C++ Logo


Advanced search

Re: Is an implementation required to define an unneeded, implicitly declared as defaulted, virtual destructor?

From: Myria <myriachan_at_[hidden]>
Date: Sat, 2 Nov 2019 07:40:20 -0700
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
> --
> Std-Discussion mailing list
> Std-Discussion_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-discussion

Received on 2019-11-02 09:42:49