# std::atomic

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< cpp‎ | atomic

Defined in header `<atomic>` |  |
template< class T > struct atomic; | (1) | (since C++11)
template< class U > struct atomic<U*>; | (2) | (since C++11)
Defined in header `<memory>` |  |
template< class U > struct atomic< std::shared_ptr <U>>; | (3) | (since C++20)
template< class U > struct atomic< std::weak_ptr <U>>; | (4) | (since C++20)
Defined in header `<stdatomic.h>` |  |
#define _Atomic(T) /* see below */ | (5) | (since C++23)

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.

The compatibility macro `_Atomic` is provided in `<stdatomic.h>` such that `_Atomic(T)` is identical to `std::atomic<T>` while both are well-formed. It is unspecified whether any declaration in namespace `std` is available when `<stdatomic.h>` is included. | (since C++23)

### Specializations

#### Primary template

The primary `std::atomic` template may be instantiated with any TriviallyCopyable type `T` satisfying both CopyConstructible and CopyAssignable. The program is ill-formed if any of following values is false:

- std::is_trivially_copyable <T>::value
- std::is_copy_constructible <T>::value
- std::is_move_constructible <T>::value
- std::is_copy_assignable <T>::value
- std::is_move_assignable <T>::value
- std::is_same <T, typename std::remove_cv <T>::type>::value

    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 and has a trivial destructor.

#### 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<U*>` for all pointer types. These specializations have standard layout, trivial default constructors,(until C++20) 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 <U>> and std::atomic< std::weak_ptr <U>> 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,(until C++20) 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 cv-unqualified floating-point types (float, double, long double and cv-unqualified extended floating-point types (since C++23)), `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 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)

### Member types

Type | Definition
`value_type` | `T` (regardless of whether specialized or not)

`difference_type`[1]

`value_type` (only for `atomic<Integral>` and `atomic<Floating>`(since C++20) specializations) |
std::ptrdiff_t (only for `std::atomic<U*>` specializations) |

1. ↑ `difference_type` is not defined in the primary `std::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)
wait (C++20) | blocks the thread until notified and the atomic value changes (public member function)
notify_one (C++20) | notifies at least one thread waiting on the atomic object (public member function)
notify_all (C++20) | notifies all threads blocked waiting on the atomic object (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

##### Specialized for integral, floating-point(since C++20) and pointer types

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)
operator+=operator-= | adds to or subtracts from the atomic value (public member function)

##### Specialized for integral and pointer types only

fetch_max (C++26) | atomically performs std::max between the argument and the value of the atomic 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 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)

##### Specialized for integral types only

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|=operator^= | performs bitwise AND, OR, XOR with the atomic value (public member function)

### Type aliases

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

##### Aliases for all `std::atomic<Integral>`

atomic_bool (C++11) | std::atomic<bool> (typedef)
atomic_char (C++11) | std::atomic<char> (typedef)
atomic_schar (C++11) | std::atomic<signed char> (typedef)
atomic_uchar (C++11) | std::atomic<unsigned char> (typedef)
atomic_short (C++11) | std::atomic<short> (typedef)
atomic_ushort (C++11) | std::atomic<unsigned short> (typedef)
atomic_int (C++11) | std::atomic<int> (typedef)
atomic_uint (C++11) | std::atomic<unsigned int> (typedef)
atomic_long (C++11) | std::atomic<long> (typedef)
atomic_ulong (C++11) | std::atomic<unsigned long> (typedef)
atomic_llong (C++11) | std::atomic<long long> (typedef)
atomic_ullong (C++11) | std::atomic<unsigned long long> (typedef)
atomic_char8_t (C++20) | std::atomic<char8_t> (typedef)
atomic_char16_t (C++11) | std::atomic<char16_t> (typedef)
atomic_char32_t (C++11) | std::atomic<char32_t> (typedef)
atomic_wchar_t (C++11) | std::atomic<wchar_t> (typedef)
atomic_int8_t (C++11)(optional) | std::atomic< std::int8_t > (typedef)
atomic_uint8_t (C++11)(optional) | std::atomic< std::uint8_t > (typedef)
atomic_int16_t (C++11)(optional) | std::atomic< std::int16_t > (typedef)
atomic_uint16_t (C++11)(optional) | std::atomic< std::uint16_t > (typedef)
atomic_int32_t (C++11)(optional) | std::atomic< std::int32_t > (typedef)
atomic_uint32_t (C++11)(optional) | std::atomic< std::uint32_t > (typedef)
atomic_int64_t (C++11)(optional) | std::atomic< std::int64_t > (typedef)
atomic_uint64_t (C++11)(optional) | std::atomic< std::uint64_t > (typedef)
atomic_int_least8_t (C++11) | std::atomic< std::int_least8_t > (typedef)
atomic_uint_least8_t (C++11) | std::atomic< std::uint_least8_t > (typedef)
atomic_int_least16_t (C++11) | std::atomic< std::int_least16_t > (typedef)
atomic_uint_least16_t (C++11) | std::atomic< std::uint_least16_t > (typedef)
atomic_int_least32_t (C++11) | std::atomic< std::int_least32_t > (typedef)
atomic_uint_least32_t (C++11) | std::atomic< std::uint_least32_t > (typedef)
atomic_int_least64_t (C++11) | std::atomic< std::int_least64_t > (typedef)
atomic_uint_least64_t (C++11) | std::atomic< std::uint_least64_t > (typedef)
atomic_int_fast8_t (C++11) | std::atomic< std::int_fast8_t > (typedef)
atomic_uint_fast8_t (C++11) | std::atomic< std::uint_fast8_t > (typedef)
atomic_int_fast16_t (C++11) | std::atomic< std::int_fast16_t > (typedef)
atomic_uint_fast16_t (C++11) | std::atomic< std::uint_fast16_t > (typedef)
atomic_int_fast32_t (C++11) | std::atomic< std::int_fast32_t > (typedef)
atomic_uint_fast32_t (C++11) | std::atomic< std::uint_fast32_t > (typedef)
atomic_int_fast64_t (C++11) | std::atomic< std::int_fast64_t > (typedef)
atomic_uint_fast64_t (C++11) | std::atomic< std::uint_fast64_t > (typedef)
atomic_intptr_t (C++11)(optional) | std::atomic< std::intptr_t > (typedef)
atomic_uintptr_t (C++11)(optional) | std::atomic< std::uintptr_t > (typedef)
atomic_size_t (C++11) | std::atomic< std::size_t > (typedef)
atomic_ptrdiff_t (C++11) | std::atomic< std::ptrdiff_t > (typedef)
atomic_intmax_t (C++11) | std::atomic< std::intmax_t > (typedef)
atomic_uintmax_t (C++11) | std::atomic< std::uintmax_t > (typedef)

##### Aliases for special-purpose types

atomic_signed_lock_free (C++20) | a signed integral atomic type that is lock-free and for which waiting/notifying is most efficient (typedef)
atomic_unsigned_lock_free (C++20) | an unsigned integral atomic type that is lock-free and for which waiting/notifying is most efficient (typedef)

Note: `std::atomic_intN_t`, `std::atomic_uintN_t`, `std::atomic_intptr_t`, and `std::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.

`std::atomic_signed_lock_free` and `std::atomic_unsigned_lock_free` are optional in freestanding implementations. | (since C++20)

### 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 <U>.

`_Atomic` is a keyword and used to provide atomic types in C.

Implementations are recommended to ensure that the representation of `_Atomic(T)` in C is same as that of `std::atomic<T>` in C++ for every possible type `T`. The mechanisms used to ensure atomicity and memory ordering should be compatible.

On GCC and Clang, some of the functionality described here requires linking against `-latomic`.

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

### Example

Run this code

    #include <atomic>
    #include <iostream>
    #include <thread>
    #include <vector>

    std::atomic_int acnt;
    int cnt;

    void f()
    {
        for (auto n{10000}; n; --n)
        {
            ++acnt;
            ++cnt;
            // Note: for this example, relaxed memory order is sufficient,
            // e.g. acnt.fetch_add(1, std::memory_order_relaxed);
        }
    }

    int main()
    {
        {
            std::vector<std::jthread> pool;
            for (int n = 0; n < 10; ++n)
                pool.emplace_back(f);
        }

        std::cout << "The atomic counter is " << acnt << '\n'
                  << "The non-atomic counter is " << cnt << '\n';
    }

Possible output:

    The atomic counter is 100000
    The non-atomic counter is 69696

### 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 | typedefs for atomic versions of optional fixed width integer types were missing | added
LWG 3012 | C++11 | `std::atomic<T>` was permitted for any `T` that is trivially copyable but not copyable | such specializations are forbidden
LWG 3949 | C++17 | the wording requiring std::atomic<bool> to have a trivial destructor was accidently dropped in C++17 | added back
LWG 4069 (P3323R1) | C++11 | support for cv-qualified `T` was questionable | disallow `T` being cv-qualified
P0558R1 | C++11 | template argument deduction for some functions for atomic types might accidently fail; invalid pointer operations were provided | specification was substantially rewritten: member typedefs `value_type` and `difference_type` are added

### See also

atomic_flag (C++11) | the lock-free boolean atomic type (class)
std::atomic<std::shared_ptr> (C++20) | atomic shared pointer (class template specialization)
std::atomic<std::weak_ptr> (C++20) | atomic weak pointer (class template specialization)
C documentation for Atomic types

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