPU's running Windows, Linux, and Mac OS X, this is a "steady clock" [C++11 20.11.3], but may not be steady on other platforms. <code> <a href="../test/cpu_timer_info.cpp">cpu_timer_info.cpp</a></code> reports whether or not the <code>high_resolution_clock</code> is steady on a particular platform. <a name="Steady-clocks">Steady clocks</a> are defined by the C++11 standard as clocks for which values never decrease as physical time advances and for which values advance at a steady rate relative to real time. That is, the clock may not be adjusted. Clocks that are steady never run backwards, even when the operating system's clock is reset backwards such as during a daylight saving time transition. </blockquote> Timings of debug builds are often several times slower than release builds, because compiler optimization is turned off and because libraries often supply very expensive error checks on debug builds. Synthetic benchmark code may be optimized way, particularly if NDEBUG is defined. It may be necessary to inspect generated code to verify this isn't happening. <h3 dir="ltr"><a name="Recommendations">Recommendations</a></h3> Think about what is important to your application. For a production process, the wall clock time may be what is most important. To study the efficiency of code, total CPU time (user + system) is often a much better measure. A useful recommendation is to never trust timings unless they are (1) at least 100 times longer than the CPU time resolution, (2) run multiple times, and (3) run on release builds. And results that are too good to be true need to be should be investigated skeptically. Shared libraries (DLLs and .so's) may incur extra time delays, including expensive disk accesses, the first time a timer or other function is called. If that would be misleading, static linking should be considered. <h2> <a name="Reference">Reference</a></h2> Specifications are given in the style of the C++ standard library (C++11, 17.5.1.4 [structure.specifications]). An additional Overview element may be provided to aid understanding. Overview elements are only informative - actual semantics are given by the other detailed specification elements. Functions not specified as <code>noexcept</code> will throw <code> std::bad_alloc</code> exceptions if a memory allocation error occurs. Other errors are reported by time values of -1. [Note: Modern hardware and operating systems have robust clock subsystems, so such errors are unusual if even possible at all. -- end note] The Timer library meets the same data race avoidance requirements as the C++11 standard library (17.6.5.9 [res.on.data.races]). Shared objects of Timer library types risk undefined behavior unless the user supplies a locking mechanism. See C++11, 17.6.4.10 [res.on.objects], Shared objects and the library. <h3> <code><boost/timer/timer.hpp></code> <a name="Synopsis">synopsis</a></h3> <table border="1" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111" width="100%"> <tr> <td bgcolor="#D7EEFF"> <blockquote> <pre>namespace boost { namespace timer { class <a href="#Class-cpu_timer">cpu_timer</a>; // wall clock, user, and system timer class <a href="#Class-auto_cpu_timer">auto_cpu_timer</a>; // automatic report() on destruction typedef boost::int_least64_t nanosecond_type; struct cpu_times { nanosecond_type wall; nanosecond_type user; nanosecond_type system; void clear(); }; const int <a name="default_places">default_places</a> = 6; std::string format(const cpu_times& times, short places, const std::string& format); std::string format(const cpu_times& times, short places = default_places); } // namespace timer } // namespace boost</pre> </blockquote> </td> </tr> </table> <h3><a name="Default-format">Default format</a></h3> The default format is " %ws wall, %us user + %ss system = %ts CPU (%p%)\n". <h3>Typedef <a name="nanosecond_type"><code>nanosecond_type</code></a></h3> The typedef <code>nanosecond_type</code> provides an implementation defined type capable of representing nanoseconds. For POSIX and Windows systems, <code> nanoseconds_type</code> is <code>boost::int_least64_t</code>. The underlying type is not based on the Boost Date-Time or Chrono library to avoid a dependency on a large library. This design choice may change at some future date. Although <code>nanosecond_type</code> is capable of representing one nanosecond, the actual resolution of common operating system timers may be much lower. For wall clock time on desktop systems circa 2010, resolution is often no better than than one microsecond. For user and system time, typical resolution is 15 milliseconds on Windows and 10 milliseconds on POSIX. <h3><a name="cpu_times">Struct <code>cpu_times</code></a></h3> Struct <code>cpu_times</code> packages the elapsed wall clock time, user process CPU time, and system process CPU time. See <a href="#Current-time-values">Current time values</a> for definitions of the source of these elapsed times. <pre>void clear();</pre> <blockquote> Effects: <code>wall = user = system = 0LL</code>. </blockquote> <h3><a name="Namespace-scope-functions">Namespace scope functions</a></h3> <pre>std::string <a name="format">format</a>(const <a href="#cpu_times">cpu_times</a>& times, short places, const std::string& format); std::string <a name="format">format</a>(const <a href="#cpu_times">cpu_times</a>& times, short places = default_places);</pre> <blockquote> Overview: Converts <code>times</code>'s values to strings representing seconds to <code>places</code> decimal places, and inserts them into the return string as controlled by <code>format</code>. Remarks: For the overload without the <code>format</code> argument, the <a href="#Default-format">default format</a> is used as <code>format</code>. Returns: A string that is a copy of <code>format</code>, except that any instances of the sequences shown below are replaced by the indicated value. Times are reported in seconds, shown to <code>std::max(0, std::min(default_places, 9))</code> decimal places. Percentage is reported to one decimal place. [Note: percentage may exceed 100% due to differences in how operating systems measure various times. --end note] <a name="Format-replacement-sequences">Format replacement sequences</a> <table border="1" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111"> <tr> <td align="center" bgcolor="#D7EEFF">Sequence</td> <td align="center" bgcolor="#D7EEFF">Replacement value</td> </tr> <tr> <td align="center"><code>%w</code></td> <td><code>times.wall</code></td> </tr> <tr> <td align="center"><code>%u</code></td> <td><code>times.user</code></td> </tr> <tr> <td align="center"><code>%s</code></td> <td><code>times.system</code></td> </tr> <tr> <td align="center"><code>%t</code></td> <td><code>times.user + times.system</code></td> </tr> <tr> <td align="center"><code>%p</code></td> <td>The percentage of <code>times.wall</code> represented by <code> times.user + times.system</code></td> </tr> </table> </blockquote> <h3><a name="Class-cpu_timer">Class <code>cpu_timer</code></a></h3> <code>cpu_timer</code> objects measure wall clock elapsed time and process elapsed time charged to the user and system. <a name="Current-time-values">Current time values</a> are the current wall clock time, user process time, and system process time as provided by the operating system: <ul> <li>Wall clock time is time as would be measured by an ordinary wristwatch or clock on the wall.</li> <li>User process time is "the CPU time charged for the execution of user instructions of the calling process." See <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/times.html"> POSIX</a>.</li> <li>System process time is "the CPU time charged for execution by the system on behalf of the calling process." See <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/times.html"> POSIX</a>.</li> </ul> <h3> <a name="cpu_timer-synopsis"> <code>cpu_timer</code> synopsis</a></h3> <table border="1" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111" width="100%"> <tr> <td bgcolor="#D7EEFF"> <pre> class <a name="cpu_timer">cpu_timer</a> { public: // constructor <a href="#cpu_timer-ctor">cpu_timer</a>() noexcept; // compiler generated; shown for exposition only ~cpu_timer() noexcept = default; cpu_timer(const cpu_timer&) noexcept = default; cpu_timer& operator=(const cpu_timer&) noexcept = default; // observers bool <a href="#is_stopped">is_stopped</a>() const noexcept; cpu_times <a href="#elapsed">elapsed</a>() const noexcept; std::string <a href="#cpu_timer-format">format</a>(int places, const std::string& format) const; std::string <a href="#cpu_timer-format">format</a>(int places = default_places) const; // actions void <a href="#start">start</a>() noexcept; void <a href="#stop">stop</a>() noexcept; void <a href="#resume">resume</a>() noexcept; };</pre> </td> </tr> </table> <h3><a name="cpu_timer-constructors"><code>cpu_timer</code> constructor</a></h3> <pre><a name="cpu_timer-ctor">cpu_timer</a>() noexcept;</pre> <blockquote> Effects: Constructs an object of type <code> cpu_timer</code>. Calls<code> start()</code>. </blockquote> <h3><a name="cpu_timer-observers"><code>cpu_timer</code> observers</a></h3> <pre>bool <a name="is_stopped">is_stopped</a>() const noexcept;</pre> <blockquote> Returns: <code>true</code> if <a href="#stop">stop()</a> was the most recent <a href="#cpu_timer-actions">action</a> function called, otherwise <code>false</code>. </blockquote> <pre>cpu_times <a name="elapsed">elapsed</a>() const noexcept;</pre> <blockquote> Returns: If <code>is_stopped()</code>, the accumulated elapsed times as of the previous <a href="#stop">stop()</a>. Otherwise, the elapsed times accumulated between the most recent call to <a href="#start">start()</a> or <a href="#resume">resume()</a> and the <a href="#Current-time-values">current time values</a>. </blockquote> <pre>std::string <a href="#cpu_timer-format">format</a>(int places, const std::string& format) const; std::string <a href="#cpu_timer-format">format</a>(int places = default_places) const;</pre> <blockquote> Overview: Returns a string for the current elapsed time as formatted by the <a href="#format">format non-member function</a>. Returns: <code>boost::<a href="#format">timer::format</a>(<a href="#elapsed">elapsed</a>(), places[, format])</code>. </blockquote> <h3><a name="cpu_timer-actions"><code>cpu_timer</code> actions</a></h3> <pre>void <a name="start">start</a>() noexcept;</pre> <blockquote> Effects: Begins accumulating elapsed time as of the <a href="#Current-time-values">current time values</a>. Postconditions: <code>!is_stopped()</code>. </blockquote> <pre>void <a name="stop">stop</a>() noexcept;</pre> <blockquote> Effects: If <code>!is_stopped()</code>, stops accumulating elapsed time as of the <a href="#Current-time-values">current time values</a>. <blockquote> [Note: This is observable via <code>elapsed()</code>. -- end note] </blockquote> Postconditions: <code>is_stopped()</code>. </blockquote> <pre>void <a name="resume">resume</a>() noexcept;</pre> <blockquote> Overview: Restarts the timer, accumulating additional elapsed time. Effects: If <code>is_stopped()</code>, resumes accumulating additional elapsed time, as of the <a href="#Current-time-values">current time values</a>. Otherwise, no effect. </blockquote> <h3><a name="Class-auto_cpu_timer">Class <code>auto_cpu_timer</code></a></h3> Class <code>auto_cpu_timer</code> adds a <code>report()</code> function to <code>class cpu_timer</code>, and automatically calls <code>report()</code> on destruction. <h3> <a name="auto_cpu_timer-synopsis"> <code>auto_cpu_timer</code> synopsis</a></h3> <table border="1" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111" width="100%"> <tr> <td bgcolor="#D7EEFF"> <pre> class <a name="auto_cpu_timer">auto_cpu_timer</a> : public <a href="#cpu_timer">cpu_timer</a> { public: explicit <a href="#auto_cpu_timer-1">auto_cpu_timer</a>(short places = default_places); <a href="#auto_cpu_timer-2">auto_cpu_timer</a>(short places, const std::string& format); explicit <a href="#auto_cpu_timer-3">auto_cpu_timer</a>(const std::string& format); <a href="#auto_cpu_timer-4">auto_cpu_timer</a>(std::ostream& os, short places, const std::string& format); explicit <a href="#auto_cpu_timer-5">auto_cpu_timer</a>(std::ostream& os, short places = default_places); <a href="#auto_cpu_timer-6">auto_cpu_timer</a>(std::ostream& os, const std::string& format); <a href="#auto_cpu_timer-destructor">~auto_cpu_timer</a>() noexcept; // compiler generated; shown for exposition only auto_cpu_timer(const auto_cpu_timer&) = default; auto_cpu_timer& operator=(const auto_cpu_timer&) = default; // <a href="#auto_cpu_timer-observers">observers</a> std::ostream& <a href="#ostream">ostream</a>() const noexcept; short <a href="#places">places</a>() const noexcept; const std::string& <a href="#format_string">format_string</a>() const noexcept; // <a href="#auto_cpu_timer-actions">actions</a> void <a href="#report">report</a>(); };</pre> </td> </tr> </table> [Note: Constructors without a <code>std::ostream&</code> argument argument imply <code> std::cout</code>. An argument default is avoided as it would require including <code><iostream></code>, with its high costs, even when the standard streams are not used. --end note] <h3><a name="auto_cpu_timer-constructors"><code>auto_cpu_timer</code> constructors</a></h3> <pre>explicit <a name="auto_cpu_timer-1">auto_cpu_timer</a>(short places = default_places); <a name="auto_cpu_timer-2">auto_cpu_timer</a>(short places, const std::string& format); explicit <a name="auto_cpu_timer-3">auto_cpu_timer</a>(const std::string& format);               <a name="auto_cpu_timer-4">auto_cpu_timer</a>(std::ostream& os, short places, const std::string& format); explicit <a name="auto_cpu_timer-5">auto_cpu_timer</a>(std::ostream& os, short places = default_places); <a name="auto_cpu_timer-6">auto_cpu_timer</a>(std::ostream& os, const std::string& format); </pre> <blockquote> Effects: Constructs an object of type <code> auto_cpu_timer</code> and stores the ostream, places, and format string data needed to establish the postconditions. Postconditions: <ul> <li>For overloads with an <code>os</code> argument, <code>ostream() == os</code>. Otherwise <code>ostream() == std::cout</code>.</li> <li><code>places() == places</code>.</li> <li>For overloads with a <code>format</code> argument, <code>format_string() == format</code>. Otherwise <code>format_string() == std::cout</code></li> </ul> </blockquote> <h3><a name="auto_cpu_timer-destructor"><code>auto_cpu_timer</code> destructor</a></h3> <pre>~auto_cpu_timer() noexcept;</pre> <blockquote> Effects: If <code>!is_stopped()</code>, stop(), <a href="#report"> report()</a>. [Note: Because the function is <code>noexcept</code>, implementation must ensure no exception escapes. --end note] </blockquote> <h3><a name="auto_cpu_timer-observers">auto_cpu_timer observers</a></h3> The observers allow testing of constructor postconditions and specification of other functionality without resorting to "for exposition only" private members. <pre>std::ostream& <a name="ostream">ostream</a>() const noexcept;</pre> <blockquote> Returns: The ostream stored by construction or subsequent copy assignment. </blockquote> <pre>short <a name="places">places</a>() const noexcept;</pre> <blockquote> Returns: The places stored by construction or subsequent copy assignment. </blockquote> <pre>const std::string& <a name="format_string">format_string</a>() const noexcept;</pre> <blockquote> Returns: The format string stored by construction or subsequent copy assignment. </blockquote> <h3><a name="auto_cpu_timer-actions"><code>auto_cpu_timer</code> actions</a></h3> <pre>void <a name="report">report</a>();</pre> <blockquote> Effects: As if: <blockquote> <pre>ostream() << timer::format(elapsed(), places(), format_string());</pre> </blockquote> [Note: It may be desirable to call <code>stop()</code> before calling <code>report()</code> because doing I/O while the timer is running might produce misleading results. <code>resume()</code> may be called afterwards to continue timing. --end note] </blockquote> <h2><a name="History">History</a></h2> Beman Dawes and Rob Stewart developed version 2 of the library. Beman did the initial development. Rob contributed many corrections, comments, and suggestions. In particular, he suggested the <code>resume()</code> and <code>format()</code> functions, resulting in improved ease-of-use for several use cases. <h2><a name="Acknowledgements">Acknowledgements</a></h2> Comments and suggestions came from Greg Rubino, Dave Abrahams, Vicente Botet, and John Maddock. <hr> Revised: 08 October 2011 � Copyright Beman Dawes, 2006 � Copyright Beman Dawes and Robert Stewart, 2011 Distributed under the Boost Software License, Version 1.0. See <a href="http://www.boost.org/LICENSE_1_0.txt">www.boost.org/ LICENSE_1_0.txt</a> </body> </html>