[util.smartptr.weak.const]

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constexpr weak_ptr() noexcept;

Postconditions: use_count() == 0.

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

Effects: If r is empty, constructs an empty weak_ptr object that stores a null pointer value; 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().

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

Postconditions: *this contains the old value of r.

r is empty, stores a null pointer value, and r.use_count() == 0.

20 Memory management library [mem]

20.3 Smart pointers [smartptr]

20.3.2 Shared-ownership pointers [util.sharedptr]

20.3.2.3 Class template weak_ptr [util.smartptr.weak]

20.3.2.3.2 Constructors [util.smartptr.weak.const]