# std::atomic_...<std::shared_ptr>

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template< class T > bool atomic_is_lock_free( const std::shared_ptr<T>* p ); | (1) | (since C++11) (deprecated in C++20)
template< class T > std::shared_ptr<T> atomic_load( const std::shared_ptr<T>* p ); | (2) | (since C++11) (deprecated in C++20)
template< class T > std::shared_ptr<T> atomic_load_explicit( const std::shared_ptr<T>* p, std::memory_order mo ); | (3) | (since C++11) (deprecated in C++20)
template< class T > void atomic_store( std::shared_ptr<T>* p, std::shared_ptr<T> r ); | (4) | (since C++11) (deprecated in C++20)
template< class T > void atomic_store_explicit( std::shared_ptr<T>* p, std::shared_ptr<T> r, std::memory_order mo); | (5) | (since C++11) (deprecated in C++20)
template< class T > std::shared_ptr<T> atomic_exchange( std::shared_ptr<T>* p, std::shared_ptr<T> r); | (6) | (since C++11) (deprecated in C++20)
template<class T> std::shared_ptr<T> atomic_exchange_explicit( std::shared_ptr<T>* p, std::shared_ptr<T> r, std::memory_order mo); | (7) | (since C++11) (deprecated in C++20)
template< class T > bool atomic_compare_exchange_weak( std::shared_ptr<T>* p, std::shared_ptr<T>* expected, std::shared_ptr<T> desired); | (8) | (since C++11) (deprecated in C++20)
template<class T> bool atomic_compare_exchange_strong( std::shared_ptr<T>* p, std::shared_ptr<T>* expected, std::shared_ptr<T> desired); | (9) | (since C++11) (deprecated in C++20)
template< class T > bool atomic_compare_exchange_strong_explicit( std::shared_ptr<T>* p, std::shared_ptr<T>* expected, std::shared_ptr<T> desired, std::memory_order success, std::memory_order failure); | (10) | (since C++11) (deprecated in C++20)
template< class T > bool atomic_compare_exchange_weak_explicit( std::shared_ptr<T>* p, std::shared_ptr<T>* expected, std::shared_ptr<T> desired, std::memory_order success, std::memory_order failure); | (11) | (since C++11) (deprecated in C++20)

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 std::atomic<std::weak_ptr>. | (since C++20)

### 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_ptr`s are never equivalent | equivalent if they store the same pointer value

### See also

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

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