NAME¶

std::atomic - std::atomic

Synopsis¶

Defined in header <memory>
template< class T > (since C++20)
struct std::atomic<std::shared_ptr<T>>;

The partial template specialization of std::atomic for std::shared_ptr<T> allows
users to manipulate shared_ptr objects atomically.

If multiple threads of execution access the same std::shared_ptr object without
synchronization and any of those accesses uses a non-const member function of
shared_ptr then a data race will occur unless all such access is performed through
an instance of std::atomic<std::shared_ptr> (or, deprecated as of C++20, through the
standalone functions for atomic access to std::shared_ptr).

Associated use_count increments are guaranteed to be part of the atomic operation.
Associated use_count decrements are sequenced after the atomic operation, but are
not required to be part of it, except for the use_count change when overriding
expected in a failed CAS. Any associated deletion and deallocation are sequenced
after the atomic update step and are not part of the atomic operation.

Note that the control block of a shared_ptr is thread-safe: different non-atomic
std::shared_ptr objects can be accessed using mutable operations, such as operator=
or reset, simultaneously by multiple threads, even when these instances are copies,
and share the same control block internally.

The type T may be an incomplete type.

Member types¶

Member type Definition
value_type std::shared_ptr<T>

Member functions¶

All non-specialized std::atomic functions are also provided by this specialization,
and no additional member functions.

atomic<shared_ptr<T>>::atomic

constexpr atomic() noexcept = default; (1)
constexpr atomic( std::nullptr_t ) noexcept : atomic() {} (2)
atomic( std::shared_ptr<T> desired ) noexcept; (3)
atomic( const atomic& ) = delete; (4)

1,2) Initializes the underlying shared_ptr<T> to the null value.
3) Initializes the underlying shared_ptr<T> to a copy of desired. As with any
std::atomic type, initialization is not an atomic operation.
4) Atomic types are not copy/move constructible.

atomic<shared_ptr<T>>::operator=

void operator=( const atomic& ) = delete; (1)
void operator=( std::shared_ptr<T> desired ) noexcept; (2)
void operator=( std::nullptr_t ) noexcept; (3)

1) Atomic types are not copy/move assignable.
2) Value assignment, equivalent to store(desired).
3) Resets the atomic shared pointer to null pointer value. Equivalent to
store(nullptr);.

atomic<shared_ptr<T>>::is_lock_free

bool is_lock_free() const noexcept;

Returns true if the atomic operations on all objects of this type are lock-free,
false otherwise.

atomic<shared_ptr<T>>::store

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

Atomically replaces the value of *this with the value of desired as if by
p.swap(desired) where p is the underlying std::shared_ptr<T>. Memory is ordered
according to order. The behavior is undefined if order is std::memory_order_consume,
std::memory_order_acquire, or std::memory_order_acq_rel.

atomic<shared_ptr<T>>::load

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

Atomically returns a copy of the underlying shared pointer. Memory is ordered
according to order. The behavior is undefined if order is std::memory_order_release
or std::memory_order_acq_rel.

atomic<shared_ptr<T>>::operator std::shared_ptr<T>

operator std::shared_ptr<T>() const noexcept;

Equivalent to return load();.

atomic<shared_ptr<T>>::exchange

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

Atomically replaces the underlying std::shared_ptr<T> with desired as if by
p.swap(desired) where p is the underlying std::shared_ptr<T> and returns a copy of
the value that p had immediately before the swap. Memory is ordered according to
order. This is an atomic read-modify-write operation.

atomic<shared_ptr<T>>::compare_exchange_weak, compare_exchange_strong

bool compare_exchange_strong( std::shared_ptr<T>& expected, std::shared_ptr<T>
desired, (1)
std::memory_order success, std::memory_order
failure ) noexcept;
bool compare_exchange_weak( std::shared_ptr<T>& expected, std::shared_ptr<T>
desired, (2)
std::memory_order success, std::memory_order
failure ) noexcept;
bool compare_exchange_strong( std::shared_ptr<T>& expected, std::shared_ptr<T>
desired, (3)
std::memory_order order =
std::memory_order_seq_cst ) noexcept;
bool compare_exchange_weak( std::shared_ptr<T>& expected, std::shared_ptr<T>
desired, (4)
std::memory_order order = std::memory_order_seq_cst
) noexcept;

1) If the underlying std::shared_ptr<T> stores the same T* as expected and shares
ownership with it, or if both underlying and expected are empty, assigns from
desired to the underlying std::shared_ptr<T>, returns true, and orders memory
according to success, otherwise assigns from the underlying std::shared_ptr<T> to
expected, returns false, and orders memory according to failure. The behavior is
undefined if failure is std::memory_order_release or std::memory_order_acq_rel. On
success, the operation is an atomic read-modify-write operation on *this and
expected is not accessed after the atomic update. On failure, the operation is an
atomic load operation on *this and expected is updated with the existing value read
from the atomic object. This update to expected's use_count is part of this atomic
operation, although the write itself (and any subsequent deallocation/destruction)
is not required to be.
2) Same as (1), but may also fail spuriously.
3) Equivalent to: return compare_exchange_strong(expected, desired, order,
fail_order);, where fail_order is the same as order except that
std::memory_order_acq_rel is replaced by std::memory_order_acquire and
std::memory_order_release is replaced by std::memory_order_relaxed.
4) Equivalent to: return compare_exchange_weak(expected, desired, order,
fail_order);, where fail_order is the same as order except that
std::memory_order_acq_rel is replaced by std::memory_order_acquire and
std::memory_order_release is replaced by std::memory_order_relaxed.

atomic<shared_ptr<T>>::wait

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

Performs an atomic waiting operation.

Compares load(order) with old and if they are equivalent then blocks until *this is
notified by notify_one() or notify_all(). This is repeated until load(order)
changes. This function is guaranteed to return only if value has changed, even if
underlying implementation unblocks spuriously.

Memory is ordered according to order. The behavior is undefined if order is
std::memory_order_release or std::memory_order_acq_rel.

Notes: two shared_ptrs are equivalent if they store the same pointer and either
share ownership or are both empty.

atomic<shared_ptr<T>>::notify_one

void notify_one() noexcept;

Performs an atomic notifying operation.

If there is a thread blocked in atomic waiting operations (i.e. wait()) on *this,
then unblocks at least one such thread; otherwise does nothing.

atomic<shared_ptr<T>>::notify_all

void notify_all() noexcept;

Performs an atomic notifying operation.

Unblocks all threads blocked in atomic waiting operations (i.e. wait()) on *this, if
there are any; otherwise does nothing.

Member constants¶

The only standard std::atomic member constant is_always_lock_free is also provided
by this specialization.

atomic<shared_ptr<T>>::is_always_lock_free

static constexpr bool is_always_lock_free = /*implementation-defined*/;

Notes¶

Feature-test macro Value Std Feature
__cpp_lib_atomic_shared_ptr 201711L (C++20) std::atomic<std::shared_ptr>

Example¶

This section is incomplete
Reason: no example

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3661 C++20 atomic<shared_ptr<T>> was not made constant-initializable
constant-initializable from nullptr
LWG 3893 C++20 LWG3661 made atomic<shared_ptr<T>> assignability restored
not assignable from nullptr_t

See also¶

atomic class template and specializations for bool, integral,
atomic floating-point,
(C++11) (since C++20) and pointer types
(class template)

Category:¶

* Todo no example