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### 854. `default_delete` converting constructor underspecified

Section: 20.3.1.2.2 [unique.ptr.dltr.dflt] Status: C++11 Submitter: Howard Hinnant Opened: 2008-06-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [unique.ptr.dltr.dflt].

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Discussion:

No relationship between `U` and `T` in the converting constructor for `default_delete` template.

Requirements: `U*` is convertible to `T*` and `has_virtual_destructor<T>`; the latter should also become a concept.

Rules out cross-casting.

The requirements for `unique_ptr` conversions should be the same as those on the deleter.

[ Howard adds 2008-11-26: ]

> I believe we need to be careful to not outlaw the following use case, and I believe the current proposed wording (`requires Convertible<U*, T*> && HasVirtualDestructor<T>`) does so:

> >     #include <memory>

> >     int main()
> >     {
> >         std::unique_ptr<int> p1(new int(1));
> >         std::unique_ptr<const int> p2(move(p1));
> >         int i = *p2;
> >     //    *p2 = i;  // should not compile
> >     }

> I've removed "`&& HasVirtualDestructor<T>`" from the `requires` clause in the proposed wording.

[ Post Summit: ]

> Alisdair: This issue has to stay in review pending a paper constraining `unique_ptr`.

> Consensus: We agree with the resolution, but `unique_ptr` needs to be constrained, too.

> Recommend Keep in Review.

[ Batavia (2009-05): ]

> Keep in Review status for the reasons cited.

[ 2009-07 Frankfurt: ]

> The proposed resolution uses concepts. Howard needs to rewrite the proposed resolution.

> Move back to Open.

[ 2009-07-26 Howard provided rewritten proposed wording and moved to Review. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

Add after 20.3.1.2.2 [unique.ptr.dltr.dflt], p1:

>     template <class U> default_delete(const default_delete<U>& other);

> > -1- Effects: ...

> > Remarks: This constructor shall participate in overload resolution if and only if `U*` is implicitly convertible to `T*`.