2751. shared_ptr deleter not specified to observe expired weak_ptr instances

Section: 20.3.2.2.3 [util.smartptr.shared.dest] Status: New Submitter: Aaron Jacobs Opened: 2016-07-21 Last modified: 2017-10-08

Priority: 4

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Discussion:

The C++14 standard contains no language that guarantees the deleter run by a shared_ptr will see all associated weak_ptr instances as expired. For example, the standard doesn't appear to guarantee that the assertion in the following snippet won't fire:

std::weak_ptr<Foo> weak;
std::shared_ptr<Foo> strong{
  new Foo,
  [&weak] (Foo* f) {
    assert(weak.expired());
    delete f;
  },
};

weak = strong;
strong.reset();

It seems clear that the intent is that associated weak_ptrs are expired, because otherwise shared_ptr deleters could resurrect a reference to an object that is being deleted.

Suggested fix: 20.3.2.2.3 [util.smartptr.shared.dest] should specify that the decrease in use_count() caused by the destructor is sequenced before the call to the deleter or the call to delete p.

[2016-11-08, Jonathan and STL suggest NAD]

STL and Jonathan feel that the example has unspecified behaviour, and the assertion is allowed to fire, and that's OK (the program's expectation is not reasonable). Otherwise it's necessary to move-construct a copy of the deleter and use that copy to destroy the owned pointer. We do not want to be required to do that.

See also 2262.

[2017-09-20, Jonathan comments]

I'd like to withdraw my NAD suggestion. The value of use_count() is already observable during the destructor via shared_ptr and weak_ptr objects that share ownership, so specifying when it changes ensures correct behaviour.

Proposed resolution: