std::atomic_...(3)

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)