20 Memory management library [mem]

20.3 Smart pointers [smartptr]

20.3.2 Shared-ownership pointers [util.sharedptr]

20.3.2.1 Class bad_weak_ptr [util.smartptr.weak.bad]

namespace std { class bad_weak_ptr : public exception { public: // see [exception] for the specification of the special member functions const char* what() const noexcept override; }; }

An exception of type bad_weak_ptr is thrown by the shared_ptr constructor taking a weak_ptr.

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const char* what() const noexcept override;

Returns: An implementation-defined ntbs.

20.3.2.2 Class template shared_ptr [util.smartptr.shared]

20.3.2.2.1 General [util.smartptr.shared.general]

The shared_ptr class template stores a pointer, usually obtained via new.

shared_ptr implements semantics of shared ownership; the last remaining owner of the pointer is responsible for destroying the object, or otherwise releasing the resources associated with the stored pointer.

A shared_ptr is said to be empty if it does not own a pointer.

namespace std { template<class T> class shared_ptr { public: using element_type = remove_extent_t<T>; using weak_type = weak_ptr<T>; // [util.smartptr.shared.const], constructors constexpr shared_ptr() noexcept; constexpr shared_ptr(nullptr_t) noexcept : shared_ptr() { } template<class Y> explicit shared_ptr(Y* p); 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); 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; shared_ptr(const shared_ptr& r) noexcept; template<class Y> shared_ptr(const shared_ptr<Y>& r) noexcept; shared_ptr(shared_ptr&& r) noexcept; template<class Y> shared_ptr(shared_ptr<Y>&& r) noexcept; template<class Y> explicit shared_ptr(const weak_ptr<Y>& r); template<class Y, class D> shared_ptr(unique_ptr<Y, D>&& r); // [util.smartptr.shared.dest], destructor ~shared_ptr(); // [util.smartptr.shared.assign], assignment shared_ptr& operator=(const shared_ptr& r) noexcept; template<class Y> shared_ptr& operator=(const shared_ptr<Y>& r) noexcept; shared_ptr& operator=(shared_ptr&& r) noexcept; template<class Y> shared_ptr& operator=(shared_ptr<Y>&& r) noexcept; template<class Y, class D> shared_ptr& operator=(unique_ptr<Y, D>&& r); // [util.smartptr.shared.mod], modifiers void swap(shared_ptr& r) noexcept; void reset() noexcept; template<class Y> void reset(Y* p); template<class Y, class D> void reset(Y* p, D d); template<class Y, class D, class A> void reset(Y* p, D d, A a); // [util.smartptr.shared.obs], observers element_type* get() const noexcept; T& operator*() const noexcept; T* operator->() const noexcept; element_type& operator[](ptrdiff_t i) const; long use_count() const noexcept; explicit operator bool() const noexcept; template<class U> bool owner_before(const shared_ptr& b) const noexcept; template<class U> bool owner_before(const weak_ptr& b) const noexcept; size_t owner_hash() const noexcept; template<class U> bool owner_equal(const shared_ptr& b) const noexcept; template<class U> bool owner_equal(const weak_ptr& b) const noexcept; }; template<class T> shared_ptr(weak_ptr<T>) -> shared_ptr<T>; template<class T, class D> shared_ptr(unique_ptr<T, D>) -> shared_ptr<T>; }

Specializations of shared_ptr shall be Cpp17CopyConstructible, Cpp17CopyAssignable, and Cpp17LessThanComparable, allowing their use in standard containers.

Specializations of shared_ptr shall be contextually convertible to bool, allowing their use in boolean expressions and declarations in conditions.

The template parameter T of shared_ptr may be an incomplete type.

[Note 1:

T can be a function type.

— end note]

[Example 1: if (shared_ptr<X> px = dynamic_pointer_cast<X>(py)) { // do something with px } — end example]

For purposes of determining the presence of a data race, member functions shall access and modify only the shared_ptr and weak_ptr objects themselves and not objects they refer to.

Changes in use_count() do not reflect modifications that can introduce data races.

For the purposes of subclause [smartptr], a pointer type Y* is said to be compatible with a pointer type T* when either Y* is convertible to T* or Y is U[N] and T is cv U[].

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.

20.3.2.2.3 Destructor [util.smartptr.shared.dest]

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~shared_ptr();

Effects:

(1.1)
If *this is empty or shares ownership with another shared_ptr instance (use_count() > 1), there are no side effects.
(1.2)
Otherwise, if *this owns an object p and a deleter d, d(p) is called.
(1.3)
Otherwise, *this owns a pointer p, and delete p is called.

[Note 1:

Since the destruction of *this decreases the number of instances that share ownership with *this by one, after *this has been destroyed all shared_ptr instances that shared ownership with *this will report a use_count() that is one less than its previous value.

— end note]

20.3.2.2.4 Assignment [util.smartptr.shared.assign]

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

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

Returns: *this.

[Note 1:

The use count updates caused by the temporary object construction and destruction are not observable side effects, so the implementation can meet the effects (and the implied guarantees) via different means, without creating a temporary.

In particular, in the example: shared_ptr<int> p(new int); shared_ptr<void> q(p); p = p; q = p; both assignments can be no-ops.

— end note]

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

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

Returns: *this.

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

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

Returns: *this.

20.3.2.2.5 Modifiers [util.smartptr.shared.mod]

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void swap(shared_ptr& r) noexcept;

Effects: Exchanges the contents of *this and r.

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void reset() noexcept;

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

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template<class Y> void reset(Y* p);

Effects: Equivalent to shared_ptr(p).swap(*this).

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template<class Y, class D> void reset(Y* p, D d);

Effects: Equivalent to shared_ptr(p, d).swap(*this).

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template<class Y, class D, class A> void reset(Y* p, D d, A a);

Effects: Equivalent to shared_ptr(p, d, a).swap(*this).

20.3.2.2.6 Observers [util.smartptr.shared.obs]

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element_type* get() const noexcept;

Returns: The stored pointer.

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T& operator*() const noexcept;

Preconditions: get() != nullptr.

Returns: *get().

Remarks: When T is an array type or cv void, it is unspecified whether this member function is declared.

If it is declared, it is unspecified what its return type is, except that the declaration (although not necessarily the definition) of the function shall be well-formed.

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T* operator->() const noexcept;

Preconditions: get() != nullptr.

Returns: get().

Remarks: When T is an array type, it is unspecified whether this member function is declared.

If it is declared, it is unspecified what its return type is, except that the declaration (although not necessarily the definition) of the function shall be well-formed.

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element_type& operator[](ptrdiff_t i) const;

Preconditions: get() != nullptr && i >= 0.

If T is U[N], i < N.

Returns: get()[i].

Throws: Nothing.

Remarks: When T is not an array type, it is unspecified whether this member function is declared.

If it is declared, it is unspecified what its return type is, except that the declaration (although not necessarily the definition) of the function shall be well-formed.

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long use_count() const noexcept;

Synchronization: None.

Returns: The number of shared_ptr objects, *this included, that share ownership with *this, or 0 when *this is empty.

[Note 1:

get() == nullptr does not imply a specific return value of use_count().

— end note]

[Note 2:

weak_ptr<T>::lock() can affect the return value of use_count().

— end note]

[Note 3:

When multiple threads might affect the return value of use_count(), the result is approximate.

In particular, use_count() == 1 does not imply that accesses through a previously destroyed shared_ptr have in any sense completed.

— end note]

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explicit operator bool() const noexcept;

Returns: get() != nullptr.

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

Returns: An unspecified value such that

(18.1)
owner_before(b) defines a strict weak ordering as defined in [alg.sorting];
(18.2)
!owner_before(b) && !b.owner_before(*this) is true if and only if owner_equal(b) is true.

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size_t owner_hash() const noexcept;

Returns: An unspecified value such that, for any object x where owner_equal(x) is true, owner_hash() == x.owner_hash() is true.

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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 and only if *this and b share ownership or are both empty.

Otherwise returns false.

Remarks: owner_equal is an equivalence relation.

20.3.2.2.7 Creation [util.smartptr.shared.create]

The common requirements that apply to all make_shared, allocate_shared, make_shared_for_overwrite, and allocate_shared_for_overwrite overloads, unless specified otherwise, are described below.

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template<class T, ...>
  shared_ptr<T> make_shared(args);
template<class T, class A, ...>
  shared_ptr<T> allocate_shared(const A& a, args);
template<class T, ...>
  shared_ptr<T> make_shared_for_overwrite(args);
template<class T, class A, ...>
  shared_ptr<T> allocate_shared_for_overwrite(const A& a, args);

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

Effects: Allocates memory for an object of type T (or U[N] when T is U[], where N is determined from args as specified by the concrete overload).

The object is initialized from args as specified by the concrete overload.

The allocate_shared and allocate_shared_for_overwrite templates use a copy of a (rebound for an unspecified value_type) to allocate memory.

If an exception is thrown, the functions have no effect.

Postconditions: r.get() != nullptr && r.use_count() == 1, where r is the return value.

Returns: A shared_ptr instance that stores and owns the address of the newly constructed object.

Throws: bad_alloc, or an exception thrown from allocate or from the initialization of the object.

Remarks:

(7.1)
Implementations should perform no more than one memory allocation.

[Note 1:
This provides efficiency equivalent to an intrusive smart pointer.
— end note]
(7.2)
When an object of an array type U is specified to have an initial value of u (of the same type), this shall be interpreted to mean that each array element of the object has as its initial value the corresponding element from u.
(7.3)
When an object of an array type is specified to have a default initial value, this shall be interpreted to mean that each array element of the object has a default initial value.
(7.4)
When a (sub)object of a non-array type U is specified to have an initial value of v, or U(l...), where l... is a list of constructor arguments, make_shared shall initialize this (sub)object via the expression ::new(pv) U(v) or ::new(pv) U(l...) respectively, where pv has type void* and points to storage suitable to hold an object of type U.
(7.5)
When a (sub)object of a non-array type U is specified to have an initial value of v, or U(l...), where l... is a list of constructor arguments, allocate_shared shall initialize this (sub)object via the expression
- (7.5.1)
 allocator_traits<A2>::construct(a2, pv, v) or
- (7.5.2)
 allocator_traits<A2>::construct(a2, pv, l...)
respectively, where pv points to storage suitable to hold an object of type U and a2 of type A2 is a rebound copy of the allocator a passed to allocate_shared such that its value_type is remove_cv_t.
(7.6)
When a (sub)object of non-array type U is specified to have a default initial value, make_shared shall initialize this (sub)object via the expression ::new(pv) U(), where pv has type void* and points to storage suitable to hold an object of type U.
(7.7)
When a (sub)object of non-array type U is specified to have a default initial value, allocate_shared shall initialize this (sub)object via the expression allocator_traits<A2>::construct(a2, pv), where pv points to storage suitable to hold an object of type U and a2 of type A2 is a rebound copy of the allocator a passed to allocate_shared such that its value_type is remove_cv_t.
(7.8)
When a (sub)object of non-array type U is initialized by make_shared_for_overwrite or
allocate_shared_for_overwrite, it is initialized via the expression ::new(pv) U, where pv has type void* and points to storage suitable to hold an object of type U.
(7.9)
Array elements are initialized in ascending order of their addresses.
(7.10)
When the lifetime of the object managed by the return value ends, or when the initialization of an array element throws an exception, the initialized elements are destroyed in the reverse order of their original construction.
(7.11)
When a (sub)object of non-array type U that was initialized by make_shared is to be destroyed, it is destroyed via the expression pv->~U() where pv points to that object of type U.
(7.12)
When a (sub)object of non-array type U that was initialized by allocate_shared is to be destroyed, it is destroyed via the expression allocator_traits<A2>::destroy(a2, pv) where pv points to that object of type remove_cv_t and a2 of type A2 is a rebound copy of the allocator a passed to allocate_shared such that its value_type is remove_cv_t.

[Note 2:

These functions will typically allocate more memory than sizeof(T) to allow for internal bookkeeping structures such as reference counts.

— end note]

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template<class T, class... Args>
  shared_ptr<T> make_shared(Args&&... args);                    // T is not array
template<class T, class A, class... Args>
  shared_ptr<T> allocate_shared(const A& a, Args&&... args);    // T is not array

Constraints: T is not an array type.

Returns: A shared_ptr to an object of type T with an initial value T(std::forward<Args>(args)...).

Remarks: The shared_ptr constructors called by these functions enable shared_from_this with the address of the newly constructed object of type T.

[Example 1: shared_ptr<int> p = make_shared<int>(); // shared_ptr to int() shared_ptr<vector<int>> q = make_shared<vector<int>>(16, 1); // shared_ptr to vector of 16 elements with value 1 — end example]

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template<class T> shared_ptr<T>
  make_shared(size_t N);                                        // T is U[]
template<class T, class A>
  shared_ptr<T> allocate_shared(const A& a, size_t N);          // T is U[]

Constraints: T is of the form U[].

Returns: A shared_ptr to an object of type U[N] with a default initial value, where U is remove_extent_t<T>.

[Example 2: shared_ptr<double[]> p = make_shared<double[]>(1024); // shared_ptr to a value-initialized double[1024] shared_ptr<double[][2][2]> q = make_shared<double[][2][2]>(6); // shared_ptr to a value-initialized double[6][2][2] — end example]

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template<class T>
  shared_ptr<T> make_shared();                                  // T is U[N]
template<class T, class A>
  shared_ptr<T> allocate_shared(const A& a);                    // T is U[N]

Constraints: T is of the form U[N].

Returns: A shared_ptr to an object of type T with a default initial value.

[Example 3: shared_ptr<double[1024]> p = make_shared<double[1024]>(); // shared_ptr to a value-initialized double[1024] shared_ptr<double[6][2][2]> q = make_shared<double[6][2][2]>(); // shared_ptr to a value-initialized double[6][2][2] — end example]

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template<class T>
  shared_ptr<T> make_shared(size_t N,
                            const remove_extent_t<T>& u);       // T is U[]
template<class T, class A>
  shared_ptr<T> allocate_shared(const A& a, size_t N,
                                const remove_extent_t<T>& u);   // T is U[]

Constraints: T is of the form U[].

Returns: A shared_ptr to an object of type U[N], where U is remove_extent_t<T> and each array element has an initial value of u.

[Example 4: shared_ptr<double[]> p = make_shared<double[]>(1024, 1.0); // shared_ptr to a double[1024], where each element is 1.0 shared_ptr<double[][2]> q = make_shared<double[][2]>(6, {1.0, 0.0}); // shared_ptr to a double[6][2], where each double[2] element is {1.0, 0.0} shared_ptr<vector<int>[]> r = make_shared<vector<int>[]>(4, {1, 2}); // shared_ptr to a vector<int>[4], where each vector has contents {1, 2} — end example]

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template<class T>
  shared_ptr<T> make_shared(const remove_extent_t<T>& u);       // T is U[N]
template<class T, class A>
  shared_ptr<T> allocate_shared(const A& a,
                                const remove_extent_t<T>& u);   // T is U[N]

Constraints: T is of the form U[N].

Returns: A shared_ptr to an object of type T, where each array element of type remove_extent_t<T> has an initial value of u.

[Example 5: shared_ptr<double[1024]> p = make_shared<double[1024]>(1.0); // shared_ptr to a double[1024], where each element is 1.0 shared_ptr<double[6][2]> q = make_shared<double[6][2]>({1.0, 0.0}); // shared_ptr to a double[6][2], where each double[2] element is {1.0, 0.0} shared_ptr<vector<int>[4]> r = make_shared<vector<int>[4]>({1, 2}); // shared_ptr to a vector<int>[4], where each vector has contents {1, 2} — end example]

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template<class T>
  shared_ptr<T> make_shared_for_overwrite();
template<class T, class A>
  shared_ptr<T> allocate_shared_for_overwrite(const A& a);

Constraints: T is not an array of unknown bound.

Returns: A shared_ptr to an object of type T.

[Example 6: struct X { double data[1024]; }; shared_ptr<X> p = make_shared_for_overwrite<X>(); // shared_ptr to a default-initialized X, where each element in X::data has an indeterminate value shared_ptr<double[1024]> q = make_shared_for_overwrite<double[1024]>(); // shared_ptr to a default-initialized double[1024], where each element has an indeterminate value — end example]

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template<class T>
  shared_ptr<T> make_shared_for_overwrite(size_t N);
template<class T, class A>
  shared_ptr<T> allocate_shared_for_overwrite(const A& a, size_t N);

Constraints: T is an array of unknown bound.

Returns: A shared_ptr to an object of type U[N], where U is remove_extent_t<T>.

[Example 7: shared_ptr<double[]> p = make_shared_for_overwrite<double[]>(1024); // shared_ptr to a default-initialized double[1024], where each element has an indeterminate value — end example]

20.3.2.2.8 Comparison [util.smartptr.shared.cmp]

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template<class T, class U>
  bool operator==(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;

Returns: a.get() == b.get().

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template<class T>
  bool operator==(const shared_ptr<T>& a, nullptr_t) noexcept;

Returns: !a.

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template<class T, class U>
  strong_ordering operator<=>(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;

Returns: compare_three_way()(a.get(), b.get()).

[Note 1:

Defining a comparison operator function allows shared_ptr objects to be used as keys in associative containers.

— end note]

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template<class T>
  strong_ordering operator<=>(const shared_ptr<T>& a, nullptr_t) noexcept;

Returns: compare_three_way()(a.get(), static_cast<typename shared_ptr<T>::element_type*>(nullptr))

20.3.2.2.9 Specialized algorithms [util.smartptr.shared.spec]

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template<class T>
  void swap(shared_ptr<T>& a, shared_ptr<T>& b) noexcept;

Effects: Equivalent to a.swap(b).

20.3.2.2.10 Casts [util.smartptr.shared.cast]

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template<class T, class U>
  shared_ptr<T> static_pointer_cast(const shared_ptr<U>& r) noexcept;
template<class T, class U>
  shared_ptr<T> static_pointer_cast(shared_ptr<U>&& r) noexcept;

Mandates: The expression static_cast<T*>((U*)nullptr) is well-formed.

Returns: shared_ptr<T>(R, static_cast<typename shared_ptr<T>::element_type*>(r.get())) where R is r for the first overload, and std::move(r) for the second.

[Note 1:

The seemingly equivalent expression shared_ptr<T>(static_cast<T*>(r.get())) will eventually result in undefined behavior, attempting to delete the same object twice.

— end note]

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template<class T, class U>
  shared_ptr<T> dynamic_pointer_cast(const shared_ptr<U>& r) noexcept;
template<class T, class U>
  shared_ptr<T> dynamic_pointer_cast(shared_ptr<U>&& r) noexcept;

Mandates: The expression dynamic_cast<T*>((U*)nullptr) is well-formed.

The expression dynamic_cast<typename shared_ptr<T>::element_type*>(r.get()) is well-formed.

Preconditions: The expression dynamic_cast<typename shared_ptr<T>::element_type*>(r.get()) has well-defined behavior.

Returns:

(6.1)
When dynamic_cast<typename shared_ptr<T>::element_type*>(r.get()) returns a non-null value p, shared_ptr<T>(R, p), where R is r for the first overload, and std::move(r) for the second.
(6.2)
Otherwise, shared_ptr<T>().

[Note 2:

The seemingly equivalent expression shared_ptr<T>(dynamic_cast<T*>(r.get())) will eventually result in undefined behavior, attempting to delete the same object twice.

— end note]

🔗

template<class T, class U>
  shared_ptr<T> const_pointer_cast(const shared_ptr<U>& r) noexcept;
template<class T, class U>
  shared_ptr<T> const_pointer_cast(shared_ptr<U>&& r) noexcept;

Mandates: The expression const_cast<T*>((U*)nullptr) is well-formed.

Returns: shared_ptr<T>(R, const_cast<typename shared_ptr<T>::element_type*>(r.get())) where R is r for the first overload, and std::move(r) for the second.

[Note 3:

The seemingly equivalent expression shared_ptr<T>(const_cast<T*>(r.get())) will eventually result in undefined behavior, attempting to delete the same object twice.

— end note]

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template<class T, class U>
  shared_ptr<T> reinterpret_pointer_cast(const shared_ptr<U>& r) noexcept;
template<class T, class U>
  shared_ptr<T> reinterpret_pointer_cast(shared_ptr<U>&& r) noexcept;

Mandates: The expression reinterpret_cast<T*>((U*)nullptr) is well-formed.

Returns: shared_ptr<T>(R, reinterpret_cast<typename shared_ptr<T>::element_type*>(r.get())) where R is r for the first overload, and std::move(r) for the second.

[Note 4:

The seemingly equivalent expression shared_ptr<T>(reinterpret_cast<T*>(r.get())) will eventually result in undefined behavior, attempting to delete the same object twice.

— end note]

20.3.2.2.11 get_deleter [util.smartptr.getdeleter]

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template<class D, class T>
  D* get_deleter(const shared_ptr<T>& p) noexcept;

Returns: If p owns a deleter d of type cv-unqualified D, returns addressof(d); otherwise returns nullptr.

The returned pointer remains valid as long as there exists a shared_ptr instance that owns d.

[Note 1:

It is unspecified whether the pointer remains valid longer than that.

This can happen if the implementation doesn't destroy the deleter until all weak_ptr instances that share ownership with p have been destroyed.

— end note]

20.3.2.2.12 I/O [util.smartptr.shared.io]

🔗

template<class E, class T, class Y>
  basic_ostream<E, T>& operator<<(basic_ostream<E, T>& os, const shared_ptr<Y>& p);

Effects: As if by: os << p.get();

Returns: os.

20.3.2.3 Class template weak_ptr [util.smartptr.weak]

20.3.2.3.1 General [util.smartptr.weak.general]

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 = remove_extent_t<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& b) const noexcept; template<class U> bool owner_before(const weak_ptr& b) const noexcept; size_t owner_hash() const noexcept; template<class U> bool owner_equal(const shared_ptr& b) const noexcept; template<class U> bool owner_equal(const weak_ptr& b) const noexcept; }; template<class T> weak_ptr(shared_ptr<T>) -> weak_ptr<T>; }

Specializations of weak_ptr shall be Cpp17CopyConstructible and Cpp17CopyAssignable, allowing their use in standard containers.

The template parameter T of weak_ptr may be an incomplete type.

20.3.2.3.2 Constructors [util.smartptr.weak.const]

🔗

constexpr weak_ptr() noexcept;

Effects: Constructs an empty weak_ptr object that stores a null pointer value.

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;

Constraints: For the second and third constructors, Y* is compatible with T*.

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().

🔗

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

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

Effects: Move constructs a weak_ptr instance from r.

Postconditions: *this contains the old value of r.

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

20.3.2.3.3 Destructor [util.smartptr.weak.dest]

🔗

~weak_ptr();

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

20.3.2.3.4 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).

Returns: *this.

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

🔗

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).

Returns: *this.

20.3.2.3.5 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).

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

Returns: An unspecified value such that

(4.1)
owner_before(b) defines a strict weak ordering as defined in [alg.sorting];
(4.2)
!owner_before(b) && !b.owner_before(*this) is true if and only if owner_equal(b) is true.

🔗

size_t owner_hash() const noexcept;

Returns: An unspecified value such that, for any object x where owner_equal(x) is true, owner_hash() == x.owner_hash() is true.

🔗

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 and only if *this and b share ownership or are both empty.

Otherwise returns false.

Remarks: owner_equal is an equivalence relation.

20.3.2.3.7 Specialized algorithms [util.smartptr.weak.spec]

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template<class T>
  void swap(weak_ptr<T>& a, weak_ptr<T>& b) noexcept;

Effects: Equivalent to a.swap(b).

20.3.2.4 Class template owner_less [util.smartptr.ownerless]

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

🔗

namespace std { template<class T = void> struct owner_less; template<class T> struct owner_less<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_less<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_less<void> { template<class T, class U> bool operator()(const shared_ptr<T>&, const shared_ptr&) const noexcept; template<class T, class U> bool operator()(const shared_ptr<T>&, const weak_ptr&) const noexcept; template<class T, class U> bool operator()(const weak_ptr<T>&, const shared_ptr&) const noexcept; template<class T, class U> bool operator()(const weak_ptr<T>&, const weak_ptr&) const noexcept; using is_transparent = unspecified; }; }

operator()(x, y) returns x.owner_before(y).

[Note 1:

Note that

(2.1)
operator() defines a strict weak ordering as defined in [alg.sorting];
(2.2)
!operator()(a, b) && !operator()(b, a) is true if and only if a.owner_equal(b) is true.

— end note]

20.3.2.5 Struct owner_hash [util.smartptr.owner.hash]

The class owner_hash provides ownership-based hashing.

🔗

namespace std { struct owner_hash { template<class T> size_t operator()(const shared_ptr<T>&) const noexcept; template<class T> size_t operator()(const weak_ptr<T>&) const noexcept; using is_transparent = unspecified; }; }

🔗

template<class T>
  size_t operator()(const shared_ptr<T>& x) const noexcept;
template<class T>
  size_t operator()(const weak_ptr<T>& x) const noexcept;

Returns: x.owner_hash().

[Note 1:

For any object y where x.owner_equal(y) is true, x.owner_hash() == y.owner_hash() is true.

— end note]

20.3.2.6 Struct owner_equal [util.smartptr.owner.equal]

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

🔗

namespace std { struct owner_equal { template<class T, class U> bool operator()(const shared_ptr<T>&, const shared_ptr&) const noexcept; template<class T, class U> bool operator()(const shared_ptr<T>&, const weak_ptr&) const noexcept; template<class T, class U> bool operator()(const weak_ptr<T>&, const shared_ptr&) const noexcept; template<class T, class U> bool operator()(const weak_ptr<T>&, const weak_ptr&) const noexcept; using is_transparent = unspecified; }; }

🔗

template<class T, class U>
  bool operator()(const shared_ptr<T>& x, const shared_ptr<U>& y) const noexcept;
template<class T, class U>
  bool operator()(const shared_ptr<T>& x, const weak_ptr<U>& y) const noexcept;
template<class T, class U>
  bool operator()(const weak_ptr<T>& x, const shared_ptr<U>& y) const noexcept;
template<class T, class U>
  bool operator()(const weak_ptr<T>& x, const weak_ptr<U>& y) const noexcept;

Returns: x.owner_equal(y).

[Note 1:

x.owner_equal(y) is true if and only if x and y share ownership or are both empty.

— end note]

20.3.2.7 Class template enable_shared_from_this [util.smartptr.enab]

A class T can inherit from enable_shared_from_this<T> to inherit the shared_from_this member functions that obtain a shared_ptr instance pointing to *this.

[Example 1: struct X: public enable_shared_from_this<X> { }; int main() { shared_ptr<X> p(new X); shared_ptr<X> q = p->shared_from_this(); assert(p == q); assert(p.owner_equal(q)); // p and q share ownership } — end example]

namespace std { template<class T> class enable_shared_from_this { protected: constexpr enable_shared_from_this() noexcept; enable_shared_from_this(const enable_shared_from_this&) noexcept; enable_shared_from_this& operator=(const enable_shared_from_this&) noexcept; ~enable_shared_from_this(); public: shared_ptr<T> shared_from_this(); shared_ptr<T const> shared_from_this() const; weak_ptr<T> weak_from_this() noexcept; weak_ptr<T const> weak_from_this() const noexcept; private: mutable weak_ptr<T> weak_this; // exposition only }; }

The template parameter T of enable_shared_from_this may be an incomplete type.

🔗

constexpr enable_shared_from_this() noexcept;
enable_shared_from_this(const enable_shared_from_this<T>&) noexcept;

Effects: Value-initializes weak_this.

🔗

enable_shared_from_this<T>& operator=(const enable_shared_from_this<T>&) noexcept;

Returns: *this.

[Note 1:

weak_this is not changed.

— end note]

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shared_ptr<T>       shared_from_this();
shared_ptr<T const> shared_from_this() const;

Returns: shared_ptr<T>(weak_this).

🔗

weak_ptr<T>       weak_from_this() noexcept;
weak_ptr<T const> weak_from_this() const noexcept;

Returns: weak_this.