std::chrono:: floor (std::chrono::duration)

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Defined in header <chrono>
template < class ToDuration, class Rep, class Period >
constexpr ToDuration floor ( const std:: chrono :: duration < Rep, Period > & d ) ;
(since C++17)

Returns the greatest duration t representable in ToDuration that is less or equal to d .

The function does not participate in the overload resolution unless ToDuration is a specialization of std::chrono::duration .

Parameters

d - duration to convert

Return value

d rounded down to a duration of type ToDuration .

Possible implementation 

namespace detail
{
    template<class> inline constexpr bool is_duration_v = false;
    template<class Rep, class Period> inline constexpr bool is_duration_v<
        std::chrono::duration<Rep, Period>> = true;
}
 
template<class To, class Rep, class Period,
         class = std::enable_if_t<detail::is_duration_v<To>>>
constexpr To floor(const duration<Rep, Period>& d)
{
    To t = std::chrono::duration_cast<To>(d);
    if (t > d)
        return t - To{1};
    return t;
}

Example

Run this code 
#include <chrono>
#include <iomanip>
#include <iostream>
 
int main()
{
    using namespace std::chrono_literals;
    std::cout << "Duration\tFloor\tRound\tCeil\n";
    for (using Sec = std::chrono::seconds;
        auto const d : {+4999ms, +5000ms, +5001ms, +5499ms, +5500ms, +5999ms,
                        -4999ms, -5000ms, -5001ms, -5499ms, -5500ms, -5999ms})
        std::cout << std::showpos << d << "\t\t"
                  << std::chrono::floor<Sec>(d) << '\t'
                  << std::chrono::round<Sec>(d) << '\t'
                  << std::chrono::ceil <Sec>(d) << '\n';
}

Output: 

Duration	Floor	Round	Ceil
+4999ms		+4s	+5s	+5s
+5000ms		+5s	+5s	+5s
+5001ms		+5s	+5s	+6s
+5499ms		+5s	+5s	+6s
+5500ms		+5s	+6s	+6s
+5999ms		+5s	+6s	+6s
-4999ms		-5s	-5s	-4s
-5000ms		-5s	-5s	-5s
-5001ms		-6s	-5s	-5s
-5499ms		-6s	-5s	-5s
-5500ms		-6s	-6s	-5s
-5999ms		-6s	-6s	-5s

See also

(C++11)
converts a duration to another, with a different tick interval
(function template)
(C++17)
converts a duration to another, rounding up
(function template)
(C++17)
converts a duration to another, rounding to nearest, ties to even
(function template)
(C++17)
converts a time_point to another, rounding down
(function template)
(C++11) (C++11)
nearest integer not greater than the given value
(function)
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