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std::chrono:: time_point_cast

From cppreference.net
C++
Date and time library
std::chrono::time_point
Defined in header <chrono>
template < class ToDuration, class Clock, class Duration >

std:: chrono :: time_point < Clock, ToDuration >

time_point_cast ( const std:: chrono :: time_point < Clock, Duration > & t ) ;
(since C++11)
(until C++14)
template < class ToDuration, class Clock, class Duration >

constexpr std:: chrono :: time_point < Clock, ToDuration >

time_point_cast ( const std:: chrono :: time_point < Clock, Duration > & t ) ;
(since C++14)

Converts a std::chrono::time_point from one duration to another.

time_point_cast participates in overload resolution only if ToDuration is a specialization of std::chrono::duration .

Contents

Parameters

t - time_point to convert from

Return value

std:: chrono :: time_point < Clock, ToDuration > (
std:: chrono :: duration_cast < ToDuration > ( t. time_since_epoch ( ) ) ) .

Example

Run this code 
#include <chrono>
#include <iostream>
using namespace std::chrono_literals;
using Clock = std::chrono::high_resolution_clock;
using Ms = std::chrono::milliseconds;
using Sec = std::chrono::seconds;
template<class Duration>
using TimePoint = std::chrono::time_point<Clock, Duration>;
inline void print_ms(const TimePoint<Ms>& time_point)
{
    std::cout << time_point.time_since_epoch().count() << " ms\n";
}
int main()
{
    TimePoint<Sec> time_point_sec{4s};
    // implicit conversion, no precision loss
    TimePoint<Ms> time_point_ms = time_point_sec;
    print_ms(time_point_ms); // 4000 ms
    time_point_ms = TimePoint<Ms>{5756ms};
    print_ms(time_point_ms); // 5756 ms
    // explicit cast, need when precision loss may happen
    // 5756 truncated to 5000
    time_point_sec = std::chrono::time_point_cast<Sec>(time_point_ms);
    print_ms(time_point_sec); // 5000 ms
}

Output: 

4000 ms
5756 ms
5000 ms

See also

(C++17)
converts a time_point to another, rounding down
(function template)
(C++17)
converts a time_point to another, rounding up
(function template)
(C++17)
converts a time_point to another, rounding to nearest, ties to even
(function template)
(C++11)
converts a duration to another, with a different tick interval
(function template)