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Subject: Re: [std-proposals] Constexpr destructors standard library update omission
From: Maciej Cencora (m.cencora_at_[hidden])
Date: 2020-07-16 17:18:25


Should have been:
void sideeffect();

struct foo
{
    constexpr foo() { val = 0; }
    ~foo() { if (val) sideeffect(); }

    int val;
};

*constinit* foo f;

int bar = (f.val = 1, throw 0, 0);

pt., 17 lip 2020 o 00:16 Maciej Cencora <m.cencora_at_[hidden]> napisał(a):

> Ah, I see what you mean.
>
> Since constinit forces constant initialization, the object is properly
> constructed before any dynamic initialization takes place, and any other
> globals can safely reference it during their dynamic initialization.
> So you can register non-trivial destructor for const init variables before
> performing dynamic initialization of other globals or executing main.
> And even in case dynamic initialization will be interrupted due to an
> exception, the destructor for such a const-init global will be called as
> expected.
> This should work safely even across different translation units.
>
> Constexpr destructor doesn't change anything in this case.
>
> void sideeffect();
>
> struct foo
> {
> constexpr foo() { val = 0; }
> ~foo() { if (val) sideeffect(); }
>
> int val;
> };
>
> foo f;
>
> int bar = (f.val = 1, throw 0, 0);
>
>
> czw., 16 lip 2020 o 23:26 Thiago Macieira <thiago_at_[hidden]>
> napisał(a):
>
>> On Thursday, 16 July 2020 13:54:24 PDT Maciej Cencora wrote:
>> > As far as I understand the latest C++ draft, constexpr destructor
>> doesn't
>> > make the type trivial. It is just that constexpr destructor is allowed
>> to
>> > be called in constexpr context (while in previous C++ standards only
>> types
>> > with trivial destructors were allowed in constexpr context).
>>
>> Sorry, I misspoke. I didn't mean it was a trivial type. I meant it was a
>> *literal* type.
>>
>> http://eel.is/c++draft/basic.types#10
>>
>> Specifically, "a type is a literal type if ... it has a constexpr
>> destructor"
>> Changed from C++17 that required trivial.
>>
>> Literal types are required (but not sufficient condition) for constexpr
>> functions.
>>
>> > So I guess this would fail to compile, because such a global would still
>> > require a runtime registration of the destructor during initialization.
>>
>> I think it would succeed in compiling but, despite the constexpr
>> destructor,
>> would always emit a dynamic initialiser to destroy this object
>> dynamically. I
>> think it makes sense, as any non-trivial destructor is necessarily doing
>> something, so it must be run
>>
>> I'd like a "don't destroy" marker to go along constinit.
>>
>> --
>> Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
>> Software Architect - Intel System Software Products
>>
>>
>>
>>



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