20 Memory management library [mem]

20.3 Smart pointers [smartptr]

20.3.2 Shared-ownership pointers [util.sharedptr]

20.3.2.2 Class template shared_ptr [util.smartptr.shared]

20.3.2.2.2 Constructors [util.smartptr.shared.const]

In the constructor definitions below, enables shared_from_this with p, for a pointer p of type Y*, means that if Y has an unambiguous and accessible base class that is a specialization of enable_shared_from_this ([util.smartptr.enab]), then remove_cv_t<Y>* shall be implicitly convertible to T* and the constructor evaluates the statement: if (p != nullptr && p->weak_this.expired()) p->weak_this = shared_ptr<remove_cv_t<Y>>(*this, const_cast<remove_cv_t<Y>*>(p));

The assignment to the weak_this member is not atomic and conflicts with any potentially concurrent access to the same object ([intro.multithread]).

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constexpr shared_ptr() noexcept;

Postconditions: use_count() == 0 && get() == nullptr.

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template<class Y> explicit shared_ptr(Y* p);

Constraints: When T is an array type, the expression delete[] p is well-formed and either T is U[N] and Y(*)[N] is convertible to T*, or T is U[] and Y(*)[] is convertible to T*.

When T is not an array type, the expression delete p is well-formed and Y* is convertible to T*.

Mandates: Y is a complete type.

Preconditions: The expression delete[] p, when T is an array type, or delete p, when T is not an array type, has well-defined behavior, and does not throw exceptions.

Effects: When T is not an array type, constructs a shared_ptr object that owns the pointer p.

Otherwise, constructs a shared_ptr that owns p and a deleter of an unspecified type that calls delete[] p.

When T is not an array type, enables shared_from_this with p.

If an exception is thrown, delete p is called when T is not an array type, delete[] p otherwise.

Postconditions: use_count() == 1 && get() == p.

Throws: bad_alloc, or an implementation-defined exception when a resource other than memory cannot be obtained.

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template<class Y, class D> shared_ptr(Y* p, D d);
template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
template<class D> shared_ptr(nullptr_t p, D d);
template<class D, class A> shared_ptr(nullptr_t p, D d, A a);

Constraints: is_move_constructible_v<D> is true, and d(p) is a well-formed expression.

For the first two overloads:

(9.1)
If T is an array type, then either T is U[N] and Y(*)[N] is convertible to T*, or T is U[] and Y(*)[] is convertible to T*.
(9.2)
If T is not an array type, then Y* is convertible to T*.

Preconditions: Construction of d and a deleter of type D initialized with std::move(d) do not throw exceptions.

The expression d(p) has well-defined behavior and does not throw exceptions.

A meets the Cpp17Allocator requirements ([allocator.requirements.general]).

Effects: Constructs a shared_ptr object that owns the object p and the deleter d.

When T is not an array type, the first and second constructors enable shared_from_this with p.

The second and fourth constructors shall use a copy of a to allocate memory for internal use.

If an exception is thrown, d(p) is called.

Postconditions: use_count() == 1 && get() == p.

Throws: bad_alloc, or an implementation-defined exception when a resource other than memory cannot be obtained.

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template<class Y> shared_ptr(const shared_ptr<Y>& r, element_type* p) noexcept;
template<class Y> shared_ptr(shared_ptr<Y>&& r, element_type* p) noexcept;

Effects: Constructs a shared_ptr instance that stores p and shares ownership with the initial value of r.

Postconditions: get() == p.

For the second overload, r is empty and r.get() == nullptr.

[Note 1:

Use of this constructor leads to a dangling pointer unless p remains valid at least until the ownership group of r is destroyed.

— end note]

[Note 2:

This constructor allows creation of an empty shared_ptr instance with a non-null stored pointer.

— end note]

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

Constraints: For the second constructor, Y* is compatible with T*.

Effects: If r is empty, constructs an empty shared_ptr object; otherwise, constructs a shared_ptr object that shares ownership with r.

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

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

Constraints: For the second constructor, Y* is compatible with T*.

Effects: Move constructs a shared_ptr instance from r.

Postconditions: *this contains the old value of r.

r is empty, and r.get() == nullptr.

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

Constraints: Y* is compatible with T*.

Effects: Constructs a shared_ptr object that shares ownership with r and stores a copy of the pointer stored in r.

If an exception is thrown, the constructor has no effect.

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

Throws: bad_weak_ptr when r.expired().

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template<class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);

Constraints: Y* is compatible with T* and unique_ptr<Y, D>::pointer is convertible to element_type*.

Effects: If r.get() == nullptr, equivalent to shared_ptr().

Otherwise, if D is not a reference type, equivalent to shared_ptr(r.release(), std::move(r.get_deleter())).

Otherwise, equivalent to shared_ptr(r.release(), ref(r.get_deleter())).

If an exception is thrown, the constructor has no effect.