32 Concurrency support library [thread] (original) (raw)

32.5 Atomic operations [atomics]

32.5.1 General [atomics.general]

Subclause [atomics] describes components for fine-grained atomic access.

This access is provided via operations on atomic objects.

32.5.3 Type aliases [atomics.alias]

The type aliases atomic_intN_t, atomic_uintN_t,atomic_intptr_t, and atomic_uintptr_tare defined if and only ifintN_t, uintN_t,intptr_t, and uintptr_tare defined, respectively.

The type aliasesatomic_signed_lock_free and atomic_unsigned_lock_freename specializations of atomicwhose template arguments are integral types, respectively signed and unsigned, and whose is_always_lock_free property is true.

[Note 1:

These aliases are optional in freestanding implementations ([compliance]).

— _end note_]

Implementations should choose for these aliases the integral specializations of atomicfor which the atomic waiting and notifying operations ([atomics.wait]) are most efficient.

32.5.4 Order and consistency [atomics.order]

namespace std { enum class memory_order : unspecified { relaxed = 0, acquire = 2, release = 3, acq_rel = 4, seq_cst = 5 };}

The enumeration memory_order specifies the detailed regular (non-atomic) memory synchronization order as defined in[intro.multithread] and may provide for operation ordering.

Its enumerated values and their meanings are as follows:

memory_order::relaxed: no operation orders memory.
memory_order::release, memory_order::acq_rel, andmemory_order::seq_cst: a store operation performs a release operation on the affected memory location.
memory_order::acquire, memory_order::acq_rel, andmemory_order::seq_cst: a load operation performs an acquire operation on the affected memory location.

[Note 1:

Atomic operations specifying memory_order::relaxed are relaxed with respect to memory ordering.

Implementations must still guarantee that any given atomic access to a particular atomic object be indivisible with respect to all other atomic accesses to that object.

— _end note_]

An atomic operation A that performs a release operation on an atomic object M synchronizes with an atomic operation B that performs an acquire operation on M and takes its value from any side effect in the release sequence headed by A.

An atomic operation A on some atomic object M iscoherence-ordered beforeanother atomic operation B on M if

A is a modification, andB reads the value stored by A, or
A precedes Bin the modification order of M, or
A and B are not the same atomic read-modify-write operation, and there exists an atomic modification X of Msuch that A reads the value stored by X andX precedes Bin the modification order of M, or
there exists an atomic modification X of Msuch that A is coherence-ordered before X andX is coherence-ordered before B.

There is a single total order Son all memory_order::seq_cst operations, including fences, that satisfies the following constraints.

First, if A and B arememory_order::seq_cst operations andA strongly happens before B, then A precedes B in S.

Second, for every pair of atomic operations A andB on an object M, where A is coherence-ordered before B, the following four conditions are required to be satisfied by S:

if A and B are bothmemory_order::seq_cst operations, then A precedes B in S; and
if A is a memory_order::seq_cst operation andB happens before a memory_order::seq_cst fence Y, then A precedes Y in S; and
if a memory_order::seq_cst fence Xhappens before A andB is a memory_order::seq_cst operation, then X precedes B in S; and
if a memory_order::seq_cst fence Xhappens before A andB happens before a memory_order::seq_cst fence Y, then X precedes Y in S.

[Note 2:

This definition ensures that S is consistent with the modification order of any atomic object M.

It also ensures that a memory_order::seq_cst load A of Mgets its value either from the last modification of Mthat precedes A in S or from some non-memory_order::seq_cst modification of Mthat does not happen before any modification of Mthat precedes A in S.

— _end note_]

[Note 3:

We do not require that S be consistent with “happens before” ([intro.races]).

This allows more efficient implementation of memory_order::acquire and memory_order::releaseon some machine architectures.

It can produce surprising results when these are mixed with memory_order::seq_cst accesses.

— _end note_]

[Note 4:

memory_order::seq_cst ensures sequential consistency only for a program that is free of data races and uses exclusively memory_order::seq_cst atomic operations.

Any use of weaker ordering will invalidate this guarantee unless extreme care is used.

In many cases, memory_order::seq_cst atomic operations are reorderable with respect to other atomic operations performed by the same thread.

— _end note_]

Implementations should ensure that no “out-of-thin-air” values are computed that circularly depend on their own computation.

[Note 5:

For example, with x and y initially zero,r1 = y.load(memory_order::relaxed); x.store(r1, memory_order::relaxed);

r2 = x.load(memory_order::relaxed); y.store(r2, memory_order::relaxed);this recommendation discourages producing r1 == r2 == 42, since the store of 42 to y is only possible if the store to x stores 42, which circularly depends on the store to y storing 42.

Note that without this restriction, such an execution is possible.

— _end note_]

[Note 6:

The recommendation similarly disallows r1 == r2 == 42 in the following example, with x and y again initially zero:

r1 = x.load(memory_order::relaxed);if (r1 == 42) y.store(42, memory_order::relaxed);

r2 = y.load(memory_order::relaxed);if (r2 == 42) x.store(42, memory_order::relaxed); — _end note_]

Atomic read-modify-write operations shall always read the last value (in the modification order) written before the write associated with the read-modify-write operation.

Recommended practice: The implementation should make atomic stores visible to atomic loads, and atomic loads should observe atomic stores, within a reasonable amount of time.

32.5.5 Lock-free property [atomics.lockfree]

#define ATOMIC_BOOL_LOCK_FREE unspecified #define ATOMIC_CHAR_LOCK_FREE unspecified #define ATOMIC_CHAR8_T_LOCK_FREE unspecified #define ATOMIC_CHAR16_T_LOCK_FREE unspecified #define ATOMIC_CHAR32_T_LOCK_FREE unspecified #define ATOMIC_WCHAR_T_LOCK_FREE unspecified #define ATOMIC_SHORT_LOCK_FREE unspecified #define ATOMIC_INT_LOCK_FREE unspecified #define ATOMIC_LONG_LOCK_FREE unspecified #define ATOMIC_LLONG_LOCK_FREE unspecified #define ATOMIC_POINTER_LOCK_FREE unspecified

The ATOMIC_..._LOCK_FREE macros indicate the lock-free property of the corresponding atomic types, with the signed and unsigned variants grouped together.

The properties also apply to the corresponding (partial) specializations of theatomic template.

A value of 0 indicates that the types are never lock-free.

A value of 1 indicates that the types are sometimes lock-free.

A value of 2 indicates that the types are always lock-free.

On a hosted implementation ([compliance]), at least one signed integral specialization of the atomic template, along with the specialization for the corresponding unsigned type ([basic.fundamental]), is always lock-free.

The functions atomic<T>::is_lock_free andatomic_is_lock_free ([atomics.types.operations]) indicate whether the object is lock-free.

In any given program execution, the result of the lock-free query is the same for all atomic objects of the same type.

Atomic operations that are not lock-free are considered to potentially block ([intro.progress]).

Recommended practice: Operations that are lock-free should also be address-free.295

The implementation of these operations should not depend on any per-process state.

[Note 1:

This restriction enables communication by memory that is mapped into a process more than once and by memory that is shared between two processes.

— _end note_]

32.5.6 Waiting and notifying [atomics.wait]

Atomic waiting operationsand atomic notifying operationsprovide a mechanism to wait for the value of an atomic object to change more efficiently than can be achieved with polling.

An atomic waiting operation may block until it is unblocked by an atomic notifying operation, according to each function's effects.

[Note 1:

Programs are not guaranteed to observe transient atomic values, an issue known as the A-B-A problem, resulting in continued blocking if a condition is only temporarily met.

— _end note_]

[Note 2:

The following functions are atomic waiting operations:

atomic<T>::wait,
atomic_flag::wait,
atomic_wait and atomic_wait_explicit,
atomic_flag_wait and atomic_flag_wait_explicit, and
atomic_ref<T>::wait.

— _end note_]

[Note 3:

The following functions are atomic notifying operations:

atomic<T>::notify_one and atomic<T>::notify_all,
atomic_flag::notify_one and atomic_flag::notify_all,
atomic_notify_one and atomic_notify_all,
atomic_flag_notify_one and atomic_flag_notify_all, and
atomic_ref<T>::notify_one and atomic_ref<T>::notify_all.

— _end note_]

A call to an atomic waiting operation on an atomic object Mis eligible to be unblockedby a call to an atomic notifying operation on Mif there exist side effects X and Y on M such that:

the atomic waiting operation has blocked after observing the result of X,
X precedes Y in the modification order of M, and
Y happens before the call to the atomic notifying operation.

32.5.7 Class template atomic_ref [atomics.ref.generic]

32.5.7.1 General [atomics.ref.generic.general]

namespace std { template<class T> struct atomic_ref { private: T* ptr; public: using value_type = remove_cv_t<T>;static constexpr size_t required_alignment = implementation-defined;static constexpr bool is_always_lock_free = implementation-defined;bool is_lock_free() const noexcept;constexpr explicit atomic_ref(T&);constexpr atomic_ref(const atomic_ref&) noexcept; atomic_ref& operator=(const atomic_ref&) = delete;constexpr void store(value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr value_type operator=(value_type) const noexcept;constexpr value_type load(memory_order = memory_order::seq_cst) const noexcept;constexpr operator value_type() const noexcept;constexpr value_type exchange(value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr bool compare_exchange_weak(value_type&, value_type, memory_order, memory_order) const noexcept;constexpr bool compare_exchange_strong(value_type&, value_type, memory_order, memory_order) const noexcept;constexpr bool compare_exchange_weak(value_type&, value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr bool compare_exchange_strong(value_type&, value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr void wait(value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr void notify_one() const noexcept;constexpr void notify_all() const noexcept;constexpr T* address() const noexcept;};}

An atomic_ref object applies atomic operations ([atomics.general]) to the object referenced by *ptr such that, for the lifetime ([basic.life]) of the atomic_ref object, the object referenced by *ptr is an atomic object ([intro.races]).

The program is ill-formed if is_trivially_copyable_v<T> is false.

The lifetime ([basic.life]) of an object referenced by *ptrshall exceed the lifetime of all atomic_refs that reference the object.

While any atomic_ref instances exist that reference the *ptr object, all accesses to that object shall exclusively occur through those atomic_ref instances.

No subobject of the object referenced by atomic_refshall be concurrently referenced by any other atomic_ref object.

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

[Note 1:

Atomic operations or the atomic_ref constructor can acquire a shared resource, such as a lock associated with the referenced object, to enable atomic operations to be applied to the referenced object.

— _end note_]

The program is ill-formed if is_always_lock_free is false andis_volatile_v<T> is true.

32.5.7.2 Operations [atomics.ref.ops]

static constexpr size_t required_alignment;

The alignment required for an object to be referenced by an atomic reference, which is at least alignof(T).

[Note 1:

Hardware could require an object referenced by an atomic_refto have stricter alignment ([basic.align]) than other objects of type T.

Further, whether operations on an atomic_refare lock-free could depend on the alignment of the referenced object.

For example, lock-free operations on std::complex<double>could be supported only if aligned to 2*alignof(double).

— _end note_]

static constexpr bool is_always_lock_free;

The static data member is_always_lock_free is trueif the atomic_ref type's operations are always lock-free, and false otherwise.

bool is_lock_free() const noexcept;

Returns: true if operations on all objects of the type atomic_ref<T>are lock-free,false otherwise.

constexpr atomic_ref(T& obj);

Preconditions: The referenced object is aligned to required_alignment.

Postconditions: *this references obj.

constexpr atomic_ref(const atomic_ref& ref) noexcept;

Postconditions: *this references the object referenced by ref.

constexpr void store(value_type desired, memory_order order = memory_order::seq_cst) const noexcept;

Constraints: is_const_v<T> is false.

Preconditions: order ismemory_order::relaxed,memory_order::release, ormemory_order::seq_cst.

Effects: Atomically replaces the value referenced by *ptrwith the value of desired.

Memory is affected according to the value of order.

constexpr value_type operator=(value_type desired) const noexcept;

Constraints: is_const_v<T> is false.

Effects: Equivalent to:store(desired);return desired;

constexpr value_type load(memory_order order = memory_order::seq_cst) const noexcept;

Preconditions: order ismemory_order::relaxed,memory_order::acquire, ormemory_order::seq_cst.

Effects: Memory is affected according to the value of order.

Returns: Atomically returns the value referenced by *ptr.

constexpr operator value_type() const noexcept;

Effects: Equivalent to: return load();

constexpr value_type exchange(value_type desired, memory_order order = memory_order::seq_cst) const noexcept;

Constraints: is_const_v<T> is false.

Effects: Atomically replaces the value referenced by *ptrwith desired.

Memory is affected according to the value of order.

This operation is an atomic read-modify-write operation ([intro.multithread]).

Returns: Atomically returns the value referenced by *ptrimmediately before the effects.

constexpr bool compare_exchange_weak(value_type& expected, value_type desired, memory_order success, memory_order failure) const noexcept;constexpr bool compare_exchange_strong(value_type& expected, value_type desired, memory_order success, memory_order failure) const noexcept;constexpr bool compare_exchange_weak(value_type& expected, value_type desired, memory_order order = memory_order::seq_cst) const noexcept;constexpr bool compare_exchange_strong(value_type& expected, value_type desired, memory_order order = memory_order::seq_cst) const noexcept;

Constraints: is_const_v<T> is false.

Preconditions: failure ismemory_order::relaxed,memory_order::acquire, ormemory_order::
seq_cst.

Effects: Retrieves the value in expected.

It then atomically compares the value representation of the value referenced by *ptr for equality with that previously retrieved from expected, and if true, replaces the value referenced by *ptrwith that in desired.

If and only if the comparison is true, memory is affected according to the value of success, and if the comparison is false, memory is affected according to the value of failure.

When only one memory_order argument is supplied, the value of success is order, and the value of failure is orderexcept that a value of memory_order::acq_rel shall be replaced by the value memory_order::acquire and a value of memory_order::release shall be replaced by the value memory_order::relaxed.

If and only if the comparison is false then, after the atomic operation, the value in expected is replaced by the value read from the value referenced by *ptrduring the atomic comparison.

If the operation returns true, these operations are atomic read-modify-write operations ([intro.races]) on the value referenced by *ptr.

Otherwise, these operations are atomic load operations on that memory.

Returns: The result of the comparison.

Remarks: A weak compare-and-exchange operation may fail spuriously.

That is, even when the contents of memory referred to by expected and ptr are equal, it may return false and store back to expected the same memory contents that were originally there.

[Note 2:

This spurious failure enables implementation of compare-and-exchange on a broader class of machines, e.g., load-locked store-conditional machines.

A consequence of spurious failure is that nearly all uses of weak compare-and-exchange will be in a loop.

When a compare-and-exchange is in a loop, the weak version will yield better performance on some platforms.

When a weak compare-and-exchange would require a loop and a strong one would not, the strong one is preferable.

— _end note_]

constexpr void wait(value_type old, memory_order order = memory_order::seq_cst) const noexcept;

Preconditions: order ismemory_order::relaxed,memory_order::acquire, ormemory_order::seq_cst.

Effects: Repeatedly performs the following steps, in order:

Evaluates load(order) and compares its value representation for equality against that of old.
If they compare unequal, returns.
Blocks until it is unblocked by an atomic notifying operation or is unblocked spuriously.

Remarks: This function is an atomic waiting operation ([atomics.wait]) on atomic object *ptr.

constexpr void notify_one() const noexcept;

Constraints: is_const_v<T> is false.

Effects: Unblocks the execution of at least one atomic waiting operation on *ptrthat is eligible to be unblocked ([atomics.wait]) by this call, if any such atomic waiting operations exist.

Remarks: This function is an atomic notifying operation ([atomics.wait]) on atomic object *ptr.

constexpr void notify_all() const noexcept;

Constraints: is_const_v<T> is false.

Effects: Unblocks the execution of all atomic waiting operations on *ptrthat are eligible to be unblocked ([atomics.wait]) by this call.

Remarks: This function is an atomic notifying operation ([atomics.wait]) on atomic object *ptr.

constexpr T* address() const noexcept;

32.5.7.3 Specializations for integral types [atomics.ref.int]

There are specializations of the atomic_ref class template for all integral types except cv bool.

For each such type integral-type, the specialization atomic_ref<_integral-type_> provides additional atomic operations appropriate to integral types.

[Note 1:

The specialization atomic_ref<bool>uses the primary template ([atomics.ref.generic]).

— _end note_]

The program is ill-formed if is_always_lock_free is false andis_volatile_v<T> is true.

namespace std { template<> struct atomic_ref<_integral-type_> { private: integral-type* ptr; public: using value_type = remove_cv_t<_integral-type_>;using difference_type = value_type;static constexpr size_t required_alignment = implementation-defined;static constexpr bool is_always_lock_free = implementation-defined;bool is_lock_free() const noexcept;constexpr explicit atomic_ref(integral-type&);constexpr atomic_ref(const atomic_ref&) noexcept; atomic_ref& operator=(const atomic_ref&) = delete;constexpr void store(value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr value_type operator=(value_type) const noexcept;constexpr value_type load(memory_order = memory_order::seq_cst) const noexcept;constexpr operator value_type() const noexcept;constexpr value_type exchange(value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr bool compare_exchange_weak(value_type&, value_type, memory_order, memory_order) const noexcept;constexpr bool compare_exchange_strong(value_type&, value_type, memory_order, memory_order) const noexcept;constexpr bool compare_exchange_weak(value_type&, value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr bool compare_exchange_strong(value_type&, value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr value_type fetch_add(value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr value_type fetch_sub(value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr value_type fetch_and(value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr value_type fetch_or(value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr value_type fetch_xor(value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr value_type fetch_max(value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr value_type fetch_min(value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr value_type operator++(int) const noexcept;constexpr value_type operator--(int) const noexcept;constexpr value_type operator++() const noexcept;constexpr value_type operator--() const noexcept;constexpr value_type operator+=(value_type) const noexcept;constexpr value_type operator-=(value_type) const noexcept;constexpr value_type operator&=(value_type) const noexcept;constexpr value_type operator|=(value_type) const noexcept;constexpr value_type operator^=(value_type) const noexcept;constexpr void wait(value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr void notify_one() const noexcept;constexpr void notify_all() const noexcept;constexpr integral-type* address() const noexcept;};}

Descriptions are provided below only for members that differ from the primary template.

The following operations perform arithmetic computations.

The correspondence among key, operator, and computation is specified in Table 153 .

constexpr value_type fetch_ _key_(value_type operand, memory_order order = memory_order::seq_cst) const noexcept;

Constraints: is_const_v<_integral-type_> is false.

Effects: Atomically replaces the value referenced by *ptr with the result of the computation applied to the value referenced by *ptrand the given operand.

Memory is affected according to the value of order.

These operations are atomic read-modify-write operations ([intro.races]).

Returns: Atomically, the value referenced by *ptrimmediately before the effects.

Remarks: Except for fetch_max and fetch_min, for signed integer types the result is as if the object value and parameters were converted to their corresponding unsigned types, the computation performed on those types, and the result converted back to the signed type.

[Note 2:

There are no undefined results arising from the computation.

— _end note_]

For fetch_max and fetch_min, the maximum and minimum computation is performed as if by max and min algorithms ([alg.min.max]), respectively, with the object value and the first parameter as the arguments.

constexpr value_type operator _op_=(value_type operand) const noexcept;

Constraints: is_const_v<_integral-type_> is false.

Effects: Equivalent to:return fetch_ key(operand) op operand;

32.5.7.4 Specializations for floating-point types [atomics.ref.float]

There are specializations of the atomic_ref class template for all floating-point types.

For each such type floating-point-type, the specialization atomic_ref<_floating-point_> provides additional atomic operations appropriate to floating-point types.

The program is ill-formed if is_always_lock_free is false andis_volatile_v<T> is true.

namespace std { template<> struct atomic_ref<_floating-point-type_> { private: floating-point-type* ptr; public: using value_type = remove_cv_t<_floating-point-type_>;using difference_type = value_type;static constexpr size_t required_alignment = implementation-defined;static constexpr bool is_always_lock_free = implementation-defined;bool is_lock_free() const noexcept;constexpr explicit atomic_ref(floating-point-type&);constexpr atomic_ref(const atomic_ref&) noexcept; atomic_ref& operator=(const atomic_ref&) = delete;constexpr void store(floating-point-type, memory_order = memory_order::seq_cst) const noexcept;constexpr value_type operator=(value_type) const noexcept;constexpr value_type load(memory_order = memory_order::seq_cst) const noexcept;constexpr operator floating-point-type() const noexcept;constexpr value_type exchange(floating-point-type, memory_order = memory_order::seq_cst) const noexcept;constexpr bool compare_exchange_weak(value_type&, floating-point-type, memory_order, memory_order) const noexcept;constexpr bool compare_exchange_strong(value_type&, floating-point-type, memory_order, memory_order) const noexcept;constexpr bool compare_exchange_weak(value_type&, floating-point-type, memory_order = memory_order::seq_cst) const noexcept;constexpr bool compare_exchange_strong(value_type&, floating-point-type, memory_order = memory_order::seq_cst) const noexcept;constexpr value_type fetch_add(floating-point-type, memory_order = memory_order::seq_cst) const noexcept;constexpr value_type fetch_sub(floating-point-type, memory_order = memory_order::seq_cst) const noexcept;constexpr value_type operator+=(value_type) const noexcept;constexpr value_type operator-=(value_type) const noexcept;constexpr void wait(floating-point-type, memory_order = memory_order::seq_cst) const noexcept;constexpr void notify_one() const noexcept;constexpr void notify_all() const noexcept;constexpr floating-point-type* address() const noexcept;};}

Descriptions are provided below only for members that differ from the primary template.

The following operations perform arithmetic computations.

The correspondence among key, operator, and computation is specified in Table 153 .

constexpr value_type fetch_ _key_(value_type operand, memory_order order = memory_order::seq_cst) const noexcept;

Constraints: is_const_v<_floating-point-type_> is false.

Effects: Atomically replaces the value referenced by *ptr with the result of the computation applied to the value referenced by *ptrand the given operand.

Memory is affected according to the value of order.

These operations are atomic read-modify-write operations ([intro.races]).

Returns: Atomically, the value referenced by *ptrimmediately before the effects.

Remarks: If the result is not a representable value for its type ([expr.pre]), the result is unspecified, but the operations otherwise have no undefined behavior.

Atomic arithmetic operations on floating-point-type should conform to the std::numeric_limits<value_type> traits associated with the floating-point type ([limits.syn]).

The floating-point environment ([cfenv]) for atomic arithmetic operations on floating- _point-type_may be different than the calling thread's floating-point environment.

constexpr value_type operator _op_=(value_type operand) const noexcept;

Constraints: is_const_v<_floating-point-type_> is false.

Effects: Equivalent to:return fetch_ key(operand) op operand;

32.5.7.5 Partial specialization for pointers [atomics.ref.pointer]

There are specializations of the atomic_ref class template for all pointer-to-object types.

For each such type pointer-type, the specialization atomic_ref<_pointer-type_> provides additional atomic operations appropriate to pointer types.

The program is ill-formed if is_always_lock_free is false andis_volatile_v<T> is true.

namespace std { template<class T> struct atomic_ref<_pointer-type_> { private: pointer-type* ptr; public: using value_type = remove_cv_t<_pointer-type_>;using difference_type = ptrdiff_t;static constexpr size_t required_alignment = implementation-defined;static constexpr bool is_always_lock_free = implementation-defined;bool is_lock_free() const noexcept;constexpr explicit atomic_ref(pointer-type&);constexpr atomic_ref(const atomic_ref&) noexcept; atomic_ref& operator=(const atomic_ref&) = delete;constexpr void store(value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr value_type operator=(value_type) const noexcept;constexpr value_type load(memory_order = memory_order::seq_cst) const noexcept;constexpr operator value_type() const noexcept;constexpr value_type exchange(value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr bool compare_exchange_weak(value_type&, value_type, memory_order, memory_order) const noexcept;constexpr bool compare_exchange_strong(value_type&, value_type, memory_order, memory_order) const noexcept;constexpr bool compare_exchange_weak(value_type&, value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr bool compare_exchange_strong(value_type&, value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr value_type fetch_add(difference_type, memory_order = memory_order::seq_cst) const noexcept;constexpr value_type fetch_sub(difference_type, memory_order = memory_order::seq_cst) const noexcept;constexpr value_type fetch_max(value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr value_type fetch_min(value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr value_type operator++(int) const noexcept;constexpr value_type operator--(int) const noexcept;constexpr value_type operator++() const noexcept;constexpr value_type operator--() const noexcept;constexpr value_type operator+=(difference_type) const noexcept;constexpr value_type operator-=(difference_type) const noexcept;constexpr void wait(value_type, memory_order = memory_order::seq_cst) const noexcept;constexpr void notify_one() const noexcept;constexpr void notify_all() const noexcept;constexpr pointer-type* address() const noexcept;};}

Descriptions are provided below only for members that differ from the primary template.

The following operations perform arithmetic computations.

The correspondence among key, operator, and computation is specified in Table 154 .

constexpr value_type fetch_ _key_(difference_type operand, memory_order order = memory_order::seq_cst) const noexcept;

Constraints: is_const_v<_pointer-type_> is false.

Mandates: remove_pointer_t<_pointer-type_> is a complete object type.

Effects: Atomically replaces the value referenced by *ptr with the result of the computation applied to the value referenced by *ptrand the given operand.

Memory is affected according to the value of order.

These operations are atomic read-modify-write operations ([intro.races]).

Returns: Atomically, the value referenced by *ptrimmediately before the effects.

Remarks: The result may be an undefined address, but the operations otherwise have no undefined behavior.

For fetch_max and fetch_min, the maximum and minimum computation is performed as if by max and minalgorithms ([alg.min.max]), respectively, with the object value and the first parameter as the arguments.

[Note 1:

If the pointers point to different complete objects (or subobjects thereof), the < operator does not establish a strict weak ordering (Table 29, [expr.rel]).

— _end note_]

constexpr value_type operator _op_=(difference_type operand) const noexcept;

Constraints: is_const_v<_pointer-type_> is false.

Effects: Equivalent to:return fetch_ key(operand) op operand;

32.5.7.6 Member operators common to integers and pointers to objects [atomics.ref.memop]

Let _referred-type_be _pointer-type_for the specializations in [atomics.ref.pointer] and be _integral-type_for the specializations in [atomics.ref.int].

constexpr value_type operator++(int) const noexcept;

Constraints: is_const_v<_referred-type_> is false.

Effects: Equivalent to: return fetch_add(1);

constexpr value_type operator--(int) const noexcept;

Constraints: is_const_v<_referred-type_> is false.

Effects: Equivalent to: return fetch_sub(1);

constexpr value_type operator++() const noexcept;

Constraints: is_const_v<_referred-type_> is false.

Effects: Equivalent to: return fetch_add(1) + 1;

constexpr value_type operator--() const noexcept;

Constraints: is_const_v<_referred-type_> is false.

Effects: Equivalent to: return fetch_sub(1) - 1;

32.5.8 Class template atomic [atomics.types.generic]

32.5.8.1 General [atomics.types.generic.general]

namespace std { template<class T> struct atomic { using value_type = T;static constexpr bool is_always_lock_free = implementation-defined;bool is_lock_free() const volatile noexcept;bool is_lock_free() const noexcept;constexpr atomic() noexcept(is_nothrow_default_constructible_v<T>);constexpr atomic(T) noexcept; atomic(const atomic&) = delete; atomic& operator=(const atomic&) = delete; atomic& operator=(const atomic&) volatile = delete; T load(memory_order = memory_order::seq_cst) const volatile noexcept;constexpr T load(memory_order = memory_order::seq_cst) const noexcept;operator T() const volatile noexcept;constexpr operator T() const noexcept;void store(T, memory_order = memory_order::seq_cst) volatile noexcept;constexpr void store(T, memory_order = memory_order::seq_cst) noexcept; T operator=(T) volatile noexcept;constexpr T operator=(T) noexcept; T exchange(T, memory_order = memory_order::seq_cst) volatile noexcept;constexpr T exchange(T, memory_order = memory_order::seq_cst) noexcept;bool compare_exchange_weak(T&, T, memory_order, memory_order) volatile noexcept;constexpr bool compare_exchange_weak(T&, T, memory_order, memory_order) noexcept;bool compare_exchange_strong(T&, T, memory_order, memory_order) volatile noexcept;constexpr bool compare_exchange_strong(T&, T, memory_order, memory_order) noexcept;bool compare_exchange_weak(T&, T, memory_order = memory_order::seq_cst) volatile noexcept;constexpr bool compare_exchange_weak(T&, T, memory_order = memory_order::seq_cst) noexcept;bool compare_exchange_strong(T&, T, memory_order = memory_order::seq_cst) volatile noexcept;constexpr bool compare_exchange_strong(T&, T, memory_order = memory_order::seq_cst) noexcept;void wait(T, memory_order = memory_order::seq_cst) const volatile noexcept;constexpr void wait(T, memory_order = memory_order::seq_cst) const noexcept;void notify_one() volatile noexcept;constexpr void notify_one() noexcept;void notify_all() volatile noexcept;constexpr void notify_all() noexcept;};}

The program is ill-formed if any of

is_trivially_copyable_v<T>,
is_copy_constructible_v<T>,
is_move_constructible_v<T>,
is_copy_assignable_v<T>,
is_move_assignable_v<T>, or
same_as<T, remove_cv_t<T>>,

is false.

[Note 1:

Type arguments that are not also statically initializable can be difficult to use.

— _end note_]

The specialization atomic<bool> is a standard-layout struct.

It has a trivial destructor.

[Note 2:

The representation of an atomic specialization need not have the same size and alignment requirement as its corresponding argument type.

— _end note_]

32.5.8.2 Operations on atomic types [atomics.types.operations]

constexpr atomic() noexcept(is_nothrow_default_constructible_v<T>);

Constraints: is_default_constructible_v<T> is true.

Effects: Initializes the atomic object with the value of T().

constexpr atomic(T desired) noexcept;

Effects: Initializes the object with the value desired.

[Note 1:

It is possible to have an access to an atomic object Arace with its construction, for example by communicating the address of the just-constructed object A to another thread viamemory_order::relaxed operations on a suitable atomic pointer variable, and then immediately accessing A in the receiving thread.

This results in undefined behavior.

— _end note_]

The static data member is_always_lock_free is trueif the atomic type's operations are always lock-free, and false otherwise.

[Note 2:

The value of is_always_lock_free is consistent with the value of the corresponding ATOMIC_..._LOCK_FREE macro, if defined.

— _end note_]

Returns: true if the object's operations are lock-free, false otherwise.

[Note 3:

The return value of the is_lock_free member function is consistent with the value of is_always_lock_free for the same type.

— _end note_]

void store(T desired, memory_order order = memory_order::seq_cst) volatile noexcept;constexpr void store(T desired, memory_order order = memory_order::seq_cst) noexcept;

Constraints: For the volatile overload of this function,is_always_lock_free is true.

Preconditions: order ismemory_order::relaxed,memory_order::release, ormemory_order::seq_cst.

Effects: Atomically replaces the value pointed to by thiswith the value of desired.

Memory is affected according to the value oforder.

T operator=(T desired) volatile noexcept;constexpr T operator=(T desired) noexcept;

Constraints: For the volatile overload of this function,is_always_lock_free is true.

Effects: Equivalent to store(desired).

T load(memory_order order = memory_order::seq_cst) const volatile noexcept;constexpr T load(memory_order order = memory_order::seq_cst) const noexcept;

Constraints: For the volatile overload of this function,is_always_lock_free is true.

Preconditions: order ismemory_order::relaxed,memory_order::acquire, ormemory_order::seq_cst.

Effects: Memory is affected according to the value of order.

Returns: Atomically returns the value pointed to by this.

operator T() const volatile noexcept;constexpr operator T() const noexcept;

Constraints: For the volatile overload of this function,is_always_lock_free is true.

Effects: Equivalent to: return load();

T exchange(T desired, memory_order order = memory_order::seq_cst) volatile noexcept;constexpr T exchange(T desired, memory_order order = memory_order::seq_cst) noexcept;

Constraints: For the volatile overload of this function,is_always_lock_free is true.

Effects: Atomically replaces the value pointed to by thiswith desired.

Memory is affected according to the value of order.

These operations are atomic read-modify-write operations ([intro.multithread]).

Returns: Atomically returns the value pointed to by this immediately before the effects.

bool compare_exchange_weak(T& expected, T desired, memory_order success, memory_order failure) volatile noexcept;constexpr bool compare_exchange_weak(T& expected, T desired, memory_order success, memory_order failure) noexcept;bool compare_exchange_strong(T& expected, T desired, memory_order success, memory_order failure) volatile noexcept;constexpr bool compare_exchange_strong(T& expected, T desired, memory_order success, memory_order failure) noexcept;bool compare_exchange_weak(T& expected, T desired, memory_order order = memory_order::seq_cst) volatile noexcept;constexpr bool compare_exchange_weak(T& expected, T desired, memory_order order = memory_order::seq_cst) noexcept;bool compare_exchange_strong(T& expected, T desired, memory_order order = memory_order::seq_cst) volatile noexcept;constexpr bool compare_exchange_strong(T& expected, T desired, memory_order order = memory_order::seq_cst) noexcept;

Constraints: For the volatile overload of this function,is_always_lock_free is true.

Preconditions: failure ismemory_order::relaxed,memory_order::acquire, ormemory_order::
seq_cst.

Effects: Retrieves the value in expected.

It then atomically compares the value representation of the value pointed to by thisfor equality with that previously retrieved from expected, and if true, replaces the value pointed to by this with that in desired.

If and only if the comparison is true, memory is affected according to the value of success, and if the comparison is false, memory is affected according to the value of failure.

When only one memory_order argument is supplied, the value of success is order, and the value offailure is order except that a value of memory_order::acq_relshall be replaced by the value memory_order::acquire and a value ofmemory_order::release shall be replaced by the valuememory_order::relaxed.

If and only if the comparison is false then, after the atomic operation, the value in expected is replaced by the value pointed to by this during the atomic comparison.

If the operation returns true, these operations are atomic read-modify-write operations ([intro.multithread]) on the memory pointed to by this.

Otherwise, these operations are atomic load operations on that memory.

Returns: The result of the comparison.

[Note 4:

For example, the effect ofcompare_exchange_strongon objects without padding bits ([basic.types.general]) isif (memcmp(this, &expected, sizeof(*this)) == 0) memcpy(this, &desired, sizeof(*this));else memcpy(&expected, this, sizeof(*this));

— _end note_]

[Example 1:

The expected use of the compare-and-exchange operations is as follows.

The compare-and-exchange operations will update expected when another iteration of the loop is needed.

expected = current.load();do { desired = function(expected);} while (!current.compare_exchange_weak(expected, desired)); — _end example_]

[Example 2:

Because the expected value is updated only on failure, code releasing the memory containing the expected value on success will work.

For example, list head insertion will act atomically and would not introduce a data race in the following code:do { p->next = head; } while (!head.compare_exchange_weak(p->next, p));

— _end example_]

Implementations should ensure that weak compare-and-exchange operations do not consistently return false unless either the atomic object has value different from expected or there are concurrent modifications to the atomic object.

Remarks: A weak compare-and-exchange operation may fail spuriously.

That is, even when the contents of memory referred to by expected and this are equal, it may return false and store back to expected the same memory contents that were originally there.

[Note 5:

This spurious failure enables implementation of compare-and-exchange on a broader class of machines, e.g., load-locked store-conditional machines.

A consequence of spurious failure is that nearly all uses of weak compare-and-exchange will be in a loop.

When a compare-and-exchange is in a loop, the weak version will yield better performance on some platforms.

When a weak compare-and-exchange would require a loop and a strong one would not, the strong one is preferable.

— _end note_]

[Note 6:

Under cases where the memcpy and memcmp semantics of the compare-and-exchange operations apply, the comparisons can fail for values that compare equal withoperator== if the value representation has trap bits or alternate representations of the same value.

Notably, on implementations conforming to ISO/IEC 60559, floating-point -0.0 and +0.0will not compare equal with memcmp but will compare equal with operator==, and NaNs with the same payload will compare equal with memcmp but will not compare equal with operator==.

— _end note_]

[Note 7:

Because compare-and-exchange acts on an object's value representation, padding bits that never participate in the object's value representation are ignored.

As a consequence, the following code is guaranteed to avoid spurious failure:struct padded { char clank = 0x42;unsigned biff = 0xC0DEFEFE;}; atomic<padded> pad = {};bool zap() { padded expected, desired{0, 0};return pad.compare_exchange_strong(expected, desired);}

— _end note_]

[Note 8:

For a union with bits that participate in the value representation of some members but not others, compare-and-exchange might always fail.

This is because such padding bits have an indeterminate value when they do not participate in the value representation of the active member.

As a consequence, the following code is not guaranteed to ever succeed:union pony { double celestia = 0.;short luna; }; atomic<pony> princesses = {};bool party(pony desired) { pony expected;return princesses.compare_exchange_strong(expected, desired);}

— _end note_]

void wait(T old, memory_order order = memory_order::seq_cst) const volatile noexcept;constexpr void wait(T old, memory_order order = memory_order::seq_cst) const noexcept;

Preconditions: order ismemory_order::relaxed,memory_order::acquire, ormemory_order::seq_cst.

Effects: Repeatedly performs the following steps, in order:

Evaluates load(order) and compares its value representation for equality against that of old.
If they compare unequal, returns.
Blocks until it is unblocked by an atomic notifying operation or is unblocked spuriously.

Remarks: This function is an atomic waiting operation ([atomics.wait]).

void notify_one() volatile noexcept;constexpr void notify_one() noexcept;

Effects: Unblocks the execution of at least one atomic waiting operation that is eligible to be unblocked ([atomics.wait]) by this call, if any such atomic waiting operations exist.

Remarks: This function is an atomic notifying operation ([atomics.wait]).

void notify_all() volatile noexcept;constexpr void notify_all() noexcept;

Effects: Unblocks the execution of all atomic waiting operations that are eligible to be unblocked ([atomics.wait]) by this call.

Remarks: This function is an atomic notifying operation ([atomics.wait]).

32.5.8.3 Specializations for integers [atomics.types.int]

There are specializations of the atomicclass template for the integral typeschar,signed char,unsigned char,short,unsigned short,int,unsigned int,long,unsigned long,long long,unsigned long long,char8_t,char16_t,char32_t,wchar_t, and any other types needed by the typedefs in the header .

For each such type integral-type, the specializationatomic<_integral-type_> provides additional atomic operations appropriate to integral types.

[Note 1:

The specialization atomic<bool>uses the primary template ([atomics.types.generic]).

— _end note_]

namespace std { template<> struct atomic<_integral-type_> { using value_type = integral-type;using difference_type = value_type;static constexpr bool is_always_lock_free = implementation-defined;bool is_lock_free() const volatile noexcept;bool is_lock_free() const noexcept;constexpr atomic() noexcept;constexpr atomic(integral-type) noexcept; atomic(const atomic&) = delete; atomic& operator=(const atomic&) = delete; atomic& operator=(const atomic&) volatile = delete;void store(integral-type, memory_order = memory_order::seq_cst) volatile noexcept;constexpr void store(integral-type, memory_order = memory_order::seq_cst) noexcept;integral-type operator=(integral-type) volatile noexcept;constexpr integral-type operator=(integral-type) noexcept;integral-type load(memory_order = memory_order::seq_cst) const volatile noexcept;constexpr integral-type load(memory_order = memory_order::seq_cst) const noexcept;operator integral-type() const volatile noexcept;constexpr operator integral-type() const noexcept;integral-type exchange(integral-type, memory_order = memory_order::seq_cst) volatile noexcept;constexpr integral-type exchange(integral-type, memory_order = memory_order::seq_cst) noexcept;bool compare_exchange_weak(integral-type&, integral-type, memory_order, memory_order) volatile noexcept;constexpr bool compare_exchange_weak(integral-type&, integral-type, memory_order, memory_order) noexcept;bool compare_exchange_strong(integral-type&, integral-type, memory_order, memory_order) volatile noexcept;constexpr bool compare_exchange_strong(integral-type&, integral-type, memory_order, memory_order) noexcept;bool compare_exchange_weak(integral-type&, integral-type, memory_order = memory_order::seq_cst) volatile noexcept;constexpr bool compare_exchange_weak(integral-type&, integral-type, memory_order = memory_order::seq_cst) noexcept;bool compare_exchange_strong(integral-type&, integral-type, memory_order = memory_order::seq_cst) volatile noexcept;constexpr bool compare_exchange_strong(integral-type&, integral-type, memory_order = memory_order::seq_cst) noexcept;integral-type fetch_add(integral-type, memory_order = memory_order::seq_cst) volatile noexcept;constexpr integral-type fetch_add(integral-type, memory_order = memory_order::seq_cst) noexcept;integral-type fetch_sub(integral-type, memory_order = memory_order::seq_cst) volatile noexcept;constexpr integral-type fetch_sub(integral-type, memory_order = memory_order::seq_cst) noexcept;integral-type fetch_and(integral-type, memory_order = memory_order::seq_cst) volatile noexcept;constexpr integral-type fetch_and(integral-type, memory_order = memory_order::seq_cst) noexcept;integral-type fetch_or(integral-type, memory_order = memory_order::seq_cst) volatile noexcept;constexpr integral-type fetch_or(integral-type, memory_order = memory_order::seq_cst) noexcept;integral-type fetch_xor(integral-type, memory_order = memory_order::seq_cst) volatile noexcept;constexpr integral-type fetch_xor(integral-type, memory_order = memory_order::seq_cst) noexcept;integral-type fetch_max( integral-type, memory_order = memory_order::seq_cst) volatile noexcept;constexpr integral-type fetch_max( integral-type, memory_order = memory_order::seq_cst) noexcept;integral-type fetch_min( integral-type, memory_order = memory_order::seq_cst) volatile noexcept;constexpr integral-type fetch_min( integral-type, memory_order = memory_order::seq_cst) noexcept;integral-type operator++(int) volatile noexcept;constexpr integral-type operator++(int) noexcept;integral-type operator--(int) volatile noexcept;constexpr integral-type operator--(int) noexcept;integral-type operator++() volatile noexcept;constexpr integral-type operator++() noexcept;integral-type operator--() volatile noexcept;constexpr integral-type operator--() noexcept;integral-type operator+=(integral-type) volatile noexcept;constexpr integral-type operator+=(integral-type) noexcept;integral-type operator-=(integral-type) volatile noexcept;constexpr integral-type operator-=(integral-type) noexcept;integral-type operator&=(integral-type) volatile noexcept;constexpr integral-type operator&=(integral-type) noexcept;integral-type operator|=(integral-type) volatile noexcept;constexpr integral-type operator|=(integral-type) noexcept;integral-type operator^=(integral-type) volatile noexcept;constexpr integral-type operator^=(integral-type) noexcept;void wait(integral-type, memory_order = memory_order::seq_cst) const volatile noexcept;constexpr void wait(integral-type, memory_order = memory_order::seq_cst) const noexcept;void notify_one() volatile noexcept;constexpr void notify_one() noexcept;void notify_all() volatile noexcept;constexpr void notify_all() noexcept;};}

The atomic integral specializations are standard-layout structs.

They each have a trivial destructor.

Descriptions are provided below only for members that differ from the primary template.

The following operations perform arithmetic computations.

The correspondence among key, operator, and computation is specified in Table 153 .

Table 153 — Atomic arithmetic computations [tab:atomic.types.int.comp]

🔗*key*	Op	Computation	*key*	Op	Computation
🔗add	+	addition	and	&	bitwise and
🔗sub	-	subtraction	or	\|	bitwise inclusive or
🔗max	maximum	xor	^	bitwise exclusive or
🔗min	minimum

T fetch_ _key_(T operand, memory_order order = memory_order::seq_cst) volatile noexcept;constexpr T fetch_ _key_(T operand, memory_order order = memory_order::seq_cst) noexcept;

Constraints: For the volatile overload of this function,is_always_lock_free is true.

Effects: Atomically replaces the value pointed to bythis with the result of the computation applied to the value pointed to by this and the given operand.

Memory is affected according to the value of order.

These operations are atomic read-modify-write operations ([intro.multithread]).

Returns: Atomically, the value pointed to by this immediately before the effects.

[Note 2:

There are no undefined results arising from the computation.

— _end note_]

T operator _op_=(T operand) volatile noexcept;constexpr T operator _op_=(T operand) noexcept;

Constraints: For the volatile overload of this function,is_always_lock_free is true.

Effects: Equivalent to: return fetch_ key(operand) op operand;

32.5.8.4 Specializations for floating-point types [atomics.types.float]

There are specializations of the atomicclass template for all cv-unqualified floating-point types.

For each such type floating-point-type, the specialization atomic<_floating-point-type_>provides additional atomic operations appropriate to floating-point types.

namespace std { template<> struct atomic<_floating-point-type_> { using value_type = floating-point-type;using difference_type = value_type;static constexpr bool is_always_lock_free = implementation-defined;bool is_lock_free() const volatile noexcept;bool is_lock_free() const noexcept;constexpr atomic() noexcept;constexpr atomic(floating-point-type) noexcept; atomic(const atomic&) = delete; atomic& operator=(const atomic&) = delete; atomic& operator=(const atomic&) volatile = delete;void store(floating-point-type, memory_order = memory_order::seq_cst) volatile noexcept;constexpr void store(floating-point-type, memory_order = memory_order::seq_cst) noexcept;floating-point-type operator=(floating-point-type) volatile noexcept;constexpr floating-point-type operator=(floating-point-type) noexcept;floating-point-type load(memory_order = memory_order::seq_cst) volatile noexcept;constexpr floating-point-type load(memory_order = memory_order::seq_cst) noexcept;operator floating-point-type() volatile noexcept;constexpr operator floating-point-type() noexcept;floating-point-type exchange(floating-point-type, memory_order = memory_order::seq_cst) volatile noexcept;constexpr floating-point-type exchange(floating-point-type, memory_order = memory_order::seq_cst) noexcept;bool compare_exchange_weak(floating-point-type&, floating-point-type, memory_order, memory_order) volatile noexcept;constexpr bool compare_exchange_weak(floating-point-type&, floating-point-type, memory_order, memory_order) noexcept;bool compare_exchange_strong(floating-point-type&, floating-point-type, memory_order, memory_order) volatile noexcept;constexpr bool compare_exchange_strong(floating-point-type&, floating-point-type, memory_order, memory_order) noexcept;bool compare_exchange_weak(floating-point-type&, floating-point-type, memory_order = memory_order::seq_cst) volatile noexcept;constexpr bool compare_exchange_weak(floating-point-type&, floating-point-type, memory_order = memory_order::seq_cst) noexcept;bool compare_exchange_strong(floating-point-type&, floating-point-type, memory_order = memory_order::seq_cst) volatile noexcept;constexpr bool compare_exchange_strong(floating-point-type&, floating-point-type, memory_order = memory_order::seq_cst) noexcept;floating-point-type fetch_add(floating-point-type, memory_order = memory_order::seq_cst) volatile noexcept;constexpr floating-point-type fetch_add(floating-point-type, memory_order = memory_order::seq_cst) noexcept;floating-point-type fetch_sub(floating-point-type, memory_order = memory_order::seq_cst) volatile noexcept;constexpr floating-point-type fetch_sub(floating-point-type, memory_order = memory_order::seq_cst) noexcept;floating-point-type operator+=(floating-point-type) volatile noexcept;constexpr floating-point-type operator+=(floating-point-type) noexcept;floating-point-type operator-=(floating-point-type) volatile noexcept;constexpr floating-point-type operator-=(floating-point-type) noexcept;void wait(floating-point-type, memory_order = memory_order::seq_cst) const volatile noexcept;constexpr void wait(floating-point-type, memory_order = memory_order::seq_cst) const noexcept;void notify_one() volatile noexcept;constexpr void notify_one() noexcept;void notify_all() volatile noexcept;constexpr void notify_all() noexcept;};}

The atomic floating-point specializations are standard-layout structs.

They each have a trivial destructor.

Descriptions are provided below only for members that differ from the primary template.

The following operations perform arithmetic addition and subtraction computations.

The correspondence among key, operator, and computation is specified in Table 153 .

T fetch_ _key_(T operand, memory_order order = memory_order::seq_cst) volatile noexcept;constexpr T fetch_ _key_(T operand, memory_order order = memory_order::seq_cst) noexcept;

Constraints: For the volatile overload of this function,is_always_lock_free is true.

Effects: Atomically replaces the value pointed to by thiswith the result of the computation applied to the value pointed to by this and the given operand.

Memory is affected according to the value of order.

These operations are atomic read-modify-write operations ([intro.multithread]).

Returns: Atomically, the value pointed to by this immediately before the effects.

Remarks: If the result is not a representable value for its type ([expr.pre]) the result is unspecified, but the operations otherwise have no undefined behavior.

Atomic arithmetic operations on _floating-point-type_should conform to the std::numeric_limits<_floating-point-type_>traits associated with the floating-point type ([limits.syn]).

The floating-point environment ([cfenv]) for atomic arithmetic operations on floating-point-type may be different than the calling thread's floating-point environment.

T operator _op_=(T operand) volatile noexcept;constexpr T operator _op_=(T operand) noexcept;

Constraints: For the volatile overload of this function,is_always_lock_free is true.

Effects: Equivalent to: return fetch_ key(operand) op operand;

Remarks: If the result is not a representable value for its type ([expr.pre]) the result is unspecified, but the operations otherwise have no undefined behavior.

Atomic arithmetic operations on _floating-point-type_should conform to the std::numeric_limits<_floating-point-type_>traits associated with the floating-point type ([limits.syn]).

The floating-point environment ([cfenv]) for atomic arithmetic operations on floating-point-type may be different than the calling thread's floating-point environment.

32.5.8.5 Partial specialization for pointers [atomics.types.pointer]

namespace std { template<class T> struct atomic<T*> { using value_type = T*;using difference_type = ptrdiff_t;static constexpr bool is_always_lock_free = implementation-defined;bool is_lock_free() const volatile noexcept;bool is_lock_free() const noexcept;constexpr atomic() noexcept;constexpr atomic(T*) noexcept; atomic(const atomic&) = delete; atomic& operator=(const atomic&) = delete; atomic& operator=(const atomic&) volatile = delete;void store(T*, memory_order = memory_order::seq_cst) volatile noexcept;constexpr void store(T*, memory_order = memory_order::seq_cst) noexcept; T* operator=(T*) volatile noexcept;constexpr T* operator=(T*) noexcept; T* load(memory_order = memory_order::seq_cst) const volatile noexcept;constexpr T* load(memory_order = memory_order::seq_cst) const noexcept;operator T*() const volatile noexcept;constexpr operator T*() const noexcept; T* exchange(T*, memory_order = memory_order::seq_cst) volatile noexcept;constexpr T* exchange(T*, memory_order = memory_order::seq_cst) noexcept;bool compare_exchange_weak(T*&, T*, memory_order, memory_order) volatile noexcept;constexpr bool compare_exchange_weak(T*&, T*, memory_order, memory_order) noexcept;bool compare_exchange_strong(T*&, T*, memory_order, memory_order) volatile noexcept;constexpr bool compare_exchange_strong(T*&, T*, memory_order, memory_order) noexcept;bool compare_exchange_weak(T*&, T*, memory_order = memory_order::seq_cst) volatile noexcept;constexpr bool compare_exchange_weak(T*&, T*, memory_order = memory_order::seq_cst) noexcept;bool compare_exchange_strong(T*&, T*, memory_order = memory_order::seq_cst) volatile noexcept;constexpr bool compare_exchange_strong(T*&, T*, memory_order = memory_order::seq_cst) noexcept; T* fetch_add(ptrdiff_t, memory_order = memory_order::seq_cst) volatile noexcept;constexpr T* fetch_add(ptrdiff_t, memory_order = memory_order::seq_cst) noexcept; T* fetch_sub(ptrdiff_t, memory_order = memory_order::seq_cst) volatile noexcept;constexpr T* fetch_sub(ptrdiff_t, memory_order = memory_order::seq_cst) noexcept; T* fetch_max(T*, memory_order = memory_order::seq_cst) volatile noexcept;constexpr T* fetch_max(T*, memory_order = memory_order::seq_cst) noexcept; T* fetch_min(T*, memory_order = memory_order::seq_cst) volatile noexcept;constexpr T* fetch_min(T*, memory_order = memory_order::seq_cst) noexcept; T* operator++(int) volatile noexcept;constexpr T* operator++(int) noexcept; T* operator--(int) volatile noexcept;constexpr T* operator--(int) noexcept; T* operator++() volatile noexcept;constexpr T* operator++() noexcept; T* operator--() volatile noexcept;constexpr T* operator--() noexcept; T* operator+=(ptrdiff_t) volatile noexcept;constexpr T* operator+=(ptrdiff_t) noexcept; T* operator-=(ptrdiff_t) volatile noexcept;constexpr T* operator-=(ptrdiff_t) noexcept;void wait(T*, memory_order = memory_order::seq_cst) const volatile noexcept;constexpr void wait(T*, memory_order = memory_order::seq_cst) const noexcept;void notify_one() volatile noexcept;constexpr void notify_one() noexcept;void notify_all() volatile noexcept;constexpr void notify_all() noexcept;};}

There is a partial specialization of the atomic class template for pointers.

Specializations of this partial specialization are standard-layout structs.

They each have a trivial destructor.

Descriptions are provided below only for members that differ from the primary template.

The following operations perform pointer arithmetic.

The correspondence among key, operator, and computation is specified in Table 154 .

T* fetch_ _key_(ptrdiff_t operand, memory_order order = memory_order::seq_cst) volatile noexcept;constexpr T* fetch_ _key_(ptrdiff_t operand, memory_order order = memory_order::seq_cst) noexcept;

Constraints: For the volatile overload of this function,is_always_lock_free is true.

Mandates: T is a complete object type.

[Note 1:

Pointer arithmetic on void* or function pointers is ill-formed.

— _end note_]

Effects: Atomically replaces the value pointed to bythis with the result of the computation applied to the value pointed to by this and the given operand.

Memory is affected according to the value of order.

These operations are atomic read-modify-write operations ([intro.multithread]).

Returns: Atomically, the value pointed to by this immediately before the effects.

Remarks: The result may be an undefined address, but the operations otherwise have no undefined behavior.

For fetch_max and fetch_min, the maximum and minimum computation is performed as if by max and minalgorithms ([alg.min.max]), respectively, with the object value and the first parameter as the arguments.

[Note 2:

If the pointers point to different complete objects (or subobjects thereof), the < operator does not establish a strict weak ordering (Table 29, [expr.rel]).

— _end note_]

T* operator _op_=(ptrdiff_t operand) volatile noexcept;constexpr T* operator _op_=(ptrdiff_t operand) noexcept;

Constraints: For the volatile overload of this function,is_always_lock_free is true.

Effects: Equivalent to: return fetch_ key(operand) op operand;

32.5.8.6 Member operators common to integers and pointers to objects [atomics.types.memop]

value_type operator++(int) volatile noexcept;constexpr value_type operator++(int) noexcept;

Constraints: For the volatile overload of this function,is_always_lock_free is true.

Effects: Equivalent to: return fetch_add(1);

value_type operator--(int) volatile noexcept;constexpr value_type operator--(int) noexcept;

Constraints: For the volatile overload of this function,is_always_lock_free is true.

Effects: Equivalent to: return fetch_sub(1);

value_type operator++() volatile noexcept;constexpr value_type operator++() noexcept;

Constraints: For the volatile overload of this function,is_always_lock_free is true.

Effects: Equivalent to: return fetch_add(1) + 1;

value_type operator--() volatile noexcept;constexpr value_type operator--() noexcept;

Constraints: For the volatile overload of this function,is_always_lock_free is true.

Effects: Equivalent to: return fetch_sub(1) - 1;

32.5.8.7 Partial specializations for smart pointers [util.smartptr.atomic]

32.5.8.7.1 General [util.smartptr.atomic.general]

The library provides partial specializations of the atomic template for shared-ownership smart pointers ([util.sharedptr]).

[Note 1:

The partial specializations are declared in header .

— _end note_]

The behavior of all operations is as specified in [atomics.types.generic], unless specified otherwise.

The template parameter T of these partial specializations may be an incomplete type.

All changes to an atomic smart pointer in [util.smartptr.atomic], and all associated use_count increments, are guaranteed to be performed atomically.

Associated use_count decrements are sequenced after the atomic operation, but are not required to be part of it.

Any associated deletion and deallocation are sequenced after the atomic update step and are not part of the atomic operation.

[Note 2:

If the atomic operation uses locks, locks acquired by the implementation will be held when any use_count adjustments are performed, and will not be held when any destruction or deallocation resulting from this is performed.

— _end note_]

[Example 1: template<typename T> class atomic_list { struct node { T t; shared_ptr<node> next;}; atomic<shared_ptr<node>> head;public: shared_ptr<node> find(T t) const { auto p = head.load();while (p && p->t != t) p = p->next;return p;} void push_front(T t) { auto p = make_shared<node>(); p->t = t; p->next = head;while (!head.compare_exchange_weak(p->next, p)) {} } }; — _end example_]

32.5.8.7.3 Partial specialization for weak_ptr [util.smartptr.atomic.weak]

namespace std { template<class T> struct atomic<weak_ptr<T>> { using value_type = weak_ptr<T>;static constexpr bool is_always_lock_free = implementation-defined;bool is_lock_free() const noexcept;constexpr atomic() noexcept; atomic(weak_ptr<T> desired) noexcept; atomic(const atomic&) = delete;void operator=(const atomic&) = delete; weak_ptr<T> load(memory_order order = memory_order::seq_cst) const noexcept;operator weak_ptr<T>() const noexcept;void store(weak_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept;void operator=(weak_ptr<T> desired) noexcept; weak_ptr<T> exchange(weak_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept;bool compare_exchange_weak(weak_ptr<T>& expected, weak_ptr<T> desired, memory_order success, memory_order failure) noexcept;bool compare_exchange_strong(weak_ptr<T>& expected, weak_ptr<T> desired, memory_order success, memory_order failure) noexcept;bool compare_exchange_weak(weak_ptr<T>& expected, weak_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept;bool compare_exchange_strong(weak_ptr<T>& expected, weak_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept;void wait(weak_ptr<T> old, memory_order order = memory_order::seq_cst) const noexcept;void notify_one() noexcept;void notify_all() noexcept;private: weak_ptr<T> p; };}

constexpr atomic() noexcept;

Effects: Value-initializes p.

atomic(weak_ptr<T> desired) noexcept;

Effects: Initializes the object with the value desired.

[Note 1:

It is possible to have an access to an atomic object A race with its construction, for example, by communicating the address of the just-constructed object Ato another thread via memory_order::relaxed operations on a suitable atomic pointer variable, and then immediately accessing A in the receiving thread.

This results in undefined behavior.

— _end note_]

void store(weak_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept;

Preconditions: order ismemory_order::relaxed,memory_order::release, ormemory_order::seq_cst.

Effects: Atomically replaces the value pointed to by this with the value of desired as if by p.swap(desired).

Memory is affected according to the value of order.

void operator=(weak_ptr<T> desired) noexcept;

Effects: Equivalent to store(desired).

weak_ptr<T> load(memory_order order = memory_order::seq_cst) const noexcept;

Preconditions: order ismemory_order::relaxed,memory_order::acquire, ormemory_order::seq_cst.

Effects: Memory is affected according to the value of order.

Returns: Atomically returns p.

operator weak_ptr<T>() const noexcept;

Effects: Equivalent to: return load();

weak_ptr<T> exchange(weak_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept;

Effects: Atomically replaces p with desiredas if by p.swap(desired).

Memory is affected according to the value of order.

This is an atomic read-modify-write operation ([intro.races]).

Returns: Atomically returns the value of p immediately before the effects.

bool compare_exchange_weak(weak_ptr<T>& expected, weak_ptr<T> desired, memory_order success, memory_order failure) noexcept;bool compare_exchange_strong(weak_ptr<T>& expected, weak_ptr<T> desired, memory_order success, memory_order failure) noexcept;

Preconditions: failure ismemory_order::relaxed,memory_order::acquire, ormemory_order::
seq_cst.

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

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

Remarks: Two weak_ptr objects are equivalent if they store the same pointer value and either share ownership or are both empty.

The weak form may fail spuriously.

If the operation returns true,expected is not accessed after the atomic update and the operation is an atomic read-modify-write operation ([intro.multithread]) on the memory pointed to by this.

Otherwise, the operation is an atomic load operation on that memory, andexpected is updated with the existing value read from the atomic object in the attempted atomic update.

The use_count update corresponding to the write to expectedis part of the atomic operation.

The write to expected itself is not required to be part of the atomic operation.

bool compare_exchange_weak(weak_ptr<T>& expected, weak_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept;

Effects: Equivalent to:return compare_exchange_weak(expected, desired, order, fail_order);where fail_order is the same as orderexcept that a value of memory_order::acq_relshall be replaced by the value memory_order::acquire and a value of memory_order::releaseshall be replaced by the value memory_order::relaxed.

bool compare_exchange_strong(weak_ptr<T>& expected, weak_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept;

Effects: Equivalent to:return compare_exchange_strong(expected, desired, order, fail_order);where fail_order is the same as orderexcept that a value of memory_order::acq_relshall be replaced by the value memory_order::acquire and a value of memory_order::releaseshall be replaced by the value memory_order::relaxed.

void wait(weak_ptr<T> old, memory_order order = memory_order::seq_cst) const noexcept;

Preconditions: order ismemory_order::relaxed,memory_order::acquire, ormemory_order::seq_cst.

Effects: Repeatedly performs the following steps, in order:

Evaluates load(order) and compares it to old.
If the two are not equivalent, returns.
Blocks until it is unblocked by an atomic notifying operation or is unblocked spuriously.

Remarks: Two weak_ptr objects are equivalent if they store the same pointer and either share ownership or are both empty.

This function is an atomic waiting operation ([atomics.wait]).

void notify_one() noexcept;

Effects: Unblocks the execution of at least one atomic waiting operation that is eligible to be unblocked ([atomics.wait]) by this call, if any such atomic waiting operations exist.

Remarks: This function is an atomic notifying operation ([atomics.wait]).

void notify_all() noexcept;

Effects: Unblocks the execution of all atomic waiting operations that are eligible to be unblocked ([atomics.wait]) by this call.

Remarks: This function is an atomic notifying operation ([atomics.wait]).

32.5.9 Non-member functions [atomics.nonmembers]

A non-member function template whose name matches the patternatomic_ f or the pattern atomic_ f_explicitinvokes the member function f, with the value of the first parameter as the object expression and the values of the remaining parameters (if any) as the arguments of the member function call, in order.

An argument for a parameter of type atomic<T>::value_type* is dereferenced when passed to the member function call.

If no such member function exists, the program is ill-formed.

[Note 1:

The non-member functions enable programmers to write code that can be compiled as either C or C++, for example in a shared header file.

— _end note_]

32.5.10 Flag type and operations [atomics.flag]

namespace std { struct atomic_flag { constexpr atomic_flag() noexcept; atomic_flag(const atomic_flag&) = delete; atomic_flag& operator=(const atomic_flag&) = delete; atomic_flag& operator=(const atomic_flag&) volatile = delete;bool test(memory_order = memory_order::seq_cst) const volatile noexcept;constexpr bool test(memory_order = memory_order::seq_cst) const noexcept;bool test_and_set(memory_order = memory_order::seq_cst) volatile noexcept;constexpr bool test_and_set(memory_order = memory_order::seq_cst) noexcept;void clear(memory_order = memory_order::seq_cst) volatile noexcept;constexpr void clear(memory_order = memory_order::seq_cst) noexcept;void wait(bool, memory_order = memory_order::seq_cst) const volatile noexcept;constexpr void wait(bool, memory_order = memory_order::seq_cst) const noexcept;void notify_one() volatile noexcept;constexpr void notify_one() noexcept;void notify_all() volatile noexcept;constexpr void notify_all() noexcept;};}

The atomic_flag type provides the classic test-and-set functionality.

It has two states, set and clear.

Operations on an object of type atomic_flag shall be lock-free.

The operations should also be address-free.

The atomic_flag type is a standard-layout struct.

It has a trivial destructor.

constexpr atomic_flag::atomic_flag() noexcept;

Effects: Initializes *this to the clear state.

bool atomic_flag_test(const volatile atomic_flag* object) noexcept;constexpr bool atomic_flag_test(const atomic_flag* object) noexcept;bool atomic_flag_test_explicit(const volatile atomic_flag* object, memory_order order) noexcept;constexpr bool atomic_flag_test_explicit(const atomic_flag* object, memory_order order) noexcept;bool atomic_flag::test(memory_order order = memory_order::seq_cst) const volatile noexcept;constexpr bool atomic_flag::test(memory_order order = memory_order::seq_cst) const noexcept;

For atomic_flag_test, let order be memory_order::seq_cst.

Preconditions: order ismemory_order::relaxed,memory_order::acquire, ormemory_order::seq_cst.

Effects: Memory is affected according to the value of order.

Returns: Atomically returns the value pointed to by object or this.

bool atomic_flag_test_and_set(volatile atomic_flag* object) noexcept;constexpr bool atomic_flag_test_and_set(atomic_flag* object) noexcept;bool atomic_flag_test_and_set_explicit(volatile atomic_flag* object, memory_order order) noexcept;constexpr bool atomic_flag_test_and_set_explicit(atomic_flag* object, memory_order order) noexcept;bool atomic_flag::test_and_set(memory_order order = memory_order::seq_cst) volatile noexcept;constexpr bool atomic_flag::test_and_set(memory_order order = memory_order::seq_cst) noexcept;

Effects: Atomically sets the value pointed to by object or by this to true.

Memory is affected according to the value oforder.

These operations are atomic read-modify-write operations ([intro.multithread]).

Returns: Atomically, the value of the object immediately before the effects.

void atomic_flag_clear(volatile atomic_flag* object) noexcept;constexpr void atomic_flag_clear(atomic_flag* object) noexcept;void atomic_flag_clear_explicit(volatile atomic_flag* object, memory_order order) noexcept;constexpr void atomic_flag_clear_explicit(atomic_flag* object, memory_order order) noexcept;void atomic_flag::clear(memory_order order = memory_order::seq_cst) volatile noexcept;constexpr void atomic_flag::clear(memory_order order = memory_order::seq_cst) noexcept;

Preconditions: order ismemory_order::relaxed,memory_order::release, ormemory_order::seq_cst.

Effects: Atomically sets the value pointed to by object or by this tofalse.

Memory is affected according to the value of order.

void atomic_flag_wait(const volatile atomic_flag* object, bool old) noexcept;constexpr void atomic_flag_wait(const atomic_flag* object, bool old) noexcept;void atomic_flag_wait_explicit(const volatile atomic_flag* object,bool old, memory_order order) noexcept;constexpr void atomic_flag_wait_explicit(const atomic_flag* object,bool old, memory_order order) noexcept;void atomic_flag::wait(bool old, memory_order order = memory_order::seq_cst) const volatile noexcept;constexpr void atomic_flag::wait(bool old, memory_order order = memory_order::seq_cst) const noexcept;

For atomic_flag_wait, let order be memory_order::seq_cst.

Let flag be object for the non-member functions andthis for the member functions.

Preconditions: order ismemory_order::relaxed,memory_order::acquire, ormemory_order::seq_cst.

Effects: Repeatedly performs the following steps, in order:

Evaluates flag->test(order) != old.
If the result of that evaluation is true, returns.
Blocks until it is unblocked by an atomic notifying operation or is unblocked spuriously.

Remarks: This function is an atomic waiting operation ([atomics.wait]).

void atomic_flag_notify_one(volatile atomic_flag* object) noexcept;constexpr void atomic_flag_notify_one(atomic_flag* object) noexcept;void atomic_flag::notify_one() volatile noexcept;constexpr void atomic_flag::notify_one() noexcept;

Effects: Unblocks the execution of at least one atomic waiting operation that is eligible to be unblocked ([atomics.wait]) by this call, if any such atomic waiting operations exist.

Remarks: This function is an atomic notifying operation ([atomics.wait]).

void atomic_flag_notify_all(volatile atomic_flag* object) noexcept;constexpr void atomic_flag_notify_all(atomic_flag* object) noexcept;void atomic_flag::notify_all() volatile noexcept;constexpr void atomic_flag::notify_all() noexcept;

Effects: Unblocks the execution of all atomic waiting operations that are eligible to be unblocked ([atomics.wait]) by this call.

Remarks: This function is an atomic notifying operation ([atomics.wait]).

#define ATOMIC_FLAG_INIT _see below_

Remarks: The macro ATOMIC_FLAG_INIT is defined in such a way that it can be used to initialize an object of type atomic_flagto the clear state.

The macro can be used in the form:atomic_flag guard = ATOMIC_FLAG_INIT;

It is unspecified whether the macro can be used in other initialization contexts.

For a complete static-duration object, that initialization shall be static.

32.5.11 Fences [atomics.fences]

This subclause introduces synchronization primitives called fences.

Fences can have acquire semantics, release semantics, or both.

A fence with acquire semantics is called an acquire fence.

A fence with release semantics is called a release fence.

A release fence A synchronizes with an acquire fence B if there exist atomic operations X and Y, both operating on some atomic objectM, such that A is sequenced before X, X modifiesM, Y is sequenced before B, and Y reads the value written by X or a value written by any side effect in the hypothetical release sequence X would head if it were a release operation.

A release fence A synchronizes with an atomic operation B that performs an acquire operation on an atomic object M if there exists an atomic operation X such that A is sequenced before X, Xmodifies M, and B reads the value written by X or a value written by any side effect in the hypothetical release sequence X would head if it were a release operation.

An atomic operation A that is a release operation on an atomic objectM synchronizes with an acquire fence B if there exists some atomic operation X on M such that X is sequenced before Band reads the value written by A or a value written by any side effect in the release sequence headed by A.

extern "C" constexpr void atomic_thread_fence(memory_order order) noexcept;

Effects: Depending on the value of order, this operation:

has no effects, if order == memory_order::relaxed;
is an acquire fence, if order == memory_order::acquire;
is a release fence, if order == memory_order::release;
is both an acquire fence and a release fence, if order == memory_order::acq_rel;
is a sequentially consistent acquire and release fence, if order == memory_order::seq_cst.

extern "C" constexpr void atomic_signal_fence(memory_order order) noexcept;

Effects: Equivalent to atomic_thread_fence(order), except that the resulting ordering constraints are established only between a thread and a signal handler executed in the same thread.

[Note 1:

atomic_signal_fence can be used to specify the order in which actions performed by the thread become visible to the signal handler.

Compiler optimizations and reorderings of loads and stores are inhibited in the same way as with atomic_thread_fence, but the hardware fence instructions that atomic_thread_fence would have inserted are not emitted.

— _end note_]

32.5.12 C compatibility [stdatomic.h.syn]

The header provides the following definitions:

template<class T> using std-atomic = std::atomic<T>; #define _Atomic(T) std-atomic<T> #define ATOMIC_BOOL_LOCK_FREE see below #define ATOMIC_CHAR_LOCK_FREE see below #define ATOMIC_CHAR16_T_LOCK_FREE see below #define ATOMIC_CHAR32_T_LOCK_FREE see below #define ATOMIC_WCHAR_T_LOCK_FREE see below #define ATOMIC_SHORT_LOCK_FREE see below #define ATOMIC_INT_LOCK_FREE see below #define ATOMIC_LONG_LOCK_FREE see below #define ATOMIC_LLONG_LOCK_FREE see below #define ATOMIC_POINTER_LOCK_FREE see below using std::memory_order; using std::memory_order_relaxed; using std::memory_order_consume; using std::memory_order_acquire; using std::memory_order_release; using std::memory_order_acq_rel; using std::memory_order_seq_cst; using std::atomic_flag; using std::atomic_bool; using std::atomic_char; using std::atomic_schar; using std::atomic_uchar; using std::atomic_short; using std::atomic_ushort; using std::atomic_int; using std::atomic_uint; using std::atomic_long; using std::atomic_ulong; using std::atomic_llong; using std::atomic_ullong; using std::atomic_char8_t; using std::atomic_char16_t; using std::atomic_char32_t; using std::atomic_wchar_t; using std::atomic_int8_t; using std::atomic_uint8_t; using std::atomic_int16_t; using std::atomic_uint16_t; using std::atomic_int32_t; using std::atomic_uint32_t; using std::atomic_int64_t; using std::atomic_uint64_t; using std::atomic_int_least8_t; using std::atomic_uint_least8_t; using std::atomic_int_least16_t; using std::atomic_uint_least16_t; using std::atomic_int_least32_t; using std::atomic_uint_least32_t; using std::atomic_int_least64_t; using std::atomic_uint_least64_t; using std::atomic_int_fast8_t; using std::atomic_uint_fast8_t; using std::atomic_int_fast16_t; using std::atomic_uint_fast16_t; using std::atomic_int_fast32_t; using std::atomic_uint_fast32_t; using std::atomic_int_fast64_t; using std::atomic_uint_fast64_t; using std::atomic_intptr_t; using std::atomic_uintptr_t; using std::atomic_size_t; using std::atomic_ptrdiff_t; using std::atomic_intmax_t; using std::atomic_uintmax_t; using std::atomic_is_lock_free; using std::atomic_load; using std::atomic_load_explicit; using std::atomic_store; using std::atomic_store_explicit; using std::atomic_exchange; using std::atomic_exchange_explicit; using std::atomic_compare_exchange_strong; using std::atomic_compare_exchange_strong_explicit; using std::atomic_compare_exchange_weak; using std::atomic_compare_exchange_weak_explicit; using std::atomic_fetch_add; using std::atomic_fetch_add_explicit; using std::atomic_fetch_sub; using std::atomic_fetch_sub_explicit; using std::atomic_fetch_and; using std::atomic_fetch_and_explicit; using std::atomic_fetch_or; using std::atomic_fetch_or_explicit; using std::atomic_fetch_xor; using std::atomic_fetch_xor_explicit; using std::atomic_flag_test_and_set; using std::atomic_flag_test_and_set_explicit; using std::atomic_flag_clear; using std::atomic_flag_clear_explicit; #define ATOMIC_FLAG_INIT see below using std::atomic_thread_fence; using std::atomic_signal_fence;

Each using-declaration for some name A in the synopsis above makes available the same entity as std::Adeclared in .

Each macro listed above other than _Atomic(T)is defined as in .

It is unspecified whether makes available any declarations in namespace std.

Each of the using-declarations forintN_t, uintN_t, intptr_t, and uintptr_tlisted above is defined if and only if the implementation defines the corresponding typedef-name in [atomics.syn].

Neither the _Atomic macro, nor any of the non-macro global namespace declarations, are provided by any C++ standard library header other than .

Recommended practice: Implementations should ensure that C and C++ representations of atomic objects are compatible, so that the same object can be accessed as both an _Atomic(T)from C code and an atomic<T> from C++ code.

The representations should be the same, and the mechanisms used to ensure atomicity and memory ordering should be compatible.