33 Concurrency support library [thread]

33.5 Atomic operations [atomics]

33.5.8 Class template atomic [atomics.types.generic]

33.5.8.1 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; // [atomics.types.operations], operations on atomic types 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; T load(memory_order = memory_order::seq_cst) const noexcept; operator T() const volatile noexcept; operator T() const noexcept; void store(T, memory_order = memory_order::seq_cst) volatile noexcept; void store(T, memory_order = memory_order::seq_cst) noexcept; T operator=(T) volatile noexcept; T operator=(T) noexcept; T exchange(T, memory_order = memory_order::seq_cst) volatile noexcept; T exchange(T, memory_order = memory_order::seq_cst) noexcept; bool compare_exchange_weak(T&, T, memory_order, memory_order) volatile noexcept; bool compare_exchange_weak(T&, T, memory_order, memory_order) noexcept; bool compare_exchange_strong(T&, T, memory_order, memory_order) volatile noexcept; 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; 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; 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; void wait(T, memory_order = memory_order::seq_cst) const noexcept; void notify_one() volatile noexcept; void notify_one() noexcept; void notify_all() volatile noexcept; void notify_all() noexcept; }; }

The template argument for T shall meet the Cpp17CopyConstructible and Cpp17CopyAssignable requirements.

The program is ill-formed if any of

(1.1)
is_trivially_copyable_v<T>,
(1.2)
is_copy_constructible_v<T>,
(1.3)
is_move_constructible_v<T>,
(1.4)
is_copy_assignable_v<T>, or
(1.5)
is_move_assignable_v<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]

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

🔗

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

Mandates: is_default_constructible_v<T> is true.

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

Initialization is not an atomic operation ([intro.multithread]).

🔗

constexpr atomic(T desired) noexcept;

Effects: Initializes the object with the value desired.

Initialization is not an atomic operation ([intro.multithread]).

[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 A to 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]

🔗

static constexpr bool is_always_lock_free = implementation-defined;

The static data member is_always_lock_free is true if 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]

🔗

bool is_lock_free() const volatile noexcept;
bool is_lock_free() const noexcept;

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;
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 is memory_order::relaxed, memory_order::release, or memory_order::seq_cst.

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

Memory is affected according to the value of order.

🔗

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

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

Effects: Equivalent to store(desired).

Returns: desired.

🔗

T load(memory_order order = memory_order::seq_cst) const volatile noexcept;
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 is memory_order::relaxed, memory_order::consume, memory_order::ac-
quire, or memory_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;
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;
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 this with 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;
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;
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;
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;
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 is memory_order::relaxed, memory_order::consume, memory_order::acquire, or memory_order::seq_cst.

Effects: Retrieves the value in expected.

It then atomically compares the value representation of the value pointed to by this for 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 of failure is order except 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 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 of compare_exchange_strong on objects without padding bits ([basic.types.general]) is if (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; // make new list node point to the current head } while (!head.compare_exchange_weak(p->next, p)); // try to insert

— 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 with operator== if the value representation has trap bits or alternate representations of the same value.

Notably, on implementations conforming to ISO/IEC/IEEE 60559, floating-point -0.0 and +0.0 will 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; // Padding here. 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; // padded }; 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;
void wait(T old, memory_order order = memory_order::seq_cst) const noexcept;

Preconditions: order is memory_order::relaxed, memory_order::consume, memory_order::ac-
quire, or memory_order::seq_cst.

Effects: Repeatedly performs the following steps, in order:

(30.1)
Evaluates load(order) and compares its value representation for equality against that of old.
(30.2)
If they compare unequal, returns.
(30.3)
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;
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;
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]).

33.5.8.3 Specializations for integers [atomics.types.int]

There are specializations of the atomic class template for the integral types char, 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 <cstdint>.

For each such type integral-type, the specialization atomic<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; void store(integral-type, memory_order = memory_order::seq_cst) noexcept; integral-type operator=(integral-type) volatile noexcept; integral-type operator=(integral-type) noexcept; integral-type load(memory_order = memory_order::seq_cst) const volatile noexcept; integral-type load(memory_order = memory_order::seq_cst) const noexcept; operator integral-type() const volatile noexcept; operator integral-type() const noexcept; integral-type exchange(integral-type, memory_order = memory_order::seq_cst) volatile noexcept; 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; 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; 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; 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; 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; 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; 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; 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; 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; integral-type fetch_xor(integral-type, memory_order = memory_order::seq_cst) noexcept; integral-type operator++(int) volatile noexcept; integral-type operator++(int) noexcept; integral-type operator--(int) volatile noexcept; integral-type operator--(int) noexcept; integral-type operator++() volatile noexcept; integral-type operator++() noexcept; integral-type operator--() volatile noexcept; integral-type operator--() noexcept; integral-type operator+=(integral-type) volatile noexcept; integral-type operator+=(integral-type) noexcept; integral-type operator-=(integral-type) volatile noexcept; integral-type operator-=(integral-type) noexcept; integral-type operator&=(integral-type) volatile noexcept; integral-type operator&=(integral-type) noexcept; integral-type operator|=(integral-type) volatile noexcept; integral-type operator|=(integral-type) noexcept; integral-type operator^=(integral-type) volatile noexcept; integral-type operator^=(integral-type) noexcept; void wait(integral-type, memory_order = memory_order::seq_cst) const volatile noexcept; void wait(integral-type, memory_order = memory_order::seq_cst) const noexcept; void notify_one() volatile noexcept; void notify_one() noexcept; void notify_all() volatile noexcept; 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 145 .

Table 145: Atomic arithmetic computations [tab:atomic.types.int.comp]

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

🔗

T fetch_key(T operand, memory_order order = memory_order::seq_cst) volatile noexcept;
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 this 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: 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]

🔗

T operator op=(T operand) volatile noexcept;
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;

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

There are specializations of the atomic class 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; void store(floating-point-type, memory_order = memory_order::seq_cst) noexcept; floating-point-type operator=(floating-point-type) volatile noexcept; floating-point-type operator=(floating-point-type) noexcept; floating-point-type load(memory_order = memory_order::seq_cst) volatile noexcept; floating-point-type load(memory_order = memory_order::seq_cst) noexcept; operator floating-point-type() volatile noexcept; operator floating-point-type() noexcept; floating-point-type exchange(floating-point-type, memory_order = memory_order::seq_cst) volatile noexcept; 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; 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; 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; 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; 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; 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; floating-point-type fetch_sub(floating-point-type, memory_order = memory_order::seq_cst) noexcept; floating-point-type operator+=(floating-point-type) volatile noexcept; floating-point-type operator+=(floating-point-type) noexcept; floating-point-type operator-=(floating-point-type) volatile noexcept; floating-point-type operator-=(floating-point-type) noexcept; void wait(floating-point-type, memory_order = memory_order::seq_cst) const volatile noexcept; void wait(floating-point-type, memory_order = memory_order::seq_cst) const noexcept; void notify_one() volatile noexcept; void notify_one() noexcept; void notify_all() volatile noexcept; 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 145 .

🔗

T fetch_key(T operand, memory_order order = memory_order::seq_cst) volatile noexcept;
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 this 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: 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;
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.

33.5.8.5 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; void store(T*, memory_order = memory_order::seq_cst) noexcept; T* operator=(T*) volatile noexcept; T* operator=(T*) noexcept; T* load(memory_order = memory_order::seq_cst) const volatile noexcept; T* load(memory_order = memory_order::seq_cst) const noexcept; operator T*() const volatile noexcept; operator T*() const noexcept; T* exchange(T*, memory_order = memory_order::seq_cst) volatile noexcept; T* exchange(T*, memory_order = memory_order::seq_cst) noexcept; bool compare_exchange_weak(T*&, T*, memory_order, memory_order) volatile noexcept; bool compare_exchange_weak(T*&, T*, memory_order, memory_order) noexcept; bool compare_exchange_strong(T*&, T*, memory_order, memory_order) volatile noexcept; 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; 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; 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; 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; T* fetch_sub(ptrdiff_t, memory_order = memory_order::seq_cst) noexcept; T* operator++(int) volatile noexcept; T* operator++(int) noexcept; T* operator--(int) volatile noexcept; T* operator--(int) noexcept; T* operator++() volatile noexcept; T* operator++() noexcept; T* operator--() volatile noexcept; T* operator--() noexcept; T* operator+=(ptrdiff_t) volatile noexcept; T* operator+=(ptrdiff_t) noexcept; T* operator-=(ptrdiff_t) volatile noexcept; T* operator-=(ptrdiff_t) noexcept; void wait(T*, memory_order = memory_order::seq_cst) const volatile noexcept; void wait(T*, memory_order = memory_order::seq_cst) const noexcept; void notify_one() volatile noexcept; void notify_one() noexcept; void notify_all() volatile noexcept; 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 146 .

Table 146: Atomic pointer computations [tab:atomic.types.pointer.comp]

🔗	*key*	Op	Computation	*key*	Op	Computation
🔗	add	+	addition	sub	-	subtraction

🔗

T* fetch_key(ptrdiff_t operand, memory_order order = memory_order::seq_cst) volatile noexcept;
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 by this 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.

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T* operator op=(ptrdiff_t operand) volatile noexcept;
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;

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

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

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

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value_type operator++() volatile noexcept;
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;

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value_type operator--() volatile noexcept;
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;

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

33.5.8.7.1 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 <memory>.

— 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]

33.5.8.7.2 Partial specialization for shared_ptr [util.smartptr.atomic.shared]

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

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constexpr atomic() noexcept;

Effects: Initializes p{}.

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atomic(shared_ptr<T> desired) noexcept;

Effects: Initializes the object with the value desired.

Initialization is not an atomic operation ([intro.multithread]).

[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 A to 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]

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

Preconditions: order is memory_order::relaxed, memory_order::release, or memory_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.

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

Effects: Equivalent to store(desired).

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void operator=(nullptr_t) noexcept;

Effects: Equivalent to store(nullptr).

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

Preconditions: order is memory_order::relaxed, memory_order::consume, memory_order::ac-
quire, or memory_order::seq_cst.

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

Returns: Atomically returns p.

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operator shared_ptr<T>() const noexcept;

Effects: Equivalent to: return load();

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

Effects: Atomically replaces p with desired as 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.

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

Preconditions: failure is memory_order::relaxed, memory_order::consume, memory_order::acquire, or memory_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 shared_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.

See [atomics.types.operations].

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, and expected 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 expected is part of the atomic operation.

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

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bool compare_exchange_weak(shared_ptr<T>& expected, shared_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 order except 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.

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bool compare_exchange_strong(shared_ptr<T>& expected, shared_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 order except 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.

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

Preconditions: order is memory_order::relaxed, memory_order::consume, memory_order::ac-
quire, or memory_order::seq_cst.

Effects: Repeatedly performs the following steps, in order:

(21.1)
Evaluates load(order) and compares it to old.
(21.2)
If the two are not equivalent, returns.
(21.3)
Blocks until it is unblocked by an atomic notifying operation or is unblocked spuriously.

Remarks: Two shared_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]).

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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]).

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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]).

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

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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; // exposition only }; }

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constexpr atomic() noexcept;

Effects: Initializes p{}.

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atomic(weak_ptr<T> desired) noexcept;

Effects: Initializes the object with the value desired.

Initialization is not an atomic operation ([intro.multithread]).

[Note 1:

This results in undefined behavior.

— end note]

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

Preconditions: order is memory_order::relaxed, memory_order::release, or memory_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.

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

Effects: Equivalent to store(desired).

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

Preconditions: order is memory_order::relaxed, memory_order::consume, memory_order::ac-
quire, or memory_order::seq_cst.

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

Returns: Atomically returns p.

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operator weak_ptr<T>() const noexcept;

Effects: Equivalent to: return load();

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

Effects: Atomically replaces p with desired as 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.

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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 is memory_order::relaxed, memory_order::consume, memory_order::acquire, or memory_order::seq_cst.

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.

See [atomics.types.operations].

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, and expected 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 expected is part of the atomic operation.

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

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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;

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

Preconditions: order is memory_order::relaxed, memory_order::consume, memory_order::ac-
quire, or memory_order::seq_cst.

Effects: Repeatedly performs the following steps, in order:

(20.1)
Evaluates load(order) and compares it to old.
(20.2)
If the two are not equivalent, returns.
(20.3)
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]).

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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]).

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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]).