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

 

C++

Compiler support

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Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Three-way comparison (C++20) three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)    is_eqis_neqis_ltis_lteqis_gtis_gteq (C++20)(C++20)(C++20)(C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20) bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator> rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)    cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17)

 

Dynamic memory management

Uninitialized memory algorithms uninitialized_copy uninitialized_fill uninitialized_move (C++17) uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) destroy (C++17) destroy_at (C++17) voidify (C++20) uninitialized_copy_n (C++11) uninitialized_fill_n uninitialized_move_n (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17) destroy_n (C++17) construct_at (C++20) Constrained uninitialized memory algorithms ranges::uninitialized_copy (C++20) ranges::uninitialized_fill (C++20) ranges::uninitialized_move (C++20) ranges::construct_at (C++20) ranges::destroy (C++20) ranges::destroy_n (C++20) ranges::destroy_at (C++20) ranges::uninitialized_copy_n (C++20) ranges::uninitialized_fill_n (C++20) ranges::uninitialized_move_n (C++20) ranges::uninitialized_default_construct (C++20) ranges::uninitialized_value_construct (C++20) ranges::uninitialized_default_construct_n (C++20)   ranges::uninitialized_value_construct_n (C++20) Allocators allocator allocator_traits (C++11) allocation_result (C++23) allocator_arg (C++11) uses_allocator (C++11) uses_allocator_construction_args (C++20) make_obj_using_allocator (C++20) uninitialized_construct_using_allocator (C++20) scoped_allocator_adaptor (C++11) pmr::polymorphic_allocator (C++17) Memory resources pmr::memory_resource (C++17) pmr::get_default_resource (C++17) pmr::set_default_resource (C++17) pmr::new_delete_resource (C++17) pmr::null_memory_resource (C++17) pmr::synchronized_pool_resource (C++17) pmr::unsynchronized_pool_resource (C++17)   pmr::monotonic_buffer_resource (C++17) pmr::pool_options (C++17) Garbage collection support declare_reachable (C++11)(until C++23) undeclare_reachable (C++11)(until C++23) declare_no_pointers (C++11)(until C++23) undeclare_no_pointers (C++11)(until C++23) pointer_safety (C++11)(until C++23) get_pointer_safety (C++11)(until C++23) Uninitialized storage raw_storage_iterator (until C++20*) get_temporary_buffer (until C++20*) return_temporary_buffer (until C++20*) Explicit lifetime management start_lifetime_asstart_lifetime_as_array (C++23)(C++23)

Smart pointers unique_ptr (C++11) shared_ptr (C++11) weak_ptr (C++11) auto_ptr (until C++17*) owner_less (C++11) owner_less<void> (C++17) enable_shared_from_this (C++11) bad_weak_ptr (C++11) default_delete (C++11) out_ptr_t (C++23) inout_ptr_t (C++23) Low level memory management operator new operator new[] operator delete operator delete[] get_new_handler set_new_handler launder (C++17) bad_alloc bad_array_new_length nothrow_t align_val_t destroying_delete_t new_handler nothrow Miscellaneous pointer_traits (C++11) to_address (C++20) addressof (C++11) align (C++11) assume_aligned (C++20) C Library malloc calloc realloc    aligned_alloc (C++17) free

 

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

Non-member functions

std::swap

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 (deprecated in C++20)

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

std::hash

Deduction guides(C++17)

 

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)
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)

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

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)