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Section: 18.104.22.168 [util.smartptr.shared.const] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-07-03 Last modified: 2019-01-01
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unique_ptr guarantees that it will not invoke its deleter if it stores a null pointer, which is useful for deleters that must not be called with a null pointer e.g.
unique_ptr<FILE, int(*)(FILE*)> fptr(file, &::fclose);
However, shared_ptr does invoke the deleter if it owns a null pointer, which is a silent change in behaviour when transferring ownership from unique_ptr to shared_ptr. That means the following leads to undefined behaviour:
std:shared_ptr<FILE> fp = std::move(fptr);
Peter Dimov's suggested fix is to construct an empty shared_ptr from a unique_ptr that contains a null pointer.
[2015-01-18 Library reflector vote]
The issue has been identified as Tentatively Ready based on eight votes in favour.
This wording is relative to N4296.
Change 22.214.171.124 [util.smartptr.shared.const] p29 as indicated:
template <class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);
Equivalent to shared_ptr(r.release(), r.get_deleter()) when D is not a reference type, otherwise shared_ptr(r.release(), ref(r.get_deleter())).