Re: [boost] Noexcept

Subject: Re: [boost] Noexcept
From: Emil Dotchevski (emildotchevski_at_[hidden])
Date: 2017-06-14 01:25:08

On Tue, Jun 13, 2017 at 6:00 PM, Gavin Lambert via Boost <
boost_at_[hidden]> wrote:

> On 14/06/2017 12:29, Emil Dotchevski wrote:
>
>> Except it won't work, you've uncovered an omission in the Noexcept API.
>> The
>> problem is that catch_<> will flag the error as handled, and in this case
>> you don't want that. I need to add another function similar to catch_<>
>> which only gets the error without handling it. Not sure what to call it,
>> or
>> maybe instead I can add a member function tr.throw_() which flags the
>> error
>> as unhandled.
>>
>> (Related: if the original throw_ was a custom type not derived from
>>> std::exception, and catch<> is used to catch it as a std::exception and
>>> throw_ it again, can it still be caught later with a
>>> catch_<custom_error>?
>>> Does the answer depend on whether throw_() or throw_(e) was used?)
>>>
>>
>>
>> throw_() is a noop, except that it converts to anything for return
>> throw_(), returning throw_return<R>::value() where R is the return type of
>> the function.
>>
>
> Ok, I assumed that throw_() was the equivalent of throw; ie. a way to flag
> a previously-caught exception as unhandled again so that it continues to
> propagate.
>
> If you're modelling this based on try-catch, that seems like essential
> functionality -- in code that wants to implement a sort of try-finally
> without using RAII classes it's not uncommon to see patterns like this:
>
> init_something();
> try
> {
>    // do something that might throw
> }
> catch (...)
> {
>    cleanup_something();
>    throw;
> }
> cleanup_something(); // if needed in the success case as well
>

Absolutely. The reason why this fell through the cracks is that with
Noexcept you could do this without a try_:

if( auto r=foo() )
  //success, all good
else
  return throw_();

It's just the case with try_ that doesn't work right now, but yes it has to
be fixed, it is essential.