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### 935. clock error handling needs to be specified

Section: 29.7 [time.clock] Status: NAD Submitter: Beman Dawes Opened: 2008-11-24 Last modified: 2019-02-26

Priority: Not Prioritized

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Discussion:

Each of the three clocks specified in Clocks 29.7 [time.clock] provides the member function:

>     static time_point now();

The semantics specified by Clock requirements 29.3 [time.clock.req] make no mention of error handling. Thus the function may throw `bad_alloc` or an implementation-defined exception (16.4.6.13 [res.on.exception.handling] paragraph 4).

Some implementations of these functions on POSIX, Windows, and presumably on other operating systems, may fail in ways only detectable at runtime. Some failures on Windows are due to supporting chipset errata and can even occur after successful calls to a clock's `now()` function.

These functions are used in cases where exceptions are not appropriate or where the specifics of the exception or cause of error need to be available to the user. See N2828, Library Support for hybrid error handling (Rev 1), for more specific discussion of use cases. Thus some change in the interface of now is required.

The proposed resolution has been implemented in the Boost version of the chrono library. No problems were encountered.

[ Batavia (2009-05): ]

> We recommend this issue be deferred until the next Committee Draft has been issued and the prerequisite paper has been accepted.

> Move to Open.

[ 2009-10 Santa Cruz: ]

> Mark as NAD future. Too late to make this change without having already accepted the hybrid error handling proposal.

[LEWG Kona 2017]

Recommend NAD. Needs a paper. Proposed resolution no longer applies.

Proposed resolution:

Accept the proposed wording of N2828, Library Support for hybrid error handling (Rev 1).

Change `Clock` requirements 29.3 [time.clock.req] as indicated:

> -2- In Table 55 `C1` and `C2` denote clock types. `t1` and `t2` are values returned by `C1::now()` where the call returning `t1` happens before (1.10) the call returning `t2` and both of these calls happen before `C1::time_point::max()`. `ec` denotes an object of type `error_code` (19.5.4.1 [syserr.errcode.overview]).

> Table 55 — Clock requirements
> Expression | Return type | Operational semantics
> --- | --- | ---
> ... | ... | ...
> `C1::now()` | `C1::time_point` | Returns a `time_point` object representing the current point in time.
> `C1::now(ec)` | `C1::time_point` | Returns a `time_point` object representing the current point in time.

Change class `system_clock` 29.7.2 [time.clock.system] as indicated:

>     static time_point now(error_code& ec=throws());

Change class `monotonic_clock` 99 [time.clock.monotonic] as indicated:

>     static time_point now(error_code& ec=throws());

Change class `high_resolution_clock` 29.7.8 [time.clock.hires] as indicated:

>     static time_point now(error_code& ec=throws());