Document #: P1901R0
Date: 2019-10-02
Reply-to: Daryl Haresign <cpp@daryl.haresign.com>
Audience: Library Evolution Working Group

Enabling the Use of weak_ptr as Keys in Unordered Associative Containers

Contents

Abstract

This paper proposes the addition of ownership based hashing and equality, to enable the use of weak_ptr as keys in unordered associative containers.

Motivation

I recently found myself wanting to store a collection of weak_ptr objects. I neither had a separate key nor any requirements on the ordering of the objects. It seemed that using an unordered_set would be a good fit. It quickly became apparent, however, that it’s simply not possible; there is no way to generate a stable hash or compare weak_ptr objects without support from the Standard Library.

Also relevant is the discussion in LWG Issue 1406, which labelled this as not a defect, and recommended a paper be brought forward to propose the addition.

Proposal

owner_hash and owner_equal

The obvious, and in my opinion best, option is to follow in the footsteps of owner_less, which can be used to allow weak_ptr keys in ordered associative containers.

N2637 was the paper through which owner_less was introduced into the standard.

The following methods and class templates would be added:

shared_ptr and weak_ptr would gain two new methods each: owner_hash and owner_equal. owner_hash would return a ownership based hash and owner_equal would compare ownership.

In addition, two new class templates would be provided: owner_hash and owner_equal.

This option has the benefit of providing all the tools necessary for having a weak_ptr key in an unordered associative container.

The addition of the methods and template specializations for shared_ptr enables the ability to look up values in an unordered associative container from either the weak_ptr or the corresponding shared_ptr. It also enables storing shared_ptr keys based on the ownership equivalence relation, if such a thing is desired.

I don't believe this proposal exposes any more implementation details than are already exposed through owner_less.

To satisfy the Cpp17Hash requirements that the probability of a collision be low, a conforming implementation could simply hash the address of the control block, per hash<void *>.

Alternatives Considered

owner_ptr / owner / owner_id

This option would instead add the following methods:

The unspecified return type would need to be something for which there is, or could be, a std::hash specialization and an equality comparator (for example, a pointer to the control block, a guid stored in the control block, etc.).

This option would look to provide the bare minimum required, leaving it up to the user to define owner_hash and owner_equal in terms of this method.

shared_ptr_key / weak_ptr_key

This option would provide wrapper classes, shared_ptr_key and weak_ptr_key, which would have a std::hash specialization and an equality comparator. They would presumably be friends of shared_ptr and weak_ptr respectively.

Do Nothing

Whilst writing this paper, I discovered that it is actually possible for end users to do this right now in terms of owner_before, it just wouldn't be efficient.

owner_hash would be implemented to return a constant value — completely going against the Cpp17Hash requirements which state thatthe probability of a collision be low.

and

owner_equal(p,q) would be implemented to return !p.owner_before(q) && !q.owner_before(p).

Wording

Class template owner_hash [util.smartptr.ownerhash]

Add this entire section.

The class template owner_hash allows ownership-based hashing. 

namespace std {
  template <class T> struct owner_hash;

  template <class T> struct owner_hash<shared_ptr<T>> {
    size_t operator()(const shared_ptr<T>&) const noexcept;
  };

  template <class T> struct owner_hash<weak_ptr<T>> {
    size_t operator()(const weak_ptr<T>&) const noexcept;
  };
}

operator()(x) shall return x.owner_hash().

Class template owner_equal [util.smartptr.ownerequal]

Add this entire section.

The class template owner_equal allows ownership-based mixed equality comparisons of shared and weak pointers. 

namespace std {
  template <class T = void> struct owner_equal;

  template <class T> struct owner_equal<shared_ptr<T>> {
    bool operator()(const shared_ptr<T>&, const shared_ptr<T>&) const noexcept;
    bool operator()(const shared_ptr<T>&, const weak_ptr<T>&) const noexcept;
    bool operator()(const weak_ptr<T>&, const shared_ptr<T>&) const noexcept;
  };

  template <class T> struct owner_equal<weak_ptr<T>> {
    bool operator()(const weak_ptr<T>&, const weak_ptr<T>&) const noexcept;
    bool operator()(const shared_ptr<T>&, const weak_ptr<T>&) const noexcept;
    bool operator()(const weak_ptr<T>&, const shared_ptr<T>&) const noexcept;
  };

  template <> struct owner_equal<void> {
    template <class T, class U>
      bool operator()(const shared_ptr<T>&, const shared_ptr<U>&) const noexcept;
    template <class T, class U>
      bool operator()(const shared_ptr<T>&, const weak_ptr<U>&) const noexcept;
    template <class T, class U>
      bool operator()(const weak_ptr<T>&, const shared_ptr<U>&) const noexcept;
    template <class T, class U>
      bool operator()(const weak_ptr<T>&, const weak_ptr<U>&) const noexcept;

    using is_transparent = unspecified;
  };
}

operator()(x, y) shall return x.owner_equal(y).

Class template shared_ptr [util.smartptr.shared]

Add to the observers section: 

size_t owner_hash() const noexcept;
template<class U>
  bool owner_equal(const shared_ptr<U>& b) const noexcept;
template<class U>
  bool owner_equal(const weak_ptr<U>& b) const noexcept;

Observers [util.smartptr.shared.obs]

Add at the end: 

size_t owner_hash() const noexcept;

Returns: An unspecified value such that

— x.owner_hash() == y.owner_hash() is true if x.owner_equal() == y.owner_equal() 

template<class U>
  bool owner_equal(const shared_ptr<U>& b) const noexcept;
template<class U>
  bool owner_equal(const weak_ptr<U>& b) const noexcept;

Returns: true if *this and b share ownership or are both empty.

Class template weak_ptr [util.smartptr.weak]

Add to the observers section: 

size_t owner_hash() const noexcept;
template<class U>
  bool owner_equal(const shared_ptr<U>& b) const noexcept;
template<class U>
  bool owner_equal(const weak_ptr<U>& b) const noexcept;

Observers [util.smartptr.weak.obs]

Add at the end: 

size_t owner_hash() const noexcept;

Returns: An unspecified value such that

— x.owner_hash() == y.owner_hash() is true if x.owner_equal() == y.owner_equal() 

template<class U>
  bool owner_equal(const shared_ptr<U>& b) const noexcept;
template<class U>
  bool owner_equal(const weak_ptr<U>& b) const noexcept;

Returns: true if *this and b share ownership or are both empty.

Class template enable_shared_from_this [util.smartptr.enab]

It might make sense to change the example assert from 

assert(!p.owner_before(q) && !q.owner_before(p)); // p and q share ownership

to 

assert(p.owner_equal(q)); // p and q share ownership

References

Acknowledgements

Thanks to Alexander Jones, Alisdair Meredith and Masud Rahman for their help reviewing, suggesting alternatives, and providing background on the history of owner_less.