23 General utilities library [utilities]

23.11 Smart pointers [smartptr]

23.11.2 Shared-ownership pointers [util.smartptr]

23.11.2.3 Class template weak_­ptr [util.smartptr.weak]

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The weak_­ptr class template stores a weak reference to an object that is already managed by a shared_­ptr. To access the object, a weak_­ptr can be converted to a shared_­ptr using the member function lock.

namespace std {
  template<class T> class weak_ptr {
  public:
    using element_type = T;

    // [util.smartptr.weak.const], constructors
    constexpr weak_ptr() noexcept;
    template<class Y> weak_ptr(const shared_ptr<Y>& r) noexcept;
    weak_ptr(const weak_ptr& r) noexcept;
    template<class Y> weak_ptr(const weak_ptr<Y>& r) noexcept;
    weak_ptr(weak_ptr&& r) noexcept;
    template<class Y> weak_ptr(weak_ptr<Y>&& r) noexcept;

    // [util.smartptr.weak.dest], destructor
    ~weak_ptr();

    // [util.smartptr.weak.assign], assignment
    weak_ptr& operator=(const weak_ptr& r) noexcept;
    template<class Y> weak_ptr& operator=(const weak_ptr<Y>& r) noexcept;
    template<class Y> weak_ptr& operator=(const shared_ptr<Y>& r) noexcept;
    weak_ptr& operator=(weak_ptr&& r) noexcept;
    template<class Y> weak_ptr& operator=(weak_ptr<Y>&& r) noexcept;

    // [util.smartptr.weak.mod], modifiers
    void swap(weak_ptr& r) noexcept;
    void reset() noexcept;

    // [util.smartptr.weak.obs], observers
    long use_count() const noexcept;
    bool expired() const noexcept;
    shared_ptr<T> lock() const noexcept;
    template<class U> bool owner_before(const shared_ptr<U>& b) const;
    template<class U> bool owner_before(const weak_ptr<U>& b) const;
  };

  template<class T> weak_ptr(shared_ptr<T>) -> weak_ptr<T>;


  // [util.smartptr.weak.spec], specialized algorithms
  template<class T> void swap(weak_ptr<T>& a, weak_ptr<T>& b) noexcept;
}
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Specializations of weak_­ptr shall be CopyConstructible and CopyAssignable, allowing their use in standard containers. The template parameter T of weak_­ptr may be an incomplete type.

23.11.2.3.1 weak_­ptr constructors [util.smartptr.weak.const]

constexpr weak_ptr() noexcept;

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Effects: Constructs an empty weak_­ptr object.

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Postconditions: use_­count() == 0.

weak_ptr(const weak_ptr& r) noexcept; template<class Y> weak_ptr(const weak_ptr<Y>& r) noexcept; template<class Y> weak_ptr(const shared_ptr<Y>& r) noexcept;

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Remarks: The second and third constructors shall not participate in overload resolution unless Y* is compatible with T*.

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Effects: If r is empty, constructs an empty weak_­ptr object; otherwise, constructs a weak_­ptr object that shares ownership with r and stores a copy of the pointer stored in r.

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Postconditions: use_­count() == r.use_­count().

weak_ptr(weak_ptr&& r) noexcept; template<class Y> weak_ptr(weak_ptr<Y>&& r) noexcept;

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Remarks: The second constructor shall not participate in overload resolution unless Y* is compatible with T*.

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Effects: Move constructs a weak_­ptr instance from r.

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Postconditions: *this shall contain the old value of r. r shall be empty. r.use_­count() == 0.

23.11.2.3.2 weak_­ptr destructor [util.smartptr.weak.dest]

~weak_ptr();
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Effects: Destroys this weak_­ptr object but has no effect on the object its stored pointer points to.

23.11.2.3.3 weak_­ptr assignment [util.smartptr.weak.assign]

weak_ptr& operator=(const weak_ptr& r) noexcept; template<class Y> weak_ptr& operator=(const weak_ptr<Y>& r) noexcept; template<class Y> weak_ptr& operator=(const shared_ptr<Y>& r) noexcept;

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Effects: Equivalent to weak_­ptr(r).swap(*this).

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Remarks: The implementation may meet the effects (and the implied guarantees) via different means, without creating a temporary.

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Returns: *this.

weak_ptr& operator=(weak_ptr&& r) noexcept; template<class Y> weak_ptr& operator=(weak_ptr<Y>&& r) noexcept;

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Effects: Equivalent to weak_­ptr(std​::​move(r)).swap(*this).

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Returns: *this.

23.11.2.3.4 weak_­ptr modifiers [util.smartptr.weak.mod]

void swap(weak_ptr& r) noexcept;

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Effects: Exchanges the contents of *this and r.

void reset() noexcept;

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Effects: Equivalent to weak_­ptr().swap(*this).

23.11.2.3.5 weak_­ptr observers [util.smartptr.weak.obs]

long use_count() const noexcept;

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Returns: 0 if *this is empty; otherwise, the number of shared_­ptr instances that share ownership with *this.

bool expired() const noexcept;

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Returns: use_­count() == 0.

shared_ptr<T> lock() const noexcept;

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

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

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Returns: An unspecified value such that

  • (4.1)

    x.owner_­before(y) defines a strict weak ordering as defined in [alg.sorting];

  • (4.2)

    under the equivalence relation defined by owner_­before, !a.owner_­before(b) && !b.owner_­before(a), two shared_­ptr or weak_­ptr instances are equivalent if and only if they share ownership or are both empty.

23.11.2.3.6 weak_­ptr specialized algorithms [util.smartptr.weak.spec]

template<class T> void swap(weak_ptr<T>& a, weak_ptr<T>& b) noexcept;
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Effects: Equivalent to a.swap(b).