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

Declarations

Since C++11, Deprecated in C++20

// 1)
template< class T >
bool atomic_is_lock_free( const std::shared_ptr<T>* p );
// 2)
template< class T >
std::shared_ptr<T> atomic_load( const std::shared_ptr<T>* p );
// 3)
template< class T >
std::shared_ptr<T> atomic_load_explicit( const std::shared_ptr<T>* p,
                                         std::memory_order mo );
// 4)
template< class T >
void atomic_store( std::shared_ptr<T>* p,
                   std::shared_ptr<T> r );
// 5)
template< class T >
void atomic_store_explicit( std::shared_ptr<T>* p,
                            std::shared_ptr<T> r,
                            std::memory_order mo);
// 6)
template< class T >
std::shared_ptr<T> atomic_exchange( std::shared_ptr<T>* p,
                                    std::shared_ptr<T> r);
// 7)
template<class T>
std::shared_ptr<T> atomic_exchange_explicit( std::shared_ptr<T>* p,
                                             std::shared_ptr<T> r,
                                             std::memory_order mo);
// 8)
template< class T >
bool atomic_compare_exchange_weak( std::shared_ptr<T>* p,
                                   std::shared_ptr<T>* expected,
                                   std::shared_ptr<T> desired);
// 9)
template<class T>
bool atomic_compare_exchange_strong( std::shared_ptr<T>* p,
                                     std::shared_ptr<T>* expected,
                                     std::shared_ptr<T> desired);
// 10)
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);
// 11)
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);

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

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	equivalent if they store the same pointer value