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Single moments in time can be specified through objects of
the class hi(time_point)tt(std::chrono::time_point).
Before using the class tt(time_point) the tthi(chrono) header file must be
included.
Like tt(duration) the class tt(time_point) requires two template arguments: A
clock type and a duration type. Usually tt(system_clock) is used as the
clock's type using tt(nanoseconds) as the default duration type (it may be
omitted if tt(nanoseconds) is the intended duration type). Otherwise specify
the duration type as the tt(time_point's) second template argument. The
following two time point definitions therefore use identifcal time point
types:
verb( time_point<standard_clock, nanoseconds> tp1;
time_point<standard_clock> tp2;)
The class tt(time_point) supports three constructors:
itemization(
itt(time_point()):nl()
the default constructor is initialized to the beginning of the clock's
emi(epoch). For tt(system_clock) it is January, 1, 1970, 00:00h, but
notice that tt(filesystem::__file_clock) uses a different epoch
(see section ref(FILECLOCK) below);
itt(time_point(time_point<Clock, Duration> const &other)):nl()
the copy constructor (cf. chapter ref(MEMORY)) initializes a
tt(time_point) object using the time point defined by tt(other). If
tt(other's) resolution uses a larger period than the period of the
constructed object then tt(other's) point in time is represented in
the constructed object's resolution (an illustration is provided
below, at the description of the member tt(time_since_epoch));
itt(time_point(time_point<Clock, Duration> const &&tmp)):nl()
the move constructor (cf. chapter ref(MEMORY)) acts comparably to the
copy constructor, converting tt(tmp's) resolution to the constructed
object while moving tt(tmp) to the constructed object.
)
The following operators and members are available:
itemization(
itt(time_point &operator+=(duration const &amount)):nl()
The amount of time represented by tt(amount) is added to the current
tt(time_point) object. This operator is also available as binary
arithmetic operator using a tt(time_point const &) and a tt(duration
const &) operand (in any order). Example:
verb( system_clock::now() + seconds{ 5 };)
itt(time_point &operator-=(duration const &amount)):nl()
The amount of time represented by tt(amount) is subtracted from the
current tt(time_point) object. This operator is also available as
binary arithmetic operator using a tt(time_point const &) and a
tt(duration const &) operand (in any order). Example:
verb( time_point<system_clock> point = system_clock::now();
point -= seconds{ 5 };)
itht(time_since_epoch)(duration time_since_epoch() const):nl()
tt(duration) is the duration type used by the time point object for
which this member is called. It returns the amount of time
since the epoch that's represented by the object.
itt(time_point min() const):nl()
a static member returning the time point's tt(duration::min)
value. Example:
verb( cout <<
time_point<system_clock>::min().time_since_epoch().count() << '\n';
// shows -9223372036854775808)
itt(time_point max() const):nl()
a static member returning the time point's tt(duration::max)
value.
)
All predefined clocks use nanoseconds as their time resolution. To express the
time in a less precise resolution take one unit of time of the less precise
resolution (e.g., tt(hours(1))) and convert it to nanoseconds. Then divide the
value returned by the time point's tt(time_since_epoch().count()) member by
tt(count) member of the less precise resolution converted to
nanoseconds. Using this procedure the number of hours passed since the
beginning of the epoch can be determined:
verb( cout << system_clock::now().time_since_epoch().count() /
nanoseconds(hours(1)).count() <<
" hours since the epoch\n";)
Time point objects based on the system clock or on the high resolution clock
can be converted to tt(std::time_t) (or the equivalent type tt(time_t))
values. Such tt(time_t) values are used when converting time to text. For such
conversions the em(manipulator) tt(put_time) (cf. section ref(IOFORMAT)) is
commonly used, but tt(put_time) must be provided with the address of a
tt(std::tm) object, which in turn can be obtained from a tt(std::time_t)
value. The whole process is fairly complex, and the core elements are
visualized in figure ref(TIMEIMG).
figure(threading/time)(Time according to bf(C++))(TIMEIMG)
The essential step eventually leading to the insertion of a time point's value
into a tt(std::ostream) consists of using
tt(system_clock::to_time_t(time_point<system_clock> const &tp)) to convert a
time point to a tt(time_t) value (instead of using tt(system_clock) the
tt(high_resolution_clock) can also be used). How a time point can be inserted
into a tt(std::ostream) is described in section ref(PUTTIME).
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