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Subject: Re: Is an implementation required to define an unneeded, implicitly declared as defaulted, virtual destructor?
From: Myria (myriachan_at_[hidden])
Date: 2019-11-02 09:40:20


delete for an incomplete type is legal, though it probably should not be.
If the completed type ends up having a nontrivial destructor, the behavior
is undefined.

On Sat, Nov 2, 2019 at 07:15 Mark Stegeman via Std-Discussion <
std-discussion_at_[hidden]> wrote:

> Hi list,
>
> I'm having some trouble determining the validity of the following C++17
> code:
>
> ----
> #include <memory>
>
> struct Base {
> virtual ~Base() = default;
> };
>
> struct Bar;
> struct Foo : Base {
> std::unique_ptr<Bar> bar_{};
> };
> ----
>
> The following standard quotes lead me to believe that this is ill-formed:
>
> [class.dtor]/4
> If a class has no user-declared destructor, a destructor is implicitly
> declared as defaulted. [...]
>
> [class.dtor]/10
> [...] If a class has a base class with a virtual destructor, its
> destructor (whether user- or implicitly-declared) is virtual.
>
> [class.dtor]/7
> A destructor that is defaulted and not defined as deleted is implicitly
> defined when it is odr-used or when it is explicitly defaulted after its
> first declaration.
>
> [class.dtor]/5
> A defaulted destructor for a class X is defined as deleted if:
> - X is a union-like class that has a variant member with a non-trivial
> destructor,
> - any potentially constructed subobject has class type M (or array
> thereof) and M has a deleted destructor or a destructor that is
> inaccessible from the defaulted destructor,
> - or, for a virtual destructor, lookup of the non-array deallocation
> function results in an ambiguity or in a function that is deleted or
> inaccessible from the defaulted destructor.
>
> [basic.def.odr]/3
> [...] A virtual member function is odr-used if it is not pure. [...]
>
> I would interpret these quotes as: ~Foo() must be implicitly defined,
> because it is virtual, defaulted, and not defined as deleted. This ~Foo()
> must invoke ~std::unique_ptr<Bar>(), which invokes
> std::default_delete<Bar>, which makes the program ill-formed, because Bar
> is incomplete.
>
> However, practically speaking, since no Foo's are created, there should be
> no reason to define ~Foo() here. The description of CWG2068 indeed suggests
> that the intended interpretation is that implementations are not required
> to define ~Foo() in this example. And the implementations I checked (MSVC,
> gcc, clang) all accept it as well. So it looks like my interpretation is
> wrong, but I haven't been able to find why.
>
> Could you tell me where my interpretation goes wrong? What have I missed?
>
> Thanks
>
> See https://stackoverflow.com/q/58543232 for a previous discussion
>
> --
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> Std-Discussion_at_[hidden]
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>



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