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2316. weak_ptr::lock() should be atomic

Section: [util.smartptr.weak.obs] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-05

Priority: 0

View all other issues in [util.smartptr.weak.obs].

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Discussion: [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, [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 [util.smartptr.shared.const]/23-27 describes the behavior with English instead of code.

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

  1. Edit [util.smartptr.weak.obs]/5 as indicated:

    shared_ptr<T> lock() const noexcept;

    -5- Returns: expired() ? shared_ptr<T>() : shared_ptr<T>(*this), executed atomically.