31 Atomic operations library [atomics]
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;
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
- is_trivially_copyable_v<T>,
- is_copy_constructible_v<T>,
- is_move_constructible_v<T>,
- is_copy_assignable_v<T>, or
- is_move_assignable_v<T>
is
false.[
Note: Type arguments that are
not also statically initializable may be difficult to use
. —
end note ]
The specialization
atomic<bool> is a standard-layout struct
.[
Note: The representation of an atomic specialization
need not have the same size and alignment requirement as
its corresponding argument type
. —
end note ]
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(). constexpr atomic(T desired) noexcept;
Effects:
Initializes the object with the value
desired. [
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
. —
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: 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: 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;
Preconditions:
The
order argument is neither
memory_order::consume,
memory_order::acquire, nor
memory_order::acq_rel. 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 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). T load(memory_order order = memory_order::seq_cst) const volatile noexcept;
T load(memory_order order = memory_order::seq_cst) const noexcept;
Preconditions:
The
order argument is neither
memory_order::release nor
memory_order::acq_rel. Constraints:
For the
volatile overload of this function,
is_always_lock_free is
true. 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.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;
Preconditions:
The
failure argument is neither
memory_order::release nor
memory_order::acq_rel. Constraints:
For the
volatile overload of this function,
is_always_lock_free is
true. 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: For example, the effect of
compare_exchange_strong
on objects without padding bits (
[basic.types]) is
if (memcmp(this, &expected, sizeof(*this)) == 0)
memcpy(this, &desired, sizeof(*this));
else
memcpy(expected, this, sizeof(*this));
—
end note ]
[
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));
—
end example ]
[
Example: 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: 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: Under cases where the
memcpy and
memcmp semantics of the compare-and-exchange
operations apply, the outcome might be failed comparisons 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: 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: 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;
void wait(T old, memory_order order = memory_order::seq_cst) const noexcept;
Preconditions:
order is neither
memory_order::release nor
memory_order::acq_rel. 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.
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
. 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
. 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> (
[cstdint.syn])
.For each such 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;
constexpr atomic() noexcept;
constexpr atomic(integral) noexcept;
atomic(const atomic&) = delete;
atomic& operator=(const atomic&) = delete;
atomic& operator=(const atomic&) volatile = delete;
void store(integral, memory_order = memory_order::seq_cst) volatile noexcept;
void store(integral, memory_order = memory_order::seq_cst) noexcept;
integral operator=(integral) volatile noexcept;
integral operator=(integral) 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;
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;
void wait(integral, memory_order = memory_order::seq_cst) const volatile noexcept;
void wait(integral, 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 key, operator, and computation correspondence is:
Table
144: 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.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: 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;
There are specializations of the
atomic
class template for the floating-point types
float,
double, and
long double.For each such type
floating-point,
the specialization
atomic<floating-point>
provides additional atomic operations appropriate to floating-point types
.
namespace std {
template<> struct atomic<floating-point> {
using value_type = floating-point;
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) noexcept;
atomic(const atomic&) = delete;
atomic& operator=(const atomic&) = delete;
atomic& operator=(const atomic&) volatile = delete;
void store(floating-point, memory_order = memory_order::seq_cst) volatile noexcept;
void store(floating-point, memory_order = memory_order::seq_cst) noexcept;
floating-point operator=(floating-point) volatile noexcept;
floating-point operator=(floating-point) noexcept;
floating-point load(memory_order = memory_order::seq_cst) volatile noexcept;
floating-point load(memory_order = memory_order::seq_cst) noexcept;
operator floating-point() volatile noexcept;
operator floating-point() noexcept;
floating-point exchange(floating-point,
memory_order = memory_order::seq_cst) volatile noexcept;
floating-point exchange(floating-point,
memory_order = memory_order::seq_cst) noexcept;
bool compare_exchange_weak(floating-point&, floating-point,
memory_order, memory_order) volatile noexcept;
bool compare_exchange_weak(floating-point&, floating-point,
memory_order, memory_order) noexcept;
bool compare_exchange_strong(floating-point&, floating-point,
memory_order, memory_order) volatile noexcept;
bool compare_exchange_strong(floating-point&, floating-point,
memory_order, memory_order) noexcept;
bool compare_exchange_weak(floating-point&, floating-point,
memory_order = memory_order::seq_cst) volatile noexcept;
bool compare_exchange_weak(floating-point&, floating-point,
memory_order = memory_order::seq_cst) noexcept;
bool compare_exchange_strong(floating-point&, floating-point,
memory_order = memory_order::seq_cst) volatile noexcept;
bool compare_exchange_strong(floating-point&, floating-point,
memory_order = memory_order::seq_cst) noexcept;
floating-point fetch_add(floating-point,
memory_order = memory_order::seq_cst) volatile noexcept;
floating-point fetch_add(floating-point,
memory_order = memory_order::seq_cst) noexcept;
floating-point fetch_sub(floating-point,
memory_order = memory_order::seq_cst) volatile noexcept;
floating-point fetch_sub(floating-point,
memory_order = memory_order::seq_cst) noexcept;
floating-point operator+=(floating-point) volatile noexcept;
floating-point operator+=(floating-point) noexcept;
floating-point operator-=(floating-point) volatile noexcept;
floating-point operator-=(floating-point) noexcept;
void wait(floating-point, memory_order = memory_order::seq_cst) const volatile noexcept;
void wait(floating-point, 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 key, operator, and computation correspondence are identified in
Table
144.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.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
should conform to the
std::numeric_limits<floating-point>
traits associated with the floating-point type (
[limits.syn])
.The floating-point environment (
[cfenv]) for atomic arithmetic operations
on
floating-point 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
should conform to the
std::numeric_limits<floating-point>
traits associated with the floating-point type (
[limits.syn])
.The floating-point environment (
[cfenv]) for atomic arithmetic operations
on
floating-point may be different than the
calling thread's floating-point environment
.
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 key, operator,
and computation correspondence is:
Table
145: 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: 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.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
. 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;
31.8.5 Member operators common to integers and pointers to objects [atomics.types.memop]
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);
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);
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;
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;
The library provides partial specializations of the
atomic template
for shared-ownership smart pointers (
[smartptr])
.The template parameter
T of these partial specializations
may be an incomplete type
.All changes to an atomic smart pointer in this subclause, 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: 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:
template<typename T> class atomic_list {
struct node {
T t;
shared_ptr<node> next;
};
atomic<shared_ptr<node>> head;
public:
auto find(T t) const {
auto p = head.load();
while (p && p->t != t)
p = p->next;
return shared_ptr<node>(move(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 ]
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;
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;
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;
};
}
constexpr atomic() noexcept;
Effects:
Initializes
p{}. atomic(shared_ptr<T> desired) noexcept;
Effects:
Initializes the object with the value
desired. [
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
. —
end note ]
void store(shared_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept;
Preconditions:
order is neither
memory_order::consume,
memory_order::acquire, nor
memory_order::acq_rel. 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=(shared_ptr<T> desired) noexcept;
Effects:
Equivalent to
store(desired). shared_ptr<T> load(memory_order order = memory_order::seq_cst) const noexcept;
Preconditions:
order is neither
memory_order::release nor
memory_order::acq_rel. Effects:
Memory is affected according to the value of
order. Returns:
Atomically returns
p. operator shared_ptr<T>() const noexcept;
Effects:
Equivalent to: return load();
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.Returns:
Atomically returns the value of
p immediately before the effects
. 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 neither
memory_order::release nor
memory_order::acq_rel. 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
.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
.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. 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. void wait(shared_ptr<T> old, memory_order order = memory_order::seq_cst) const noexcept;
Preconditions:
order is
neither
memory_order::release nor
memory_order::acq_rel. 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
shared_ptr objects are equivalent
if they store the same pointer and either share ownership or are both empty
. 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
. void notify_all() noexcept;
Effects:
Unblocks the execution of all atomic waiting operations
that are eligible to be unblocked (
[atomics.wait]) by this call
.
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:
Initializes
p{}. atomic(weak_ptr<T> desired) noexcept;
Effects:
Initializes the object with the value
desired. [
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
. —
end note ]
void store(weak_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept;
Preconditions:
order is neither
memory_order::consume,
memory_order::acquire, nor
memory_order::acq_rel. 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 is neither
memory_order::release nor
memory_order::acq_rel. 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
desired
as if by
p.swap(desired). Memory is affected according to the value of
order.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 is neither
memory_order::release nor
memory_order::acq_rel. 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, 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
.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
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. 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
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. void wait(weak_ptr<T> old, memory_order order = memory_order::seq_cst) const noexcept;
Preconditions:
order is
neither
memory_order::release nor
memory_order::acq_rel. 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
. 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
. void notify_all() noexcept;
Effects:
Unblocks the execution of all atomic waiting operations
that are eligible to be unblocked (
[atomics.wait]) by this call
.