std::atomic

Defined in header `<atomic>`
template< class T > struct atomic;	(1)	(since C++11)
template< class T > struct atomic<T*>;	(2)	(since C++11)
Defined in header `<memory>`
template<class T> struct atomic<std::shared_ptr<T>>;	(3)	(since C++20)
template<class T> struct atomic<std::weak_ptr<T>>;	(4)	(since C++20)

Each instantiation and full specialization of the std::atomic template defines an atomic type. 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.

std::atomic is neither copyable nor movable.

Specializations

Primary template

The primary std::atomic template may be instantiated with any TriviallyCopyable type T:

struct Counters { int a; int b; }; // user-defined trivially-copyable type
std::atomic<Counters> cnt;         // specialization for the user-defined type

std::atomic<bool> uses the primary template. It is guaranteed to be a standard layout struct.

Partial specializations

The standard library provides partial specializations of the std::atomic template for the following types with additional properties that the primary template does not have:

2) Partial specializations std::atomic<T*> for all pointer types. These specializations have standard layout, trivial default constructors, and trivial destructors. Besides the operations provided for all atomic types, these specializations additionally support atomic arithmetic operations appropriate to pointer types, such as fetch_add, fetch_sub.

3-4) Partial specializations std::atomic<std::shared_ptr<T>> and std::atomic<std::weak_ptr<T>> are provided for std::shared_ptr and std::weak_ptr.

See std::atomic<std::shared_ptr> and std::atomic<std::weak_ptr> for details.

(since C++20)

Specializations for integral types

When instantiated with one of the following integral types, std::atomic provides additional atomic operations appropriate to integral types such as fetch_add, fetch_sub, fetch_and, fetch_or, fetch_xor:

The character types char, char8_t (since C++20), char16_t, char32_t, and wchar_t;
The standard signed integer types: signed char, short, int, long, and long long;
The standard unsigned integer types: unsigned char, unsigned short, unsigned int, unsigned long, and unsigned long long;
Any additional integral types needed by the typedefs in the header <cstdint>.

Additionally, the resulting std::atomic<Integral> specialization has standard layout, a trivial default constructor, and a trivial destructor. Signed integer arithmetic is defined to use two's complement; there are no undefined results.

Specializations for floating-point types

When instantiated with one of the floating-point types float, double, and long double, std::atomic provides additional atomic operations appropriate to floating-point types such as fetch_add and fetch_sub.

Additionally, the resulting std::atomic<Floating> specialization has standard layout, a trivial default constructor, and a trivial destructor.

No operations result in undefined behavior even if the result is not representable in the floating-point type. The floating-point environment in effect may be different from the calling thread's floating-point environment.

(since C++20)

Type aliases

Type aliases are provided for bool and all integral types listed above, as follows:

Type alias	Definition
std::atomic_bool	std::atomic<bool>
std::atomic_char	std::atomic<char>
std::atomic_schar	std::atomic<signed char>
std::atomic_uchar	std::atomic<unsigned char>
std::atomic_short	std::atomic<short>
std::atomic_ushort	std::atomic<unsigned short>
std::atomic_int	std::atomic<int>
std::atomic_uint	std::atomic<unsigned int>
std::atomic_long	std::atomic<long>
std::atomic_ulong	std::atomic<unsigned long>
std::atomic_llong	std::atomic<long long>
std::atomic_ullong	std::atomic<unsigned long long>
std::atomic_char8_t	std::atomic<char8_t> (C++20)
std::atomic_char16_t	std::atomic<char16_t>
std::atomic_char32_t	std::atomic<char32_t>
std::atomic_wchar_t	std::atomic<wchar_t>
std::atomic_int8_t	std::atomic<std::int8_t>
std::atomic_uint8_t	std::atomic<std::uint8_t>
std::atomic_int16_t	std::atomic<std::int16_t>
std::atomic_uint16_t	std::atomic<std::uint16_t>
std::atomic_int32_t	std::atomic<std::int32_t>
std::atomic_uint32_t	std::atomic<std::uint32_t>
std::atomic_int64_t	std::atomic<std::int64_t>
std::atomic_uint64_t	std::atomic<std::uint64_t>
std::atomic_int_least8_t	std::atomic<std::int_least8_t>
std::atomic_uint_least8_t	std::atomic<std::uint_least8_t>
std::atomic_int_least16_t	std::atomic<std::int_least16_t>
std::atomic_uint_least16_t	std::atomic<std::uint_least16_t>
std::atomic_int_least32_t	std::atomic<std::int_least32_t>
std::atomic_uint_least32_t	std::atomic<std::uint_least32_t>
std::atomic_int_least64_t	std::atomic<std::int_least64_t>
std::atomic_uint_least64_t	std::atomic<std::uint_least64_t>
std::atomic_int_fast8_t	std::atomic<std::int_fast8_t>
std::atomic_uint_fast8_t	std::atomic<std::uint_fast8_t>
std::atomic_int_fast16_t	std::atomic<std::int_fast16_t>
std::atomic_uint_fast16_t	std::atomic<std::uint_fast16_t>
std::atomic_int_fast32_t	std::atomic<std::int_fast32_t>
std::atomic_uint_fast32_t	std::atomic<std::uint_fast32_t>
std::atomic_int_fast64_t	std::atomic<std::int_fast64_t>
std::atomic_uint_fast64_t	std::atomic<std::uint_fast64_t>
std::atomic_intptr_t	std::atomic<std::intptr_t>
std::atomic_uintptr_t	std::atomic<std::uintptr_t>
std::atomic_size_t	std::atomic<std::size_t>
std::atomic_ptrdiff_t	std::atomic<std::ptrdiff_t>
std::atomic_intmax_t	std::atomic<std::intmax_t>
std::atomic_uintmax_t	std::atomic<std::uintmax_t>

Note: std::atomic_intN_t, std::atomic_uintN_t, std::atomic_intptr_t, and atomic_uintptr_t are defined if and only if std::intN_t, std::uintN_t, std::intptr_t, and std::uintptr_t are defined, respectively.

Member types

Member type	Definition
`value_type`	see below
`difference_type`	`value_type` (only for `atomic<Integral>` and `atomic<Floating>` (since C++20) specializations) std::ptrdiff_t (only for `atomic<T*>` specializations)

For every std::atomic<X> (whether or not specialized), std::atomic<X>::value_type is X.

difference_type is not defined in the primary atomic template or in the partial specializations for std::shared_ptr and std::weak_ptr.

Member functions

(constructor)	constructs an atomic object (public member function)
operator=	stores a value into an atomic object (public member function)
is_lock_free	checks if the atomic object is lock-free (public member function)
store	atomically replaces the value of the atomic object with a non-atomic argument (public member function)
load	atomically obtains the value of the atomic object (public member function)
operator T	loads a value from an atomic object (public member function)
exchange	atomically replaces the value of the atomic object and obtains the value held previously (public member function)
compare_exchange_weakcompare_exchange_strong	atomically compares the value of the atomic object with non-atomic argument and performs atomic exchange if equal or atomic load if not (public member function)
Constants
is_always_lock_free [static] (C++17)	indicates that the type is always lock-free (public static member constant)

Specialized member functions

fetch_add	atomically adds the argument to the value stored in the atomic object and obtains the value held previously (public member function)
fetch_sub	atomically subtracts the argument from the value stored in the atomic object and obtains the value held previously (public member function)
fetch_and	atomically performs bitwise AND between the argument and the value of the atomic object and obtains the value held previously (public member function)
fetch_or	atomically performs bitwise OR between the argument and the value of the atomic object and obtains the value held previously (public member function)
fetch_xor	atomically performs bitwise XOR between the argument and the value of the atomic object and obtains the value held previously (public member function)
operator++operator++(int)operator--operator--(int)	increments or decrements the atomic value by one (public member function)
operator+=operator-=operator&=operator\|=operator^=	adds, subtracts, or performs bitwise AND, OR, XOR with the atomic value (public member function)

Notes

There are non-member function template equivalents for all member functions of std::atomic. Those non-member functions may be additionally overloaded for types that are not specializations of std::atomic, but are able to guarantee atomicity. The only such type in the standard library is std::shared_ptr<T>.

On gcc and clang, some of the functionality described here requires linking against -latomic.

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 2441	C++11		added specializations for the (optional) fixed width integer types
P0558R1	C++11		specification was substantially rewritten to resolve numerous issues in particular, member typedefs value_type and difference_type are added

Language
Standard Library Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

Types
atomic (C++11)
atomic_ref (C++20)
Functions
atomic_storeatomic_store_explicit (C++11)(C++11)
atomic_loadatomic_load_explicit (C++11)(C++11)
atomic_exchangeatomic_exchange_explicit (C++11)(C++11)
atomic_compare_exchange_weakatomic_compare_exchange_weak_explicitatomic_compare_exchange_strongatomic_compare_exchange_strong_explicit (C++11)(C++11)(C++11)(C++11)
atomic_fetch_addatomic_fetch_add_explicit (C++11)(C++11)
atomic_fetch_subatomic_fetch_sub_explicit (C++11)(C++11)
atomic_fetch_andatomic_fetch_and_explicit (C++11)(C++11)
atomic_fetch_oratomic_fetch_or_explicit (C++11)(C++11)
atomic_fetch_xoratomic_fetch_xor_explicit (C++11)(C++11)
atomic_is_lock_free (C++11)
Atomic flags
atomic_flag (C++11)
atomic_flag_test_and_setatomic_flag_test_and_set_explicit (C++11)(C++11)
atomic_flag_clearatomic_flag_clear_explicit (C++11)(C++11)
Initialization
atomic_init (C++11)
ATOMIC_VAR_INIT (C++11)
ATOMIC_FLAG_INIT (C++11)
Memory ordering
memory_order (C++11)
kill_dependency (C++11)
atomic_thread_fence (C++11)
atomic_signal_fence (C++11)

Member functions
atomic::atomic
atomic::operator=
atomic::is_lock_free
atomic::store
atomic::load
atomic::operator T
atomic::exchange
atomic::compare_exchange_strongatomic::compare_exchange_weak
Constants
atomic::is_always_lock_free (C++17)
Specialized member functions
atomic::fetch_add
atomic::fetch_sub
atomic::fetch_and
atomic::fetch_or
atomic::fetch_xor
atomic::operator++atomic::operator++(int)atomic::operator--atomic::operator--(int)
atomic::operator+=atomic::operator-=atomic::operator&=atomic::operator\|=atomic::operator^=