pon., 17 kwi 2023 o 12:54 Andrey Semashev via Std-Discussion
<std-discussion_at_[hidden]> napisał(a):
>
> On 4/17/23 13:47, Marcin Jaczewski wrote:
> > pon., 17 kwi 2023 o 12:35 Andrey Semashev via Std-Discussion
> > <std-discussion_at_[hidden]> napisał(a):
> >>
> >> On 4/17/23 13:27, Marcin Jaczewski via Std-Discussion wrote:
> >>> niedz., 16 kwi 2023 o 21:44 Ville Voutilainen via Std-Discussion
> >>> <std-discussion_at_[hidden]> napisał(a):
> >>>>
> >>>> On Sun, 16 Apr 2023 at 22:41, Edward Catmur <ecatmur_at_[hidden]> wrote:
> >>>>
> >>>>>>>> If we have a solution that doesn't do that prevention
> >>>>>>>> and solves the coroutine problem, I'm all ears.
> >>>>>>> There's Andrey's solution, but the overhead is substantial.
> >>>>>>
> >>>>>> I don't quite follow how that overhead is avoidable just by wrecking
> >>>>>> the composability of these types, or at all.
> >>>>>
> >>>>>
> >>>>> To recap: the compiler knows when a destructor is being called by normal or exceptional scope exit. Since the compiler and library are written together, the former can pass that information to the facility by private ("magic") means. However, this may be trickier to implement if the destructor of another class intervenes.
> >>>>
> >>>> Right. The "good old" "let's have multiple destructors, and give them
> >>>> parameters that tell them who's who". That doesn't compose,
> >>>> so the conclusion is to desperately pretend that we don't need composition.
> >>>>
> >>>> Nothing good comes out of that.
> >>>>
> >>>
> >>> Why are multiple destructors bad? Especially if they were handled like
> >>> `const` overloads.
> >>> Besides, if a new destructor is no-go, why not add a new operator that
> >>> simply is called before the destructor?
> >>> like `operator unwind()` that is called before a proper destructor is
> >>> used in case of unwind.
> >>
> >> It doesn't matter how you name it. If it doesn't work through other
> >> types (e.g. if the scope guard is a member or accessed through a
> >> pointer) then we have a problem with composability. If it does work,
> >> then we're back to the status quo, since the operator will be called
> >> under the same conditions as the destructor, and therefore it will need
> >> to check for uncaught_exceptions().
> >>
> >
> > `operator unwind() = default;`?
>
> And what would this mean? That it forwards or that it doesn't?
> Regardless of how you answer, my previous reply applies.
>

Apply same operator to all members.

> > And if it's not a direct member then we have the same problem with
> > composible as "copy",
> > that I suppose is solved problem?
>
> I'm not sure what you mean here. Default copy constructors/assignments
> do forward, but there's no problem with that, so I don't see the relation.
>

As the same logic applies in both cases, `T(const T&) = default` apply
copy for each member.
`operator throw_unwind() = default;` is equal to:
```
x.operator throw_unwind();
y.operator throw_unwind();
z.operator throw_unwind();
```
Of course if `x` is a pointer then this operator is noop. But we can
easy work around this by composition like:
```
struct mixing_unwind_deref
{
    template<typename T>
    operator throw_unwind(this T& self) { if (self) (*self).operator
throw_unwind(); }
};

struct MyPointer : std::unique_ptr<scope_gaurd>, mixing_unwind_deref
{

};
```

> > You miss an important thing `unwind` will be called only when an
> > exception unwind code.
>
> This doesn't help if you still need to check for uncaught_exceptions().
>

But what would be the point for checking this? even more when its have
wrong value
when call in coroutine? `uncaught_exceptions` was needed to check if
it was throw unwind.
This operator gives this info directly. What exactly need I have for
this function?

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