881. shared_ptr conversion issue

Section: 23.11.3.1 [util.smartptr.shared.const] Status: C++11 Submitter: Peter Dimov Opened: 2008-08-30 Last modified: 2016-02-10

Priority: Not Prioritized

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Discussion:

We've changed shared_ptr<Y> to not convert to shared_ptr<T> when Y* doesn't convert to T* by resolving issue 687. This only fixed the converting copy constructor though. N2351 later added move support, and the converting move constructor is not constrained.

[ San Francisco: ]

We might be able to move this to NAD, Editorial once shared_ptr is conceptualized, but we want to revisit this issue to make sure.

[ 2009-07 Frankfurt ]

Moved to Ready.

This issue now represents the favored format for specifying constrained templates.

Proposed resolution:

We need to change the Requires clause of the move constructor:

shared_ptr(shared_ptr&& r); 
template<class Y> shared_ptr(shared_ptr<Y>&& r); 

Requires Remarks: For the second constructor Y* shall be convertible to T*. The second constructor shall not participate in overload resolution unless Y* is convertible to T*.

in order to actually make the example in 687 compile (it now resolves to the move constructor).