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; void store(T, memory_order = memory_order_seq_cst) volatile noexcept; void store(T, memory_order = memory_order_seq_cst) 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; atomic() noexcept = default; constexpr atomic(T) noexcept; atomic(const atomic&) = delete; atomic& operator=(const atomic&) = delete; atomic& operator=(const atomic&) volatile = delete; T operator=(T) volatile noexcept; T operator=(T) noexcept; }; }

[ Note

: *end note*

]Many operations are volatile-qualified.

The “volatile as device register”
semantics have not changed in the standard.

This qualification means that volatility is
preserved when applying these operations to volatile objects.

It does not mean that
operations on non-volatile objects become volatile.

— ```
atomic() noexcept = default;
```

```
constexpr atomic(T desired) noexcept;
```

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

[ Note

: *end note*

]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.

— ```
#define ATOMIC_VAR_INIT(value) see below
```

```
static constexpr bool is_always_lock_free = implementation-defined;
```

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

```
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;
```

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

```
T load(memory_order order = memory_order_seq_cst) const volatile noexcept;
T load(memory_order order = memory_order_seq_cst) const noexcept;
```

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

```
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;
```

Memory is affected according to the value of order.

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

```
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;
```

It then atomically
compares the contents of the memory pointed to by this
for equality with that previously retrieved from expected,
and if true, replaces the contents of the memory 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 contents of the memory in expected are replaced by the value
read from the memory 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.

[ Note

: *end note*

]For example, the effect of
compare_exchange_strong is

— if (memcmp(this, &expected, sizeof(*this)) == 0) memcpy(this, &desired, sizeof(*this)); else memcpy(expected, this, sizeof(*this));

[ Example

: *end example*

]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));—

[ Example

: *end example*

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

E.

g.

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

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

: *end note*

]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.

— [ Note

: *end note*

]The memcpy and memcmp semantics of the compare-and-exchange
operations may result in failed comparisons for values that compare equal with
operator== if the underlying type has padding bits, trap bits, or alternate
representations of the same value.

Thus, compare_exchange_strong should be used
with extreme care.

On the other hand, compare_exchange_weak should converge
rapidly.

— There are specializations of the atomic
template for the integral types
char,
signed char,
unsigned char,
short,
unsigned short,
int,
unsigned int,
long,
unsigned long,
long long,
unsigned long long,
char16_t,
char32_t,
wchar_t,
and any other types needed by the typedefs in the header <cstdint>.

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

namespace std { template <> struct atomic<integral> { using value_type = integral; 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; void store(integral, memory_order = memory_order_seq_cst) volatile noexcept; void store(integral, memory_order = memory_order_seq_cst) noexcept; integral load(memory_order = memory_order_seq_cst) const volatile noexcept; integral load(memory_order = memory_order_seq_cst) const noexcept; operator integral() const volatile noexcept; operator integral() const noexcept; integral exchange(integral, memory_order = memory_order_seq_cst) volatile noexcept; integral exchange(integral, memory_order = memory_order_seq_cst) noexcept; bool compare_exchange_weak(integral&, integral, memory_order, memory_order) volatile noexcept; bool compare_exchange_weak(integral&, integral, memory_order, memory_order) noexcept; bool compare_exchange_strong(integral&, integral, memory_order, memory_order) volatile noexcept; bool compare_exchange_strong(integral&, integral, memory_order, memory_order) noexcept; bool compare_exchange_weak(integral&, integral, memory_order = memory_order_seq_cst) volatile noexcept; bool compare_exchange_weak(integral&, integral, memory_order = memory_order_seq_cst) noexcept; bool compare_exchange_strong(integral&, integral, memory_order = memory_order_seq_cst) volatile noexcept; bool compare_exchange_strong(integral&, integral, memory_order = memory_order_seq_cst) noexcept; integral fetch_add(integral, memory_order = memory_order_seq_cst) volatile noexcept; integral fetch_add(integral, memory_order = memory_order_seq_cst) noexcept; integral fetch_sub(integral, memory_order = memory_order_seq_cst) volatile noexcept; integral fetch_sub(integral, memory_order = memory_order_seq_cst) noexcept; integral fetch_and(integral, memory_order = memory_order_seq_cst) volatile noexcept; integral fetch_and(integral, memory_order = memory_order_seq_cst) noexcept; integral fetch_or(integral, memory_order = memory_order_seq_cst) volatile noexcept; integral fetch_or(integral, memory_order = memory_order_seq_cst) noexcept; integral fetch_xor(integral, memory_order = memory_order_seq_cst) volatile noexcept; integral fetch_xor(integral, memory_order = memory_order_seq_cst) noexcept; atomic() noexcept = default; constexpr atomic(integral) noexcept; atomic(const atomic&) = delete; atomic& operator=(const atomic&) = delete; atomic& operator=(const atomic&) volatile = delete; integral operator=(integral) volatile noexcept; integral operator=(integral) noexcept; integral operator++(int) volatile noexcept; integral operator++(int) noexcept; integral operator--(int) volatile noexcept; integral operator--(int) noexcept; integral operator++() volatile noexcept; integral operator++() noexcept; integral operator--() volatile noexcept; integral operator--() noexcept; integral operator+=(integral) volatile noexcept; integral operator+=(integral) noexcept; integral operator-=(integral) volatile noexcept; integral operator-=(integral) noexcept; integral operator&=(integral) volatile noexcept; integral operator&=(integral) noexcept; integral operator|=(integral) volatile noexcept; integral operator|=(integral) noexcept; integral operator^=(integral) volatile noexcept; integral operator^=(integral) noexcept; }; }

The atomic integral specializations
are standard-layout structs.

They each have a trivial default constructor
and a trivial destructor.

The following operations perform arithmetic computations.

The key, operator, and computation correspondence is:

Table 138 — Atomic arithmetic computations

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;
```

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]).

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

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; void store(T*, memory_order = memory_order_seq_cst) volatile noexcept; void store(T*, memory_order = memory_order_seq_cst) 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; atomic() noexcept = default; constexpr atomic(T*) noexcept; atomic(const atomic&) = delete; atomic& operator=(const atomic&) = delete; atomic& operator=(const atomic&) volatile = delete; T* operator=(T*) volatile noexcept; T* operator=(T*) 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; }; }

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 default constructor and a trivial destructor.

The following operations perform pointer arithmetic.

The key, operator,
and computation correspondence is:

Table 139 — Atomic pointer computations

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;
```

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]).

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

```
T operator++(int) volatile noexcept;
T operator++(int) noexcept;
```

Effects: Equivalent to: return fetch_add(1);

```
T operator--(int) volatile noexcept;
T operator--(int) noexcept;
```

Effects: Equivalent to: return fetch_sub(1);

```
T operator++() volatile noexcept;
T operator++() noexcept;
```

```
T operator--() volatile noexcept;
T operator--() noexcept;
```