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### 1089. Unify "Throws: Nothing." specifications

Section: 32 [thread] Status: C++11 Submitter: Howard Hinnant Opened: 2009-03-22 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses JP 76 [CD1]

A description for "Throws: Nothing." are not unified.

At the part without throw, "Throws: Nothing." should be described.

Add "Throws: Nothing." to the following.

- 32.4.3.7 [thread.thread.static] p1
- 32.5.5.2 [thread.lock.guard] p4
- 32.5.5.4.2 [thread.lock.unique.cons] p6
- 32.6.4 [thread.condition.condvar] p7 and p8
- 32.6.5 [thread.condition.condvarany] p6, p7, p19, p21 and p25

[ Summit: ]

> Pass on to editor.

[ Post Summit: Editor declares this non-editorial. ]

[ 2009-08-01 Howard provided wording: ]

> The definition of "Throws: Nothing." that I added is probably going to be controversial, but I beg you to consider it seriously.

> > In C++ there are three "flow control" options for a function:

> > 1. It can return, either with a value, or with `void`.
> > 2. It can call a function which never returns, such as `std::exit` or `std::terminate`.
> > 3. It can throw an exception.

> > The above list can be abbreviated with:

> > 1. Returns.
> > 2. Ends program.
> > 3. Throws exception.

> > In general a function can have the behavior of any of these 3, or any combination of any of these three, depending upon run time data.

> > 1. R
> > 2. E
> > 3. T
> > 4. RE
> > 5. RT
> > 6. ET
> > 7. RET

> > A function with no throw spec, and no documentation, is in general a RET function. It may return, it may end the program, or it may throw. When we specify a function with an empty throw spec:

> > >     void f() throw();

> > We are saying that `f()` is an RE function: It may return or end the program, but it will not throw.

> > I posit that there are very few places in the library half of the standard where we intend for functions to be able to end the program (call `terminate`). And none of those places where we do say `terminate` could be called, do we currently say "Throws: Nothing.".

> > I believe that if we define "Throws: Nothing." to mean R, we will both clarify many, many places in the standard, and give us a good rationale for choosing between "Throws: Nothing." (R) and `throw()` (RE) in the future. Indeed, this may give us motivation to change several `throw()`s to "Throws: Nothing.".

> I did not add the following changes as JP 76 requested as I believe we want to allow these functions to throw:

> > Add a paragraph under 32.5.5.2 [thread.lock.guard] p4:

> > >     explicit lock_guard(mutex_type& m);

> > > Throws: Nothing.

> > Add a paragraph under 32.5.5.4.2 [thread.lock.unique.cons] p6:

> > >     explicit unique_lock(mutex_type& m);

> > > Throws: Nothing.

> > Add a paragraph under 32.6.5 [thread.condition.condvarany] p19, p21 and p25:

> > >     template <class Lock, class Rep, class Period>
> > >       bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);

> > > Throws: Nothing.

> > >     template <class Lock, class Duration, class Predicate>
> > >       bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& rel_time, Predicate pred);

> > > Throws: Nothing.

> > >     template <class Lock, class Rep, class Period, class Predicate>
> > >       bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);

> > > Throws: Nothing.

[ 2009-10 Santa Cruz: ]

> Defer pending further developments with exception restriction annotations.

[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-02-24 Pete moved to Open: ]

> A "Throws: Nothing" specification is not the place to say that a function is not allowed to call `exit()`. While I agree with the thrust of the proposed resolution, "doesn't throw exceptions" is a subset of "always returns normally". If it's important to say that most library functions don't call `exit()`, say so.

[ 2010 Pittsburgh: ]

> Move to Ready except for the added paragraph to 16.3.2.4 [structure.specifications].

Proposed resolution:

Add a paragraph under 32.4.3.7 [thread.thread.static] p1:

>     unsigned hardware_concurrency();

> -1- Returns: ...

> Throws: Nothing.

Add a paragraph under 32.6.4 [thread.condition.condvar] p7 and p8:

> [Informational, not to be incluced in the WP: The POSIX spec allows only:

> [EINVAL]
>   The value `cond` does not refer to an initialized condition variable. — end informational]

>     void notify_one();

> -7- Effects: ...

> Throws: Nothing.

>     void notify_all();

> -8- Effects: ...

> Throws: Nothing.

Add a paragraph under 32.6.5 [thread.condition.condvarany] p6 and p7:

>     void notify_one();

> -6- Effects: ...

> Throws: Nothing.

>     void notify_all();

> -7- Effects: ...

> Throws: Nothing.