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;
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;
~weak_ptr();
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;
void swap(weak_ptr& r) noexcept;
void reset() noexcept;
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>;
template<class T> void swap(weak_ptr<T>& a, weak_ptr<T>& b) noexcept;
}
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;
Effects: Constructs an empty weak_ptr object.
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;
Remarks: The second and third constructors shall not participate in overload resolution unless Y* is compatible with T*.
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.
Postconditions: use_count() == r.use_count().
weak_ptr(weak_ptr&& r) noexcept; template<class Y> weak_ptr(weak_ptr<Y>&& r) noexcept;
Remarks: The second constructor shall not participate in overload resolution unless Y* is compatible with T*.
Effects: Move constructs a weak_ptr instance from r.
Postconditions: *this shall contain the old value of r. r shall be empty. r.use_count() == 0.
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;
Effects: Equivalent to weak_ptr(r).swap(*this).
Remarks: The implementation may meet the effects (and the implied guarantees) via different means, without creating a temporary.
weak_ptr& operator=(weak_ptr&& r) noexcept; template<class Y> weak_ptr& operator=(weak_ptr<Y>&& r) noexcept;
Effects: Equivalent to weak_ptr(std::move(r)).swap(*this).
23.11.2.3.4 weak_ptr modifiers [util.smartptr.weak.mod]void swap(weak_ptr& r) noexcept;
Effects: Exchanges the contents of *this and r.
void reset() noexcept;
Effects: Equivalent to weak_ptr().swap(*this).
23.11.2.3.5 weak_ptr observers [util.smartptr.weak.obs]long use_count() const noexcept;
Returns: 0 if *this is empty; otherwise, the number of shared_ptr instances that share ownership with *this.
bool expired() const noexcept;
Returns: use_count() == 0.
shared_ptr<T> lock() const noexcept;
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;
Returns: An unspecified value such that
x.owner_before(y) defines a strict weak ordering as defined in [alg.sorting];
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.
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