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

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< cpp‎ | memory‎ | shared ptr
 
 
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Defined in header <memory>
template< class T >
bool atomic_is_lock_free( const std::shared_ptr <T>* p );
(1) (since C++11)
(deprecated in C++20)
(removed in C++26)
template< class T >
std::shared_ptr <T> atomic_load( const std::shared_ptr <T>* p );
(2) (since C++11)
(deprecated in C++20)
(removed in C++26)
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)
(removed in C++26)
template< class T >
void atomic_store( std::shared_ptr <T>* p, std::shared_ptr <T> r );
(4) (since C++11)
(deprecated in C++20)
(removed in C++26)
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)
(removed in C++26)
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)
(removed in C++26)
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)
(removed in C++26)
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)
(removed in C++26)
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)
(removed in C++26)
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)
(removed in C++26)
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)
(removed in C++26)

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 , if mo is std::memory_order_release or std::memory_order_acq_rel , the behavior is undefined.
4) Equivalent to atomic_store_explicit(p, r, std::memory_order_seq_cst ).
5) Stores the shared pointer r in the shared pointer pointed-to by p atomically, as if by p->swap(r).
As with the non-specialized std::atomic_store_explicit , if mo is std::memory_order_acquire or std::memory_order_acq_rel , the behavior is undefined.
6) Equivalent to atomic_exchange_explicit(p, r, std::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, as if by 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 ).
10,11) 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 expected is a null pointer, or failure is std::memory_order_release or std::memory_order_acq_rel , the behavior is undefined.

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

Contents

[edit] 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

[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.

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)
(until C++26)

These functions were removed in favor of the specializations of the std::atomic template: std::atomic <std::shared_ptr > and std::atomic <std::weak_ptr >.

(since C++26)

[edit] Example

This section is incomplete
Reason: no example

[edit] 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 2172 C++11 expected could be a null pointer the behavior is undefined in this case
LWG 2980 C++11 empty shared_ptrs were never equivalent equivalent if they store the same pointer value

[edit] See also

checks if the atomic type's operations are lock-free
(function template) [edit]
atomically replaces the value of the atomic object with a non-atomic argument
(function template) [edit]
atomically obtains the value stored in an atomic object
(function template) [edit]
atomically replaces the value of the atomic object with non-atomic argument and returns the old value of the atomic
(function template) [edit]
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) [edit]

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