[time.clock.req]

# 30 Time library [[time]](./#time)

## 30.3 *Cpp17Clock* requirements [time.clock.req]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L976)

A clock is a bundle consisting of a duration, atime_point, and a function now() to get the current time_point[.](#1.sentence-1)

The origin of the clock's time_point is referred to as the clock's [*epoch*](#def:epoch "30.3 Cpp17Clock requirements [time.clock.req]")[.](#1.sentence-2)

A clock shall meet the requirements in Table [131](#tab:time.clock "Table 131: Cpp17Clock requirements")[.](#1.sentence-3)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L982)

In Table [131](#tab:time.clock "Table 131: Cpp17Clock requirements") C1 and C2 denote clock types[.](#2.sentence-1)

t1 andt2 are values returned by C1​::​now() where the call returning t1 happens
before ([[intro.multithread]](intro.multithread "6.10.2 Multi-threaded executions and data races")) the call returning t2 and both of these calls
occur
before C1​::​time_point​::​max()[.](#2.sentence-2)

[*Note [1](#note-1)*:

This means C1 did not wrap around between t1 andt2[.](#2.sentence-3)

â *end note*]

Table [131](#tab:time.clock) — *Cpp17Clock* requirements [[tab:time.clock]](./tab:time.clock)  

| [ð](#tab:time.clock-row-1)<br>**Expression** | **Return type** | **Operational semantics** |
| --- | --- | --- |
| [ð](#tab:time.clock-row-2)<br>C1​::​rep | An arithmetic type or a class emulating an arithmetic type | The representation type of C1​::​duration[.](#tab:time.clock-row-2-column-3-sentence-1) |
| [ð](#tab:time.clock-row-3)<br>C1​::​period | a specialization of ratio | The tick period of the clock in seconds[.](#tab:time.clock-row-3-column-3-sentence-1) |
| [ð](#tab:time.clock-row-4)<br>C1​::​duration | chrono​::​duration<C1​::​rep, C1​::​period> | The duration type of the clock[.](#tab:time.clock-row-4-column-3-sentence-1) |
| [ð](#tab:time.clock-row-5)<br>C1​::​time_point | chrono​::​time_point<C1> or chrono​::​time_point<C2, C1​::​duration> | The time_point type of the clock[.](#tab:time.clock-row-5-column-3-sentence-1)<br>C1 and C2 shall  refer to the same epoch[.](#tab:time.clock-row-5-column-3-sentence-2) |
| [ð](#tab:time.clock-row-6)<br>C1​::​is_steady | const bool | true if t1 <= t2 is always true and the time between clock  ticks is constant, otherwise false[.](#tab:time.clock-row-6-column-3-sentence-1) |
| [ð](#tab:time.clock-row-7)<br>C1​::​now() | C1​::​time_point | Returns a time_point object representing the current point in time[.](#tab:time.clock-row-7-column-3-sentence-1) |

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L1032)

[*Note [2](#note-2)*:

The relative difference in durations between those reported by a given clock and the
SI definition is a measure of the quality of implementation[.](#3.sentence-1)

â *end note*]

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L1038)

A type TC meets the *Cpp17TrivialClock* requirements if

- [(4.1)](#4.1)

TC meets the [*Cpp17Clock*](#:Cpp17Clock "30.3 Cpp17Clock requirements [time.clock.req]") requirements,

- [(4.2)](#4.2)

the types TC​::​rep, TC​::​duration, and TC​::​time_point meet the [*Cpp17EqualityComparable*](utility.arg.requirements#:Cpp17EqualityComparable "16.4.4.2 Template argument requirements [utility.arg.requirements]") (Table [28](utility.arg.requirements#tab:cpp17.equalitycomparable "Table 28: Cpp17EqualityComparable requirements")) and[*Cpp17LessThanComparable*](utility.arg.requirements#:Cpp17LessThanComparable "16.4.4.2 Template argument requirements [utility.arg.requirements]") (Table [29](utility.arg.requirements#tab:cpp17.lessthancomparable "Table 29: Cpp17LessThanComparable requirements"))
and [*Cpp17Swappable*](swappable.requirements#:Cpp17Swappable "16.4.4.3 Swappable requirements [swappable.requirements]") ([[swappable.requirements]](swappable.requirements "16.4.4.3 Swappable requirements"))
requirements and the requirements of
numeric types ([[numeric.requirements]](numeric.requirements "29.2 Numeric type requirements")),
  [*Note [3](#note-3)*:
  This means, in particular,
that operations on these types will not throw exceptions[.](#4.2.sentence-1)
 â *end note*]

- [(4.3)](#4.3)

the function TC​::​now() does not throw exceptions, and

- [(4.4)](#4.4)

the type TC​::​time_point​::​clock meets the *Cpp17TrivialClock* requirements, recursively[.](#4.sentence-1)
