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Section: 18.104.22.168 [util.smartptr.weak.obs] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-06
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20.11.3 [util.smartptr.shared]/4 says: "For purposes of determining the presence of a data race, member functions shall access and modify only the shared_ptr and weak_ptr objects themselves and not objects they refer to. Changes in use_count() do not reflect modifications that can introduce data races." This requires shared_ptr/weak_ptr implementations to protect their strong and weak refcounts with atomic operations, without the Standardese having to say this elsewhere. However, 22.214.171.124 [util.smartptr.weak.obs]/5 describes weak_ptr::lock() with "Returns: expired() ? shared_ptr<T>() : shared_ptr<T>(*this)." Even after considering the blanket wording about data races, this specification is insufficient. If this conditional expression were literally implemented, the use_count() could change from nonzero to zero after testing expired(), causing shared_ptr<T>(*this) to throw bad_weak_ptr when the intention is for weak_ptr::lock() to return empty or nonempty without throwing — indeed, weak_ptr::lock() is marked as noexcept.
We all know what weak_ptr::lock() should do, the Standardese just doesn't say it. shared_ptr(const weak_ptr<Y>&)'s specification is not really affected because 126.96.36.199 [util.smartptr.shared.const]/23-27 describes the behavior with English instead of code.
[Issaquah 2014-02-11: Move to Immediate]
This wording is relative to N3691.
Edit 188.8.131.52 [util.smartptr.weak.obs]/5 as indicated:
shared_ptr<T> lock() const noexcept;
-5- Returns: expired() ? shared_ptr<T>() : shared_ptr<T>(*this).