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

 

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Memory management library

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std::shared_ptr

Member functions

shared_ptr::shared_ptr

shared_ptr::~shared_ptr

shared_ptr::operator=

Modifiers

shared_ptr::reset

shared_ptr::swap

Observers

shared_ptr::get

shared_ptr::operator*shared_ptr::operator->

shared_ptr::operator[] (C++17)

shared_ptr::use_count

shared_ptr::unique (until C++20*)

shared_ptr::operator bool

shared_ptr::owner_before

shared_ptr::owner_hash (C++26)

shared_ptr::owner_equal (C++26)

Non-member functions

swap(std::shared_ptr)

make_sharedmake_shared_for_overwrite (C++20)

allocate_sharedallocate_shared_for_overwrite (C++20)

static_pointer_castdynamic_pointer_castconst_pointer_castreinterpret_pointer_cast (C++17)

get_deleter

operator==operator!=operator<operator<=operator>operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)

operator<<

atomic_xxx functions (until C++26*)

Helper classes

atomic<std::shared_ptr> (C++20)

hash<std::shared_ptr>

Deduction guides(C++17)

 

Defined in header <memory>
template<class T > bool atomic_is_lock_free(conststd::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(conststd::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     (conststd::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.

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.

8-11)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.

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)

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

See also

atomic_is_lock_free (C++11)	checks if the atomic type's operations are lock-free (function template)
atomic_storeatomic_store_explicit (C++11)(C++11)	atomically replaces the value of the atomic object with a non-atomic argument (function template)
atomic_loadatomic_load_explicit (C++11)(C++11)	atomically obtains the value stored in an atomic object (function template)
atomic_exchangeatomic_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_weakatomic_compare_exchange_weak_explicitatomic_compare_exchange_strongatomic_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)