20 General utilities library [utilities]

20.7 Smart pointers [smartptr]

20.7.2 Shared-ownership pointers [util.smartptr]

20.7.2.1 Class bad_weak_ptr [util.smartptr.weakptr]

namespace std {
  class bad_weak_ptr: public std::exception {
  public:
    bad_weak_ptr() noexcept;
  };
} // namespace std
1

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

bad_weak_ptr() noexcept;

2

Postconditions: what() returns "bad_weak_ptr".

20.7.2.2 Class template shared_ptr [util.smartptr.shared]

1

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 object is empty if it does not own a pointer.

namespace std {
  template<class T> class shared_ptr {
  public:
    typedef T element_type;

    // [util.smartptr.shared.const], constructors:
    constexpr shared_ptr() noexcept;
    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, T *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> shared_ptr(auto_ptr<Y>&& r);
    template <class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);
    constexpr shared_ptr(nullptr_t) : shared_ptr() { }

    // [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> shared_ptr& operator=(auto_ptr<Y>&& r);
    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:
    T* get() const noexcept;
    T& operator*() const noexcept;
    T* operator->() const noexcept;
    long use_count() const noexcept;
    bool unique() const noexcept;
    explicit operator bool() const noexcept;
    template<class U> bool owner_before(shared_ptr<U> const& b) const;
    template<class U> bool owner_before(weak_ptr<U> const& b) const;
  };

  // [util.smartptr.shared.create], shared_ptr creation
  template<class T, class... Args> shared_ptr<T> make_shared(Args&&... args);
  template<class T, class A, class... Args>
    shared_ptr<T> allocate_shared(const A& a, Args&&... args);

  // [util.smartptr.shared.cmp], shared_ptr comparisons:
  template<class T, class U>
    bool operator==(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
  template<class T, class U>
    bool operator!=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
  template<class T, class U>
    bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
  template<class T, class U>
    bool operator>(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
  template<class T, class U>
    bool operator<=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
  template<class T, class U>
    bool operator>=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;

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

  // [util.smartptr.shared.spec], shared_ptr specialized algorithms:
  template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>& b) noexcept;

  // [util.smartptr.shared.cast], shared_ptr casts:
  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> dynamic_pointer_cast(const shared_ptr<U>& r) noexcept;
  template<class T, class U>
    shared_ptr<T> const_pointer_cast(const shared_ptr<U>& r) noexcept;

  // [util.smartptr.getdeleter], shared_ptr get_deleter:
  template<class D, class T> D* get_deleter(const shared_ptr<T>& p) noexcept;

  // [util.smartptr.shared.io], shared_ptr I/O:
  template<class E, class T, class Y>
    basic_ostream<E, T>& operator<< (basic_ostream<E, T>& os, const shared_ptr<Y>& p);
} // namespace std
2

Specializations of shared_ptr shall be CopyConstructible, CopyAssignable, and LessThanComparable, allowing their use in standard containers. Specializations of shared_ptr shall be 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.

3

[ Example: 

if(shared_ptr<X> px = dynamic_pointer_cast<X>(py)) {
  // do something with px
}

 — end example ]

4

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.

20.7.2.2.1 shared_ptr constructors [util.smartptr.shared.const]

constexpr shared_ptr() noexcept;

1

Effects: Constructs an empty shared_ptr object.

2

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

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

3

Requires: p shall be convertible to T*. Y shall be a complete type. The expression delete p shall be well formed, shall have well defined behavior, and shall not throw exceptions.

4

Effects: Constructs a shared_ptr object that owns the pointer p.

5

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

6

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

7

Exception safety: If an exception is thrown, delete p is called.

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);

8

Requires: p shall be convertible to T*. D shall be CopyConstructible. The copy constructor and destructor of D shall not throw exceptions. The expression d(p) shall be well formed, shall have well defined behavior, and shall not throw exceptions. A shall be an allocator ([allocator.requirements]). The copy constructor and destructor of A shall not throw exceptions.

9

Effects: Constructs a shared_ptr object that owns the object p and the deleter d. The second and fourth constructors shall use a copy of a to allocate memory for internal use.

10

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

11

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

12

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

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

13

Effects: Constructs a shared_ptr instance that stores p and shares ownership with r.

14

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

15

[ Note: To avoid the possibility of a dangling pointer, the user of this constructor must ensure that p remains valid at least until the ownership group of r is destroyed.  — end note ]

16

[ Note: This constructor allows creation of an empty shared_ptr instance with a non-NULL stored pointer.  — end note ]

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

17

Requires: The second constructor shall not participate in the overload resolution unless Y* is implicitly convertible to T*.

18

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

19

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

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

20

Remark: The second constructor shall not participate in overload resolution unless Y* is convertible to T*.

21

Effects: Move-constructs a shared_ptr instance from r.

22

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

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

23

Requires: Y* shall be convertible to T*.

24

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

25

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

26

Throws: bad_weak_ptr when r.expired().

27

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

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

28

Requires: r.release() shall be convertible to T*. Y shall be a complete type. The expression delete r.release() shall be well formed, shall have well defined behavior, and shall not throw exceptions.

29

Effects: Constructs a shared_ptr object that stores and owns r.release().

30

Postconditions: use_count() == 1 && r.get() == 0.

31

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

32

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

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

33

Effects: Equivalent to shared_ptr(r.release(), r.get_deleter()) when D is not a reference type, otherwise shared_ptr(r.release(), ref(r.get_deleter())).

34

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

20.7.2.2.2 shared_ptr destructor [util.smartptr.shared.dest]

~shared_ptr();

1

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.

2

[ Note: 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 ]

20.7.2.2.3 shared_ptr assignment [util.smartptr.shared.assign]

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

1

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

2

Returns: *this.

3

[ Note: The use count updates caused by the temporary object construction and destruction are not observable side effects, so the implementation may 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 may be no-ops.  — end note ]

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

4

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

5

Returns: *this.

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

6

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

7

Returns: *this

20.7.2.2.4 shared_ptr modifiers [util.smartptr.shared.mod]

void swap(shared_ptr& r) noexcept;

1

Effects: Exchanges the contents of *this and r.

void reset() noexcept;

2

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

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

3

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

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

4

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

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

5

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

20.7.2.2.5 shared_ptr observers [util.smartptr.shared.obs]

T* get() const noexcept;

1

Returns: the stored pointer.

T& operator*() const noexcept;

2

Requires: get() != 0.

3

Returns: *get().

4

Remarks: When T is 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.

T* operator->() const noexcept;

5

Requires: get() != 0.

6

Returns: get().

long use_count() const noexcept;

7

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

8

[ Note: use_count() is not necessarily efficient. — end note ]

bool unique() const noexcept;

9

Returns: use_count() == 1.

10

[ Note: unique() may be faster than use_count(). If you are using unique() to implement copy on write, do not rely on a specific value when get() == 0.  — end note ]

explicit operator bool() const noexcept;

11

Returns: get() != 0.

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

12

Returns: An unspecified value such that

  • (12.1)

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

  • (12.2)

    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.

20.7.2.2.6 shared_ptr creation [util.smartptr.shared.create]

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

1

Requires: The expression ::new (pv) T(std::forward<Args>(args)...), where pv has type void* and points to storage suitable to hold an object of type T, shall be well formed. A shall be an allocator ([allocator.requirements]). The copy constructor and destructor of A shall not throw exceptions.

2

Effects: Allocates memory suitable for an object of type T and constructs an object in that memory via the placement new expression ::new (pv) T(std::forward<Args>(args)...). The template allocate_shared uses a copy of a to allocate memory. If an exception is thrown, the functions have no effect.

3

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

4

Postconditions: get() != 0 && use_count() == 1

5

Throws: bad_alloc, or an exception thrown from A::allocate or from the constructor of T.

6

Remarks: Implementations are encouraged, but not required, to perform no more than one memory allocation. [ Note: This provides efficiency equivalent to an intrusive smart pointer.  — end note ]

7

[ Note: These functions will typically allocate more memory than sizeof(T) to allow for internal bookkeeping structures such as the reference counts.  — end note ]

20.7.2.2.7 shared_ptr comparison [util.smartptr.shared.cmp]

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

1

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

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

2

Returns: less<V>()(a.get(), b.get()), where V is the composite pointer type ([expr.rel]) of T* and U*.

3

[ Note: Defining a comparison operator allows shared_ptr objects to be used as keys in associative containers.  — end note ]

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

4

Returns: !a.

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

5

Returns: (bool)a.

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

6

Returns: The first function template returns less<T*>()(a.get(), nullptr). The second function template returns less<T*>()(nullptr, a.get()).

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

7

Returns: The first function template returns nullptr < a. The second function template returns a < nullptr.

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

8

Returns: The first function template returns !(nullptr < a). The second function template returns !(a < nullptr).

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

9

Returns: The first function template returns !(a < nullptr). The second function template returns !(nullptr < a).

20.7.2.2.8 shared_ptr specialized algorithms [util.smartptr.shared.spec]

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

1

Effects: Equivalent to a.swap(b).

20.7.2.2.9 shared_ptr casts [util.smartptr.shared.cast]

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

1

Requires: The expression static_cast<T*>(r.get()) shall be well formed.

2

Returns: If r is empty, an empty shared_ptr<T>; otherwise, a shared_ptr<T> object that stores static_cast<T*>(r.get()) and shares ownership with r.

3

Postconditions: w.get() == static_cast<T*>(r.get()) and w.use_count() == r.use_count(), where w is the return value.

4

[ Note: 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 ]

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

5

Requires: The expression dynamic_cast<T*>(r.get()) shall be well formed and shall have well defined behavior.

6

Returns:

  • (6.1)

    When dynamic_cast<T*>(r.get()) returns a nonzero value, a shared_ptr<T> object that stores a copy of it and shares ownership with r;

  • (6.2)

    Otherwise, an empty shared_ptr<T> object.

7

Postcondition: w.get() == dynamic_cast<T*>(r.get()), where w is the return value.

8

[ Note: 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;

9

Requires: The expression const_cast<T*>(r.get()) shall be well formed.

10

Returns: If r is empty, an empty shared_ptr<T>; otherwise, a shared_ptr<T> object that stores const_cast<T*>(r.get()) and shares ownership with r.

11

Postconditions: w.get() == const_cast<T*>(r.get()) and w.use_count() == r.use_count(), where w is the return value.

12

[ Note: 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 ]

20.7.2.2.10 get_deleter [util.smartptr.getdeleter]

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

1

Returns: If p owns a deleter d of type cv-unqualified D, returns &d; otherwise returns 0. The returned pointer remains valid as long as there exists a shared_ptr instance that owns d. [ Note: 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 ]

20.7.2.2.11 shared_ptr I/O [util.smartptr.shared.io]

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

1

Effects: os << p.get();.

2

Returns: os.

20.7.2.3 Class template weak_ptr [util.smartptr.weak]

1

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:
    typedef T element_type;

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

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

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

  // [util.smartptr.weak.spec], specialized algorithms
  template<class T> void swap(weak_ptr<T>& a, weak_ptr<T>& b) noexcept;
} // namespace std
2

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.

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

constexpr weak_ptr() noexcept;

1

Effects: Constructs an empty weak_ptr object.

2

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;

3

Requires: The second and third constructors shall not participate in the overload resolution unless Y* is implicitly convertible to T*.

4

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.

5

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

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

~weak_ptr();

1

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

20.7.2.3.3 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;

1

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

2

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

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

void swap(weak_ptr& r) noexcept;

1

Effects: Exchanges the contents of *this and r.

void reset() noexcept;

2

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

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

long use_count() const noexcept;

1

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

2

[ Note: use_count() is not necessarily efficient. — end note ]

bool expired() const noexcept;

3

Returns: use_count() == 0.

4

[ Note: expired() may be faster than use_count().  — end note ]

shared_ptr<T> lock() const noexcept;

5

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

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

6

Returns: An unspecified value such that

  • (6.1)

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

  • (6.2)

    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.

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

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

1

Effects: Equivalent to a.swap(b).

20.7.2.3.7 Class template owner_less [util.smartptr.ownerless]

1

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

namespace std {
  template<class T> struct owner_less;

  template<class T> struct owner_less<shared_ptr<T> > {
    typedef bool result_type;
    typedef shared_ptr<T> first_argument_type;
    typedef shared_ptr<T> second_argument_type;
    bool operator()(shared_ptr<T> const&, shared_ptr<T> const&) const;
    bool operator()(shared_ptr<T> const&, weak_ptr<T> const&) const;
    bool operator()(weak_ptr<T> const&, shared_ptr<T> const&) const;
  };

  template<class T> struct owner_less<weak_ptr<T> > {
    typedef bool result_type;
    typedef weak_ptr<T> first_argument_type;
    typedef weak_ptr<T> second_argument_type;
    bool operator()(weak_ptr<T> const&, weak_ptr<T> const&) const;
    bool operator()(shared_ptr<T> const&, weak_ptr<T> const&) const;
    bool operator()(weak_ptr<T> const&, shared_ptr<T> const&) const;
  };
}
2

operator()(x,y) shall return x.owner_before(y). [ Note: Note that

  • (2.1)

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

  • (2.2)

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

20.7.2.4 Class template enable_shared_from_this [util.smartptr.enab]

1

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.

2

[ Example:

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 < q ) && !(q < p)); // 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(enable_shared_from_this const&) noexcept;
    enable_shared_from_this& operator=(enable_shared_from_this const&) noexcept;
    ~enable_shared_from_this();
  public:
    shared_ptr<T> shared_from_this();
    shared_ptr<T const> shared_from_this() const;
  };
} // namespace std
3

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;

4

Effects: Constructs an enable_shared_from_this<T> object.

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

5

Returns: *this.

~enable_shared_from_this();

6

Effects: Destroys *this.

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

7

Requires: enable_shared_from_this<T> shall be an accessible base class of T. *this shall be a subobject of an object t of type T. There shall be at least one shared_ptr instance p that owns &t.

8

Returns: A shared_ptr<T> object r that shares ownership with p.

9

Postconditions: r.get() == this.

10

[ Note: A possible implementation is shown below: 

template<class T> class enable_shared_from_this {
private:
  weak_ptr<T> __weak_this;
protected:
  constexpr enable_shared_from_this() : __weak_this() { }
  enable_shared_from_this(enable_shared_from_this const &) { }
  enable_shared_from_this& operator=(enable_shared_from_this const &) { return *this; }
  ~enable_shared_from_this() { }
public:
  shared_ptr<T> shared_from_this() { return shared_ptr<T>(__weak_this); }
  shared_ptr<T const> shared_from_this() const { return shared_ptr<T const>(__weak_this); }
};
11

The shared_ptr constructors that create unique pointers can detect the presence of an enable_shared_from_this base and assign the newly created shared_ptr to its __weak_this member.  — end note ]

20.7.2.5 shared_ptr atomic access [util.smartptr.shared.atomic]

1

Concurrent access to a shared_ptr object from multiple threads does not introduce a data race if the access is done exclusively via the functions in this section and the instance is passed as their first argument.

2

The meaning of the arguments of type memory_order is explained in [atomics.order].

template<class T> bool atomic_is_lock_free(const shared_ptr<T>* p);

3

Requires: p shall not be null.

4

Returns: true if atomic access to *p is lock-free, false otherwise.

5

Throws: Nothing.

template<class T> shared_ptr<T> atomic_load(const shared_ptr<T>* p);

6

Requires: p shall not be null.

7

Returns: atomic_load_explicit(p, memory_order_seq_cst).

8

Throws: Nothing.

template<class T> shared_ptr<T> atomic_load_explicit(const shared_ptr<T>* p, memory_order mo);

9

Requires: p shall not be null.

10

Requires: mo shall not be memory_order_release or memory_order_acq_rel.

11

Returns: *p.

12

Throws: Nothing.

template<class T> void atomic_store(shared_ptr<T>* p, shared_ptr<T> r);

13

Requires: p shall not be null.

14

Effects: atomic_store_explicit(p, r, memory_order_seq_cst).

15

Throws: Nothing.

template<class T> void atomic_store_explicit(shared_ptr<T>* p, shared_ptr<T> r, memory_order mo);

16

Requires: p shall not be null.

17

Requires: mo shall not be memory_order_acquire or memory_order_acq_rel.

18

Effects: p->swap(r).

19

Throws: Nothing.

template<class T> shared_ptr<T> atomic_exchange(shared_ptr<T>* p, shared_ptr<T> r);

20

Requires: p shall not be null.

21

Returns: atomic_exchange_explicit(p, r, memory_order_seq_cst).

22

Throws: Nothing.

template<class T> shared_ptr<T> atomic_exchange_explicit(shared_ptr<T>* p, shared_ptr<T> r, memory_order mo);

23

Requires: p shall not be null.

24

Effects: p->swap(r).

25

Returns: The previous value of *p.

26

Throws: Nothing.

template<class T> bool atomic_compare_exchange_weak( shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w);

27

Requires: p shall not be null.

28

Returns: atomic_compare_exchange_weak_explicit(p, v, w, memory_order_seq_cst, memory_order_seq_cst).

29

Throws: Nothing.

template<class T> bool atomic_compare_exchange_strong( shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w);

30

Returns: atomic_compare_exchange_strong_explicit(p, v, w, memory_order_seq_cst, memory_order_seq_cst).

template<class T> bool atomic_compare_exchange_weak_explicit( shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w, memory_order success, memory_order failure); template<class T> bool atomic_compare_exchange_strong_explicit( shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w, memory_order success, memory_order failure);

31

Requires: p shall not be null.

32

Requires: failure shall not be memory_order_release, memory_order_acq_rel, or stronger than success.

33

Effects: If *p is equivalent to *v, assigns w to *p and has synchronization semantics corresponding to the value of success, otherwise assigns *p to *v and has synchronization semantics corresponding to the value of failure.

34

Returns: true if *p was equivalent to *v, false otherwise.

35

Throws: Nothing.

36

Remarks: two shared_ptr objects are equivalent if they store the same pointer value and share ownership.

37

Remarks: the weak forms may fail spuriously. See [atomics.types.operations].

20.7.2.6 Smart pointer hash support [util.smartptr.hash]

template <class T, class D> struct hash<unique_ptr<T, D> >;

1

Requires: The template specialization shall meet the requirements of class template hash ([unord.hash]). For an object p of type UP, where UP is unique_ptr<T, D>, hash<UP>()(p) shall evaluate to the same value as hash<typename UP::pointer>()(p.get()). The specialization hash<typename UP::pointer> shall be well-formed.

template <class T> struct hash<shared_ptr<T> >;

2

Requires: The template specialization shall meet the requirements of class template hash ([unord.hash]). For an object p of type shared_ptr<T>, hash<shared_ptr<T> >()(p) shall evaluate to the same value as hash<T*>()(p.get()).