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std::atomic_...(3) C++ Standard Libary 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 ob- ject 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 ac- cess functions (std::atomic_load, std::atomic_store, etc.) Note that the control block of a shared_ptr is thread-safe: differ- ent std::shared_ptr objects can be accessed using mutable operations, such as operator= or reset, simultaneously by multiple threads, even when these in- stances 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-spe- cialized 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_or- der_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, ex- pected, 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::mem- ory_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 ex- change 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 deref- erence 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 retroac- tively 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 ar- gument (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 argu- ment 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 argu- ment 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) http://cppreference.com 2022.07.31 std::atomic_...(3)
NAME | Synopsis | Parameters | Exceptions | Return value | Notes | Example | See also
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