NAME¶

std::atomic_... - std::atomic_...

Synopsis¶

template< class T > (since C++11)
bool atomic_is_lock_free( const std::shared_ptr<T>* p ); (1) (deprecated in
C++20)
template< class T > (since C++11)
std::shared_ptr<T> atomic_load( const std::shared_ptr<T>* p ); (2) (deprecated in
C++20)
template< class T >
(since C++11)
std::shared_ptr<T> atomic_load_explicit( const (3) (deprecated in
std::shared_ptr<T>* p, C++20)

std::memory_order mo );
template< class T >
(since C++11)
void atomic_store( std::shared_ptr<T>* p, (4) (deprecated in
C++20)
std::shared_ptr<T> r );
template< class T >
(since C++11)
void atomic_store_explicit( std::shared_ptr<T>* p, (5) (deprecated in
std::shared_ptr<T> r, C++20)

std::memory_order mo);
template< class T >
(since C++11)
std::shared_ptr<T> atomic_exchange( std::shared_ptr<T>* p, (6) (deprecated in
C++20)
std::shared_ptr<T> r);
template<class T>

std::shared_ptr<T> atomic_exchange_explicit( std::shared_ptr<T>* (since C++11)
p, (7) (deprecated in
std::shared_ptr<T> r, C++20)

std::memory_order mo);
template< class T >
(since C++11)
bool atomic_compare_exchange_weak( std::shared_ptr<T>* p, (8) (deprecated in
std::shared_ptr<T>* expected, C++20)

std::shared_ptr<T> desired);
template<class T>
(since C++11)
bool atomic_compare_exchange_strong( std::shared_ptr<T>* p, (9) (deprecated in
std::shared_ptr<T>* expected, C++20)

std::shared_ptr<T> desired);
template< class T >

bool atomic_compare_exchange_strong_explicit(
std::shared_ptr<T>* p, (since C++11)
std::shared_ptr<T>* expected, (10) (deprecated in
std::shared_ptr<T> desired, C++20)
std::memory_order success,

std::memory_order failure);
template< class T >

bool atomic_compare_exchange_weak_explicit( std::shared_ptr<T>*
p, (since C++11)
std::shared_ptr<T>* expected, (11) (deprecated in
std::shared_ptr<T> desired, C++20)
std::memory_order success,

std::memory_order failure);

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
these functions, which are overloads of the corresponding atomic access functions
(std::atomic_load, std::atomic_store, etc.)

Note that the control block of a shared_ptr is thread-safe: different
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.

1) Determines whether atomic access to the shared pointer pointed-to by p is
lock-free.
2) Equivalent to atomic_load_explicit(p, std::memory_order_seq_cst)
3) Returns the shared pointer pointed-to by p. As with the non-specialized
std::atomic_load_explicit, mo cannot be std::memory_order_release or
std::memory_order_acq_rel
4) Equivalent to atomic_store_explicit(p, r, memory_order_seq_cst)
5) Stores the shared pointer r in the shared pointer pointed-to by p atomically,
effectively executing p->swap(r). As with the non-specialized
std::atomic_store_explicit, mo cannot be std::memory_order_acquire or
std::memory_order_acq_rel.
6) Equivalent to atomic_exchange_explicit(p, r, memory_order_seq_cst)
7) Stores the shared pointer r in the shared pointer pointed to by p and returns the
value formerly pointed-to by p, atomically. Effectively executes p->swap(r) and
returns a copy of r after the swap.
8) Equivalent to atomic_compare_exchange_weak_explicit(p, expected, desired,
std::memory_order_seq_cst, std::memory_order_seq_cst)
9) Equivalent to atomic_compare_exchange_strong_explicit(p, expected, desired,
std::memory_order_seq_cst, std::memory_order_seq_cst)
10) Compares the shared pointers pointed-to by p and expected. If they are
equivalent (store the same pointer value, and either share ownership of the same
object or are both empty), assigns desired into *p using the memory ordering
constraints specified by success and returns true. If they are not equivalent,
assigns *p into *expected using the memory ordering constraints specified by failure
and returns false.
11) Same as 10), but may fail spuriously.

All these functions invoke undefined behavior if p is a null pointer.

Parameters¶

p, expected - a pointer to a std::shared_ptr
r, desired - a std::shared_ptr
mo, success, failure - memory ordering selectors of type std::memory_order

Exceptions¶

These functions do not throw exceptions.

Return value¶

1) true if atomic access is implemented using lock-free instructions
2,3) A copy of the pointed-to shared pointer.
4,5) (none)
6,7) A copy of the formerly pointed-to shared pointer
8,9,10,11) true if the shared pointers were equivalent and the exchange was
performed, false otherwise.

Notes¶

These functions are typically implemented using mutexes, stored in a global hash
table where the pointer value is used as the key.

To avoid data races, once a shared pointer is passed to any of these functions, it
cannot be accessed non-atomically. In particular, you cannot dereference such a
shared_ptr without first atomically loading it into another shared_ptr object, and
then dereferencing through the second object.

The Concurrency TS offers atomic smart pointer classes atomic_shared_ptr and
atomic_weak_ptr as a replacement for the use of these functions.

These functions were deprecated in favor of the specializations of the
std::atomic template: std::atomic<std::shared_ptr> and (since C++20)
std::atomic<std::weak_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 2980 C++11 empty shared_ptrs are never equivalent if they store the same
equivalent pointer value

See also¶

atomic_is_lock_free checks if the atomic type's operations are
(C++11) lock-free
(function template)
atomic_store atomically replaces the value of the atomic
atomic_store_explicit object with a non-atomic argument
(C++11) (function template)
(C++11)
atomic_load atomically obtains the value stored in an
atomic_load_explicit atomic object
(C++11) (function template)
(C++11)
atomic_exchange atomically replaces the value of the atomic
atomic_exchange_explicit object with non-atomic argument and returns
(C++11) the old value of the atomic
(C++11) (function template)
atomic_compare_exchange_weak
atomic_compare_exchange_weak_explicit atomically compares the value of the atomic
atomic_compare_exchange_strong object with non-atomic argument and performs
atomic_compare_exchange_strong_explicit atomic exchange if equal or atomic load if
(C++11) not
(C++11) (function template)
(C++11)
(C++11)