std::chrono...on::duration(3)

💖 std::chrono...on::duration(3) 💖

okay so this is the page for std::chrono...on::duration(3) from the C++ Standard Libary manual — yes, “Libary” with one r, we are not fixing the lore 💅. The main character is std::chrono::duration::duration, and bestie it is literally the constructor page for std::chrono::duration::duration.

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🌸 NAME 🌸

std::chrono::duration::duration — std::chrono::duration::duration. That’s the whole name, no cap.

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✨ Synopsis ✨

constexpr duration() = default;                         (1) (since C++11)
duration( const duration& ) = default;                  (2) (since C++11)
template< class Rep2 >                                  (3) (since C++11)
constexpr explicit duration( const Rep2& r );
template< class Rep2, class Period2 >                   (4) (since C++11)
constexpr duration( const duration<Rep2, Period2>& d );

Constructs a new duration from one of several optional data sources.

1. The default constructor.
2. The copy constructor.
3. Constructs a duration with r ticks.

This overload participates in overload resolution only if all following conditions are satisfied:

• is_convertible<const Rep2&, Rep>::value is true.
• Any of the following conditions is satisfied:^[1]
  • std::chrono::treat_as_floating_point<Rep>::value is true.
  • std::chrono::treat_as_floating_point<Rep2>::value is false.

4. Constructs a duration by converting d to an appropriate period and tick count, as if by std::chrono::duration_cast<duration>(d).count().

This overload participates in overload resolution only if no overflow is induced in the conversion, and any of the following conditions is satisfied:^[2]

• std::chrono::treat_as_floating_point<Rep>::value is true.
• All following conditions are satisfied:
  • std::ratio_divide<Period2, Period>::den is 1.
  • std::chrono::treat_as_floating_point<Rep2>::value is false.

^[1] ↑ That is, a duration with an integer tick count cannot be constructed from a floating-point value, but a duration with a floating-point tick count can be constructed from an integer value.

^[2] ↑ That is, either the duration uses floating-point ticks, or Period2 is exactly divisible by Period.

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🎀 Parameters 🎀

• r — a tick count
• d — a duration to copy from

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🦋 Example 🦋

The following code shows several examples (both valid and invalid) of how to construct durations:

// Run this code

#include <chrono>

int main()
{
    std::chrono::hours h(1); // one hour
    std::chrono::milliseconds ms{3}; // 3 milliseconds
    std::chrono::duration<int, std::kilo> ks(3); // 3000 seconds

    // error: treat_as_floating_point<int>::value == false,
    // This duration allows whole tick counts only
//  std::chrono::duration<int, std::kilo> d3(3.5);

    // 30Hz clock using fractional ticks
    std::chrono::duration<double, std::ratio<1, 30>> hz30(3.5);

    // 3000 microseconds constructed from 3 milliseconds
    std::chrono::microseconds us = ms;
    // error: 1/1000000 is not divisible by 1/1000
//  std::chrono::milliseconds ms2 = us
    std::chrono::duration<double, std::milli> ms2 = us; // 3.0 milliseconds
}

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🌈 Defect reports 🌈

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• LWG 2094 — applied to C++11. Behavior as published: for overload (4), std::ratio_divide<Period2, period>::num might overflow when evaluating std::ratio_divide<Period2, period>::den. Correct behavior: overload (4) does not participate in overload resolution in this case.
• LWG 3050 — applied to C++11. Behavior as published: convertibility constraint used non-const xvalue. Correct behavior: use const lvalues instead.

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🌟 See also 🌟

• operator= — assigns the contents (public member function)

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byeeee bestie 💅💖 — source/version and date vibes: http://cppreference.com, 2024.06.10, page std::chrono...on::duration(3).