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std::atomic_ref - cppreference.com

template< class T > struct atomic_ref;

(since C++20)

The std::atomic_ref class template applies atomic operations to the object it references.

For the lifetime of the std::atomic_ref object, the object it references is considered an atomic object. If one thread writes to an atomic object while another thread reads from it, the behavior is well-defined (see memory model for details on data races). In addition, accesses to atomic objects may establish inter-thread synchronization and order non-atomic memory accesses as specified by std::memory_order.

The lifetime of an object must exceed the lifetime of all std::atomic_refs that references the object. While any std::atomic_ref instance referencing an object exists, the object must be exclusively accessed through these std::atomic_ref instances. No subobject of an object referenced by an std::atomic_ref object may be concurrently referenced by any other std::atomic_ref object.

Atomic operations applied to an object through an std::atomic_ref are atomic with respect to atomic operations applied through any other std::atomic_ref referencing the same object.

Like references in the core language, constness is shallow for std::atomic_ref - it is possible to modify the referenced value through a const std::atomic_ref object.

If any of the following conditions are satisfied, the program is ill-formed:

std::is_trivially_copyable_v<T> is false.
is_always_lock_free is false and std::is_volatile_v<T> is true.

std::atomic_ref is CopyConstructible.

[edit] Nested types Type Definition value_type std::remove_cv_t<T> difference_type

value_type, if T is an arithmetic type other than cv bool.
Otherwise, std::ptrdiff_t, if T is a pointer-to-object type.
Otherwise, not defined.

[edit] Data members Member Description T* ptr the pointer to the referenced object
(exposition-only member object*) indicates that the type is always lock-free
(public static member constant) [edit] indicates the required alignment of an object to be referenced by atomic_ref
(public static member constant) [edit] [edit] Member functions constructs an atomic_ref object
(public member function) [edit] stores a value into the object referenced by an atomic_ref object
(public member function) [edit] checks if the atomic_ref object is lock-free
(public member function) [edit] atomically replaces the value of the referenced object with a non-atomic argument
(public member function) [edit] atomically obtains the value of the referenced object
(public member function) [edit] loads a value from the referenced object
(public member function) [edit] atomically replaces the value of the referenced object and obtains the value held previously
(public member function) [edit] atomically compares the value of the referenced object with non-atomic argument and performs atomic exchange if equal or atomic load if not
(public member function) [edit] blocks the thread until notified and the atomic value changes
(public member function) [edit] notifies at least one thread waiting on the atomic object
(public member function) [edit] notifies all threads blocked waiting on the atomic object
(public member function) [edit] returns the object's address
(public member function) [edit] Provided only when T is an arithmetic type other than cv bool or a pointer-to-object type atomically adds the argument to the value stored in the referenced object and obtains the value held previously
(public member function) [edit] atomically subtracts the argument from the value stored in the referenced object and obtains the value held previously
(public member function) [edit] atomically adds to or subtracts from the referenced value
(public member function) [edit] Provided only when T is an integral type other than cv bool or a pointer-to-object type atomically performs std::max between the argument and the value of the referenced object and obtains the value held previously
(public member function) [edit] atomically performs std::min between the argument and the value of the referenced object and obtains the value held previously
(public member function) [edit] atomically increments or decrements the referenced object by one
(public member function) [edit] Provided only when T is an integral type other than cv bool atomically performs bitwise AND between the argument and the value of the referenced object and obtains the value held previously
(public member function) [edit] atomically performs bitwise OR between the argument and the value of the referenced object and obtains the value held previously
(public member function) [edit] atomically performs bitwise XOR between the argument and the value of the referenced object and obtains the value held previously
(public member function) [edit] atomically performs bitwise AND, OR, XOR with the referenced value
(public member function) [edit] [edit] Specializations

The standard specifies that std::atomic_ref has following specializations:

template<> struct atomic_ref</*integral-type*/>;

(1) (since C++20)

template<> struct atomic_ref</*floating-point-type*/>;

(2) (since C++20) template< class /*pointer-type*/ > requires /* see below */ struct atomic_ref</*pointer-type*/>; (3) (since C++20)

1) /*integral-type*/ denotes a possibly cv-qualified integral type other than cv bool.

2) /*floating-point-type*/ denotes a possibly cv-qualified floating-point type.

3) The partial specialization is provided for /*pointer-type*/ types that are possibly cv-qualified pointer-to-object types.

[edit] Notes

Implementations may merge the specified specializations. E.g. MSVC STL merges all of them into the primary template.

When T is cv void or a function type, std::atomic_ref<T*> (i.e. std::atomic_ref<void*>, std::atomic_ref<int(*)()> etc.) does not have difference_type or any operation requiring pointer arithmetic or relational comparison(since C++26).

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior LWG 3508
(P3323R1) C++20 atomic_ref<T> had unimplementable operations
if T is a const type or pointer-to-non-object type these operations are either constained
or not provided for unsuitable T [edit] See also atomic class template and specializations for bool, integral, floating-point,(since C++20) and pointer types
(class template) [edit]

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