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Subject: Re: [std-proposals] extern noexcept
From: Nevin Liber (nevin_at_[hidden])
Date: 2019-09-24 11:23:48


On Tue, Sep 24, 2019 at 9:28 AM Walt Karas via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> On 2019-09-24 01:06, Walt Karas via Std-Proposals wrote:
> >
> > (Side question:? shouldn't the Standard require a diagnostic if a
> > noexcept function calls noexcept(false) functions on some codepath?)
>
> Andrey Semashev via Std-Proposals wrote:
> No, because that function (1) may not actually throw, (2) may be inside
> a try block or may not be called in runtime and (3) program termination
> in case of an exception may be intended.
>
> But points 1 and 2 contradict the premise of the question.
>

Why?

noexcept(false) means potentially throwing, not guaranteed throwing.

What would you do with code like:

try {
    std::deque<int> d;
} catch (int) { /* ... */ }

Should that compile, even though an exception can still escape depending on
the implementation of deque? If so, how would the compiler know what
exceptions deque::deque() might throw?

> try/catch statements would I suppose be required to have catch (...) in
> noexcept functions. For point 3, is there really a guarantee this will
> cause termination?
>

Yes. [except.spec] <http://eel.is/c++draft/except.spec#5>: "Whenever an
exception is thrown and the search for a handler ([except.handle])
encounters the outermost block of a function with a non-throwing exception
specification, the function std::terminate is called ([except.terminate]).
[ Note: An implementation shall not reject an expression merely because,
when executed, it throws or might throw an exception from a function with a
non-throwing exception specification. — end note ]".

> Seems like the exception could still be caught higher in the call stack,
> and some objects could potentially not get destroyed.
>

I don't understand this. Are you saying noexcept should just be ignored
(not even UB)?

Anyway, that ship sailed eight years ago.

-- 
 Nevin ":-)" Liber  <mailto:nevin_at_[hidden] <nevin_at_[hidden]>>
+1-847-691-1404


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