std:: atomic_ref

Defined in header <atomic>
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_ref s 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 .

Contents 1 Nested types 2 Data members 3 Member functions 3.1 Provided only when T is an arithmetic type other than cv bool or a pointer-to-object type 3.2 Provided only when T is an integral type other than cv bool or a pointer-to-object type 3.3 Provided only when T is an integral type other than cv bool 4 Specializations 5 Notes 6 Defect reports 7 See also

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.

Data members

Member	Description
T * ptr	the pointer to the referenced object ( exposition-only member object* )
is_always_lock_free [static]	indicates that the type is always lock-free (public static member constant)
required_alignment [static]	indicates the required alignment of an object to be referenced by atomic_ref (public static member constant)

Member functions

(constructor)	constructs an atomic_ref object (public member function)
operator=	stores a value into the object referenced by an atomic_ref object (public member function)
is_lock_free	checks if the atomic_ref object is lock-free (public member function)
store	atomically replaces the value of the referenced object with a non-atomic argument (public member function)
load	atomically obtains the value of the referenced object (public member function)
operator value_type	loads a value from the referenced object (public member function)
exchange	atomically replaces the value of the referenced object and obtains the value held previously (public member function)
compare_exchange_weak compare_exchange_strong	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)
wait	blocks the thread until notified and the atomic value changes (public member function)
notify_one	notifies at least one thread waiting on the atomic object (public member function)
notify_all	notifies all threads blocked waiting on the atomic object (public member function)
address (C++26)	returns the object's address (public member function)
Provided only when T is an arithmetic type other than cv bool or a pointer-to-object type
fetch_add	atomically adds the argument to the value stored in the referenced object and obtains the value held previously (public member function)
fetch_sub	atomically subtracts the argument from the value stored in the referenced object and obtains the value held previously (public member function)
operator+= operator-=	atomically adds to or subtracts from the referenced value (public member function)
Provided only when T is an integral type other than cv bool or a pointer-to-object type
fetch_max (C++26)	atomically performs std::max between the argument and the value of the referenced object and obtains the value held previously (public member function)
fetch_min (C++26)	atomically performs std::min between the argument and the value of the referenced object and obtains the value held previously (public member function)
operator++ operator++ (int) operator-- operator-- (int)	atomically increments or decrements the referenced object by one (public member function)
Provided only when T is an integral type other than cv bool
fetch_and	atomically performs bitwise AND between the argument and the value of the referenced object and obtains the value held previously (public member function)
fetch_or	atomically performs bitwise OR between the argument and the value of the referenced object and obtains the value held previously (public member function)
fetch_xor	atomically performs bitwise XOR between the argument and the value of the referenced object and obtains the value held previously (public member function)
operator&= operator|= operator^=	atomically performs bitwise AND, OR, XOR with the referenced value (public member function)

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)

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) .

Feature-test macro	Value	Std	Feature
__cpp_lib_atomic_ref	201806L	(C++20)	std::atomic_ref
__cpp_lib_constexpr_atomic	202411L	(C++26)	constexpr std::atomic and std::atomic_ref

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

See also

atomic (C++11)

atomic class template and specializations for bool, integral, floating-point, (since C++20) and pointer types (class template)