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Difference between revisions of "cpp/memory/shared ptr/atomic"

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< cpp‎ | memory‎ | shared ptr
m (apply P2869R3 (C++26): Remove Deprecated shared_ptr Atomic Access APIs)
(Added LWG issue #2172 DR.)
 
(3 intermediate revisions by 2 users not shown)
Line 79: Line 79:
 
@4@ Equivalent to {{c|atomic_store_explicit(p, r, std::memory_order_seq_cst)}}.
 
@4@ Equivalent to {{c|atomic_store_explicit(p, r, std::memory_order_seq_cst)}}.
  
−
@5@ Stores the shared pointer {{c|r}} in the shared pointer pointed-to by {{c|p}} atomically, equivalent to {{c|p->swap(r)}}.
+
@5@ Stores the shared pointer {{c|r}} in the shared pointer pointed-to by {{c|p}} atomically, {{lt|cpp/language/as if}} by {{c|p->swap(r)}}.
 
@@ As with the non-specialized {{lc|std::atomic_store_explicit}}, if {{c|mo}} is {{c|std::memory_order_release}} or {{c|std::memory_order_acq_rel}}, the behavior is undefined.
 
@@ As with the non-specialized {{lc|std::atomic_store_explicit}}, if {{c|mo}} is {{c|std::memory_order_release}} or {{c|std::memory_order_acq_rel}}, the behavior is undefined.
  
 
@6@ Equivalent to {{c|atomic_exchange_explicit(p, r, std::memory_order_seq_cst)}}.
 
@6@ Equivalent to {{c|atomic_exchange_explicit(p, r, std::memory_order_seq_cst)}}.
  
−
@7@ Stores the shared pointer {{c|r}} in the shared pointer pointed to by {{c|p}} and returns the value formerly pointed-to by {{c|p}}, atomically. Equivalent to {{c|p->swap(r)}} and returns a copy of {{c|r}} after the swap.
+
@7@ Stores the shared pointer {{c|r}} in the shared pointer pointed to by {{c|p}} and returns the value formerly pointed-to by {{c|p}}, atomically, {{lt|cpp/language/as if}} by {{c|p->swap(r)}} and returns a copy of {{c|r}} after the swap.
  
 
@8@ Equivalent to
 
@8@ Equivalent to
Line 128: Line 128:
 
The [[cpp/experimental/concurrency|Concurrency TS]] offers atomic smart pointer classes {{tt|atomic_shared_ptr}} and {{tt|atomic_weak_ptr}} as a replacement for the use of these functions.
 
The [[cpp/experimental/concurrency|Concurrency TS]] offers atomic smart pointer classes {{tt|atomic_shared_ptr}} and {{tt|atomic_weak_ptr}} as a replacement for the use of these functions.
  
−
{{rrev|since=c++20|
+
{{rrev multi|since1=c++20|until1=c++26|rev1=
 
These functions were deprecated in favor of the specializations of the {{lc|std::atomic}} template: {{c/core|std::atomic<std::shared_ptr>}} and {{c/core|std::atomic<std::weak_ptr>}}.
 
These functions were deprecated in favor of the specializations of the {{lc|std::atomic}} template: {{c/core|std::atomic<std::shared_ptr>}} and {{c/core|std::atomic<std::weak_ptr>}}.
 +
|rev2=
 +
These functions were removed in favor of the specializations of the {{lc|std::atomic}} template: {{c/core|std::atomic<std::shared_ptr>}} and {{c/core|std::atomic<std::weak_ptr>}}.
 
}}
 
}}
  
Line 137: Line 139:
 
===Defect reports===
 
===Defect reports===
 
{{dr list begin}}
 
{{dr list begin}}
−
{{dr list item|wg=lwg|dr=2980|std=C++11|before=empty {{tt|shared_ptr}}s are never equivalent|after=equivalent if they store the same pointer value}}
+
{{dr list item|wg=lwg|dr=2172|std=C++11|before={{c|expected}} could be a null pointer|after=the behavior is undefined in this case}}
 +
{{dr list item|wg=lwg|dr=2980|std=C++11|before=empty {{tt|shared_ptr}}s were never equivalent|after=equivalent if they store the same pointer value}}
 
{{dr list end}}
 
{{dr list end}}
  

Latest revision as of 23:04, 20 October 2024

 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
Dynamic memory management
Uninitialized memory algorithms
Constrained uninitialized memory algorithms
Allocators
Garbage collection support
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)



 
std::shared_ptr
Member functions
Modifiers
Observers
(C++17)
(until C++20*)
(C++26)
(C++26)
Non-member functions
(C++20)
(C++20)
(C++17)
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)
atomic_xxx
functions (until C++26*)
Helper classes
(C++20)
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)
(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_release 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

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