std::atomic_...std::shared_ptr - cppreference.com (original) (raw)

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. Returns the shared pointer pointed-to by p.

3. Stores the shared pointer r in the shared pointer pointed-to by p atomically, as if by p->swap(r).

4. Stores the shared pointer r in the shared pointer pointed to by p and returns the value formerly pointed-to by p, atomically, as if by p->swap(r) and returns a copy of r after the swap.

5. Equivalent to

6. Equivalent to

10,11) 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.

atomic_compare_exchange_weak_explicit may fail spuriously.

If p is a null pointer, the behaviors of these functions are all undefined.

Contents

• 1 Parameters
• 2 Exceptions
• 3 Return value
• 4 Notes
• 5 Example
• 6 Defect reports
• 7 See also

[edit] Parameters

[edit] Exceptions

These functions do not throw exceptions.

[edit] 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.

8-11) true if the shared pointers were equivalent and the exchange was performed, false otherwise.

[edit] Notes

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

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

[edit] Example

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also