boost-react-native-bundle

<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>User's Guide</title> <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../chrono.html" title="Chapter 6. Boost.Chrono 2.0.4"> <link rel="prev" href="../chrono.html" title="Chapter 6. Boost.Chrono 2.0.4"> <link rel="next" href="reference.html" title="Reference"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> <td align="center"><a href="../../../index.html">Home</a></td> <td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="../chrono.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../chrono.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="reference.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="chrono.users_guide"></a><a class="link" href="users_guide.html" title="User's Guide">User's Guide</a> </h2></div></div></div> <div class="toc"><dl class="toc"> <dt><a href="users_guide.html#chrono.users_guide.getting_started">Getting Started</a></dt> <dt><a href="users_guide.html#chrono.users_guide.tutorial">Tutorial</a></dt> <dt><a href="users_guide.html#chrono.users_guide.examples">Examples</a></dt> <dt><a href="users_guide.html#chrono.users_guide.ext_references">External Resources</a></dt> </dl></div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="chrono.users_guide.getting_started"></a><a class="link" href="users_guide.html#chrono.users_guide.getting_started" title="Getting Started">Getting Started</a> </h3></div></div></div> <div class="toc"><dl class="toc"> <dt><a href="users_guide.html#chrono.users_guide.getting_started.install">Installing Chrono</a></dt> <dt><a href="users_guide.html#chrono.users_guide.getting_started.hello_world__">Hello World! </a></dt> </dl></div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="chrono.users_guide.getting_started.install"></a><a class="link" href="users_guide.html#chrono.users_guide.getting_started.install" title="Installing Chrono">Installing Chrono</a> </h4></div></div></div> <h6> <a name="chrono.users_guide.getting_started.install.h0"></a> <a name="chrono.users_guide.getting_started.install.getting__emphasis_role__bold__boost_chrono__emphasis__"></a><a class="link" href="users_guide.html#chrono.users_guide.getting_started.install.getting__emphasis_role__bold__boost_chrono__emphasis__">Getting Boost.Chrono </a> </h6> Boost.Chrono is in the latest Boost release in the folder <code class="computeroutput">/boost/chrono</code>. Documentation, tests and examples folder are at <code class="computeroutput">boost/libs/chrono/</code>. You can also access the latest (unstable?) state from the <a href="https://svn.boost.org/svn/boost-trunk" target="_top">Boost trunk</a> directories boost/chrono and libs/chrono. Just go to <a href="http://svn.boost.org/trac/boost/wiki/BoostSubversion" target="_top">here</a> and follow the instructions there for anonymous SVN access. <h6> <a name="chrono.users_guide.getting_started.install.h1"></a> <a name="chrono.users_guide.getting_started.install.where_to_install_boost_chrono__"></a><a class="link" href="users_guide.html#chrono.users_guide.getting_started.install.where_to_install_boost_chrono__">Where to install Boost.Chrono? </a> </h6> The simple way is to decompress (or checkout from SVN) the files in your BOOST_ROOT directory. <h6> <a name="chrono.users_guide.getting_started.install.h2"></a> <a name="chrono.users_guide.getting_started.install.building_boost_chrono_"></a><a class="link" href="users_guide.html#chrono.users_guide.getting_started.install.building_boost_chrono_">Building Boost.Chrono </a> </h6> Boost.Chrono can be configured as a header-only library defining <a class="link" href="reference.html#chrono.reference.cpp0x.chrono_chrono_hpp.conf.header_only" title="How to Build Boost.Chrono as a Header Only Library?"><code class="computeroutput">BOOST_CHRONO_HEADER_ONLY</code></a>. However Boost.Chrono depends on the non header-only library Boost.System, so that you will need to link with boost_system. Boost.System has un undocumented feature (use of macro BOOST_ERROR_CODE_HEADER_ONLY) to make it header only. If <a class="link" href="reference.html#chrono.reference.cpp0x.chrono_chrono_hpp.conf.header_only" title="How to Build Boost.Chrono as a Header Only Library?"><code class="computeroutput">BOOST_CHRONO_HEADER_ONLY</code></a> is not defined you need to compile it and build the library before use, for example using: <pre class="programlisting">bjam libs/chrono/build </pre> <h6> <a name="chrono.users_guide.getting_started.install.h3"></a> <a name="chrono.users_guide.getting_started.install.requirements"></a><a class="link" href="users_guide.html#chrono.users_guide.getting_started.install.requirements">Requirements</a> </h6> In particular, Boost.Chrono depends on: <div class="variablelist"> <dl class="variablelist"> <dt><a href="http://www.boost.org/libs/config" target="_top">Boost.Config</a></dt> <dd> for configuration purposes, ... </dd> <dt><a href="http://www.boost.org/libs/exception" target="_top">Boost.Exception</a></dt> <dd> for throw_exception, ... </dd> <dt><a href="http://www.boost.org/libs/integer" target="_top">Boost.Integer</a></dt> <dd> for cstdint conformance, ... </dd> <dt><a href="http://www.boost.org/libs/mpl" target="_top">Boost.MPL</a></dt> <dd> for MPL Assert and bool, logical ... </dd> <dt><a href="http://www.boost.org/libs/operators" target="_top">Boost.Operators</a></dt> <dd> for operators, ... </dd> <dt><a href="http://www.boost.org/libs/ratio" target="_top">Boost.Ratio</a></dt> <dd> for ratio, milli, micro, ... </dd> <dt><a href="http://www.boost.org/libs/system" target="_top">Boost.System</a></dt> <dd> for error_code, ... </dd> <dt><a href="http://www.boost.org/libs/type_traits" target="_top">Boost.TypeTraits</a></dt> <dd> for is_base, is_convertible, common_type, ... </dd> <dt><a href="http://www.boost.org/libs/utility" target="_top">Boost.Utility/EnableIf</a></dt> <dd> for enable_if, ... </dd> </dl> </div> <h6> <a name="chrono.users_guide.getting_started.install.h4"></a> <a name="chrono.users_guide.getting_started.install.building_an_executable_that_uses_boost_chrono_"></a><a class="link" href="users_guide.html#chrono.users_guide.getting_started.install.building_an_executable_that_uses_boost_chrono_">Building an Executable that Uses Boost.Chrono </a> </h6> In addition to link with the Boost.Chrono library you need also to link with the Boost.System library. If Boost.System is configured defining BOOST_ERROR_CODE_HEADER_ONLY you will no need to link with it as the dependent part is header only then. <h6> <a name="chrono.users_guide.getting_started.install.h5"></a> <a name="chrono.users_guide.getting_started.install.exception_safety_"></a><a class="link" href="users_guide.html#chrono.users_guide.getting_started.install.exception_safety_">Exception safety </a> </h6> All functions in the library are exception-neutral and provide strong guarantee of exception safety as long as the underlying parameters provide it. <h6> <a name="chrono.users_guide.getting_started.install.h6"></a> <a name="chrono.users_guide.getting_started.install.thread_safety_"></a><a class="link" href="users_guide.html#chrono.users_guide.getting_started.install.thread_safety_">Thread safety </a> </h6> All functions in the library are thread-unsafe except when noted explicitly. As Boost.Chrono doesn't use mutable global variables the thread-safety analysis is limited to the access to each instance variable. It is not thread safe to use a function that modifies the access to a user variable if another can be reading or writing it. <h6> <a name="chrono.users_guide.getting_started.install.h7"></a> <a name="chrono.users_guide.getting_started.install.tested_compilers_"></a><a class="link" href="users_guide.html#chrono.users_guide.getting_started.install.tested_compilers_">Tested compilers </a> </h6> The implementation will eventually work with most C++03 conforming compilers. Currently I use to test with on: Windows with <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> MSVC 10.0 </li></ul></div> MinGW with <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> <li class="listitem"> GCC 4.5.0 </li> <li class="listitem"> GCC 4.5.0 -std=c++0x </li> <li class="listitem"> GCC 4.5.2 </li> <li class="listitem"> GCC 4.5.2 -std=c++0x </li> <li class="listitem"> GCC 4.6.0 </li> <li class="listitem"> GCC 4.6.0 -std=c++0x </li> <li class="listitem"> GCC 4.8.0 </li> <li class="listitem"> GCC 4.8.0 -std=c++0x </li> </ul></div> Ubuntu with * GCC 4.4.6 * GCC 4.4.6 -std=c++0x * GCC 4.5.4 * GCC 4.5.4 -std=c++0x * GCC 4.6.1 * GCC 4.6.1 -std=c++0x * Intel 12.1.3 * Intel 12.1.3 -std=c++0x OsX with <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> <li class="listitem"> GCC 4.1.2 </li> <li class="listitem"> GCC 4.6.2 </li> <li class="listitem"> GCC 4.6.2 -std=c++0x </li> <li class="listitem"> GCC 4.7.0 </li> <li class="listitem"> GCC 4.7.0 -std=c++0x </li> <li class="listitem"> GCC 4.7.1 </li> <li class="listitem"> GCC 4.7.1 -std=c++0x </li> <li class="listitem"> GCC 4.7.2 </li> <li class="listitem"> GCC 4.7.2 -std=c++0x </li> <li class="listitem"> GCC 4.8.0 </li> <li class="listitem"> GCC 4.8.0 -std=c++0x </li> <li class="listitem"> GCC 4.8.1 </li> <li class="listitem"> GCC 4.8.1 -std=c++0x </li> <li class="listitem"> clang 3.1 </li> <li class="listitem"> clang 3.1 -std=c++0x -stdlib=libc++ </li> <li class="listitem"> clang 3.2 </li> <li class="listitem"> clang 3.2 -std=c++11 -stdlib=libc++ </li> </ul></div> The committed code is tested with much more compilers. There are two compilers (VACPP and Borland) that don't provide the needed features. Other as Intel and Sun have some issues with i/o. While everything compiles and link correctly, there are some runtime issues I have not cached yet. See the regression tests for details. <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"> Please let us know how this works on other platforms/compilers. </td></tr> </table></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"> Please send any questions, comments and bug reports to boost <at> lists <dot> boost <dot> org. </td></tr> </table></div> </div> <div class="section"><div class="titlepage"><div><div><h4 class="title"> <a name="chrono.users_guide.getting_started.hello_world__"></a><a class="link" href="users_guide.html#chrono.users_guide.getting_started.hello_world__" title="Hello World!">Hello World! </a> </h4></div></div></div></div> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="chrono.users_guide.tutorial"></a><a class="link" href="users_guide.html#chrono.users_guide.tutorial" title="Tutorial">Tutorial</a> </h3></div></div></div> <div class="toc"><dl class="toc"> <dt><a href="users_guide.html#chrono.users_guide.tutorial.duration">Duration</a></dt> <dt><a href="users_guide.html#chrono.users_guide.tutorial.clocks">Clocks</a></dt> <dt><a href="users_guide.html#chrono.users_guide.tutorial.time_point">Time Point</a></dt> <dt><a href="users_guide.html#chrono.users_guide.tutorial.specific_clocks">Specific Clocks</a></dt> <dt><a href="users_guide.html#chrono.users_guide.tutorial.i_o">I/O</a></dt> </dl></div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="chrono.users_guide.tutorial.duration"></a><a class="link" href="users_guide.html#chrono.users_guide.tutorial.duration" title="Duration">Duration</a> </h4></div></div></div> <div class="toc"><dl class="toc"> <dt><a href="users_guide.html#chrono.users_guide.tutorial.duration.so_what_exactly_is_a__duration__and_how_do_i_use_one_">So What Exactly is a <code class="computeroutput">duration</code> and How Do I Use One?</a></dt> <dt><a href="users_guide.html#chrono.users_guide.tutorial.duration.what_happens_if_i_assign__m3___us3__to__minutes__instead_of__microseconds__">What Happens if I Assign <code class="computeroutput">m3 + us3</code> to <code class="computeroutput">minutes</code> Instead of <code class="computeroutput">microseconds</code>?</a></dt> <dt><a href="users_guide.html#chrono.users_guide.tutorial.duration.but_what_if_the_truncation_behavior_is_what_i_want_to_do_">But What if the Truncation Behavior is What I Want to Do?</a></dt> <dt><a href="users_guide.html#chrono.users_guide.tutorial.duration.round">Rounding functions</a></dt> <dt><a href="users_guide.html#chrono.users_guide.tutorial.duration.trafficking_in_floating_point_durations">Trafficking in floating-point Durations</a></dt> <dt><a href="users_guide.html#chrono.users_guide.tutorial.duration.how_expensive_is_all_of_this_">How Expensive is All of this?</a></dt> <dt><a href="users_guide.html#chrono.users_guide.tutorial.duration.how_complicated_is_it_to_build_a_function_taking_a__duration__parameter_">How Complicated is it to Build a Function Taking a <code class="computeroutput">duration</code> Parameter?</a></dt> <dt><a href="users_guide.html#chrono.users_guide.tutorial.duration.is_it_possible_for_the_user_to_pass_a___duration_to_a_function_with_the_units_being_ambiguous_">Is it possible for the user to pass a <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a> to a function with the units being ambiguous?</a></dt> <dt><a href="users_guide.html#chrono.users_guide.tutorial.duration.can_durations_overflow_">Can Durations Overflow?</a></dt> </dl></div> The <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a> is the heart of this library. The interface that the user will see in everyday use is nearly identical to that of Boost.DateTime time <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a>s authored by Jeff Garland, both in syntax and in behavior. This has been a very popular boost library for 7 years. There is an enormous positive history with this interface. The library consists of six units of time <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a>: <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> <li class="listitem"> <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput">hours</code></a> </li> <li class="listitem"> <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput">minutes</code></a> </li> <li class="listitem"> <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput">seconds</code></a> </li> <li class="listitem"> <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput">milliseconds</code></a> </li> <li class="listitem"> <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput">microseconds</code></a> </li> <li class="listitem"> <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput">nanoseconds</code></a> </li> </ul></div> These units were chosen as a subset of the boost library because they are the most common units used when sleeping, waiting on a condition variable, or waiting to obtain the lock on a mutex. Each of these units is nothing but a thin wrapper around a signed integral count. That is, when you construct <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput">minutes</code></a><code class="computeroutput">(3)</code>, all that happens is a <code class="computeroutput">3</code> is stored inside <code class="computeroutput">minutes</code>. When you construct <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput">microseconds</code></a><code class="computeroutput">(3)</code>, all that happens is a <code class="computeroutput">3</code> is stored inside <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput">microseconds</code></a>. The only context in which these different types differ is when being converted to one another. At this time, unit-specific compile-time conversion constants are used to convert the source unit to the target unit. Only conversions from coarser units to finer units are allowed (in Boost). This restriction ensures that all conversions are always exact. That is, <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput">microseconds</code></a> can always represent any value <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput">minutes</code></a> has. In Boost.DateTime, these units are united via inheritance. Boost.Chrono instead unites these units through the class template <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a>. That is, in Boost.Chrono all six of the above units are nothing but typedefs to different instantiations of <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a>. This change from Boost.DateTime has a far reaching positive impact, while not changing the syntax of the everyday use at all. The most immediate positive impact is that the library can immediately generate any unit, with any precision it needs. This is sometimes necessary when doing comparisons or arithmetic between <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a>s of differing precision, assuming one wants the comparison and arithmetic to be exact. A secondary benefit is that by publishing the class template <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a> interface, user code can very easily create <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a>s with any precision they desire. The <code class="computeroutput">ratio</code> utility is used to specify the precision, so as long as the precision can be expressed by a rational constant with respect to seconds, this framework can exactly represent it (one third of a second is no problem, and neither is one third of a <code class="computeroutput">femto</code> second). All of this utility and flexibility comes at no cost just by making use of the no-run-time-overhead <code class="computeroutput">ratio</code> facility. In Boost.DateTime, <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput">hours</code></a> does not have the same representation as <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput">nanoseconds</code></a>. The former is usually represented with a <code class="computeroutput">long</code> whereas a <code class="computeroutput">long long</code> is required for the latter. The reason for this is simply range. You don't need many hours to cover an extremely large range of time. But this isn't true of nanoseconds. Being able to reduce the sizeof overhead for some units when possible, can be a significant performance advantage. Boost.Chrono continues, and generalizes that philosophy. Not only can one specify the precision of a <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a>, one can also specify its representation. This can be any integral type, or even a floating-point type. Or it can be a user-defined type which emulates an arithmetic type. The six predefined units all use signed integral types as their representation. And they all have a minimum range of ± 292 years. <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput">nanoseconds</code></a> needs 64 bits to cover that range. <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput">hours</code></a> needs only 23 bits to cover that range. <div class="section"> <div class="titlepage"><div><div><h5 class="title"> <a name="chrono.users_guide.tutorial.duration.so_what_exactly_is_a__duration__and_how_do_i_use_one_"></a><a class="link" href="users_guide.html#chrono.users_guide.tutorial.duration.so_what_exactly_is_a__duration__and_how_do_i_use_one_" title="So What Exactly is a duration and How Do I Use One?">So What Exactly is a <code class="computeroutput">duration</code> and How Do I Use One?</a> </h5></div></div></div> A <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a> has a representation and a tick period (precision). <pre class="programlisting">template <class Rep, class Period = <code class="computeroutput">ratio</code><1> > class duration; </pre> The representation is simply any arithmetic type, or an emulation of such a type. The representation stores a count of ticks. This count is the only data member stored in a <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a>. If the representation is floating-point, it can store fractions of a tick to the precision of the representation. The tick period is represented by a <code class="computeroutput">ratio</code> and is encoded into the <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a>'s type, instead of stored. The tick period only has an impact on the behavior of the <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a> when a conversion between different <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a>s is attempted. The tick period is completely ignored when simply doing arithmetic among like <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a>s. Example: <pre class="programlisting">typedef boost::chrono::<a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a><long, boost::ratio<60> > minutes; minutes m1(3); // m1 stores 3 minutes m2(2); // m2 stores 2 minutes m3 = m1 + m2; // m3 stores 5 typedef boost::chrono::<a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a><long long, boost::micro> microseconds; microseconds us1(3); // us1 stores 3 microseconds us2(2); // us2 stores 2 microseconds us3 = us1 + us2; // us3 stores 5 microseconds us4 = m3 + us3; // us4 stores 300000005 </pre> In the final line of code above, there is an implicit conversion from minutes to microseconds, resulting in a relatively large number of microseconds. If you need to access the tick count within a <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a>, there is a member <code class="computeroutput">count()</code> which simply returns the stored tick count. <pre class="programlisting">long long tc = us4.count(); // tc is 300000005 </pre> These <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a>'s have very simple, very predictable, and very observable behavior. After all, this is really nothing but the time-tested interface of Jeff's boost time <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a> library (unified with templates instead of inheritance). </div> <div class="section"> <div class="titlepage"><div><div><h5 class="title"> <a name="chrono.users_guide.tutorial.duration.what_happens_if_i_assign__m3___us3__to__minutes__instead_of__microseconds__"></a><a class="link" href="users_guide.html#chrono.users_guide.tutorial.duration.what_happens_if_i_assign__m3___us3__to__minutes__instead_of__microseconds__" title="What Happens if I Assign m3 + us3 to minutes Instead of microseconds?">What Happens if I Assign <code class="computeroutput">m3 + us3</code> to <code class="computeroutput">minutes</code> Instead of <code class="computeroutput">microseconds</code>?</a> </h5></div></div></div> <pre class="programlisting">minutes m4 = m3 + us3; </pre> It won't compile! The rationale is that implicit truncation error should not be allowed to happen. If this were to compile, then <code class="computeroutput">m4</code> would hold <code class="computeroutput">5</code>, the same value as <code class="computeroutput">m3</code>. The value associated with <code class="computeroutput">us3</code> has been effectively ignored. This is similar to the problem of assigning a double to an <code class="computeroutput">int</code>: the fractional part gets silently discarded. </div> <div class="section"> <div class="titlepage"><div><div><h5 class="title"> <a name="chrono.users_guide.tutorial.duration.but_what_if_the_truncation_behavior_is_what_i_want_to_do_"></a><a class="link" href="users_guide.html#chrono.users_guide.tutorial.duration.but_what_if_the_truncation_behavior_is_what_i_want_to_do_" title="But What if the Truncation Behavior is What I Want to Do?">But What if the Truncation Behavior is What I Want to Do?</a> </h5></div></div></div> There is a <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_cast" title="Non-Member Function duration_cast(duration)"><code class="computeroutput">duration_cast</code></a> facility to explicitly ask for this behavior: <pre class="programlisting">minutes m4 = boost::chrono::<a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_cast" title="Non-Member Function duration_cast(duration)"><code class="computeroutput">duration_cast</code></a><minutes>(m3 + us3); // m4.count() == 5 </pre> In general, one can perform <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a> arithmetic at will. If <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_cast" title="Non-Member Function duration_cast(duration)"><code class="computeroutput">duration_cast</code></a> isn't used, and it compiles, the arithmetic is exact. If one wants to override this exact arithmetic behavior, <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_cast" title="Non-Member Function duration_cast(duration)"><code class="computeroutput">duration_cast</code></a> can be used to explicitly specify that desire. The <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_cast" title="Non-Member Function duration_cast(duration)"><code class="computeroutput">duration_cast</code></a> has the same efficiency as the implicit conversion, and will even be exact as often as it can. You can use <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_cast" title="Non-Member Function duration_cast(duration)"><code class="computeroutput">duration_cast</code></a><code class="computeroutput"><></code> to convert the <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a> into whatever units you desire. This facility will round down (truncate) if an exact conversion is not possible. For example: <pre class="programlisting">boost::chrono::<a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput">nanoseconds</code></a> start; boost::chrono::<a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput">nanoseconds</code></a> end; typedef boost::chrono::<a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput">milliseconds</code></a> ms; ms d = boost::chrono::<a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_cast" title="Non-Member Function duration_cast(duration)"><code class="computeroutput">duration_cast</code></a><ms>(end - start); // d now holds the number of milliseconds from start to end. std::cout << ms.count() << "ms\n"; </pre> We can convert to <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput">nanoseconds</code></a>, or some integral-based duration which <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput">nanoseconds</code></a> will always exactly convert to, then <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_cast" title="Non-Member Function duration_cast(duration)"><code class="computeroutput">duration_cast</code></a><code class="computeroutput"><></code> is unnecessary: <pre class="programlisting">typedef boost::chrono::<a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_typedefs" title="duration typedefs"><code class="computeroutput">nanoseconds</code></a> ns; ns d = end - start; std::cout << ns.count() << "ns\n"; </pre> If you need seconds with a floating-point representation you can also eliminate the <a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration_cast" title="Non-Member Function duration_cast(duration)"><code class="computeroutput">duration_cast</code></a><code class="computeroutput"><></code>: <pre class="programlisting">typedef boost::chrono::<a class="link" href="reference.html#chrono.reference.cpp0x.duration_hpp.duration" title="Class Template duration<>"><code class="computeroutput">duration</code></a><double> sec; // seconds, stored with a double sec d = end - start; std::cout << sec.count() << "s\n"; </pre> If you're not sure if you need <a class="link" href="reference.h