Namespaces
Variants
Views
Actions

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

From cppreference.com
< cpp‎ | memory‎ | shared ptr
(+)
 
(Added LWG issue #2172 DR.)
 
(31 intermediate revisions by 12 users not shown)
Line 1: Line 1:
−
{{cpp/title | atomic_is_lock_free | atomic_load | atomic_store | atomic_...}}
+
{{cpp/title|atomic_...{{dsc small|<std::shared_ptr>}}}}
 
{{cpp/memory/shared_ptr/navbar}}
 
{{cpp/memory/shared_ptr/navbar}}
−
{{ddcl list begin}}
+
 
−
{{ddcl list item | num=1 | notes={{mark since c++11}} | 1=  
+
{{dcl begin}}
 +
{{dcl header|memory}}
 +
{{dcl|num=1|since=c++11|deprecated=c++20|until=c++26|1=  
 
template< class T >
 
template< class T >
 
bool atomic_is_lock_free( const std::shared_ptr<T>* p );
 
bool atomic_is_lock_free( const std::shared_ptr<T>* p );
 
}}
 
}}
−
{{ddcl list item | num=2 | notes={{mark since c++11}} | 1=  
+
{{dcl|num=2|since=c++11|deprecated=c++20|until=c++26|1=  
 
template< class T >
 
template< class T >
 
std::shared_ptr<T> atomic_load( const std::shared_ptr<T>* p );
 
std::shared_ptr<T> atomic_load( const std::shared_ptr<T>* p );
 
}}
 
}}
−
{{ddcl list item | num=3 | notes={{mark since c++11}} | 1=  
+
{{dcl|num=3|since=c++11|deprecated=c++20|until=c++26|1=  
 
template< class T >
 
template< class T >
−
std::shared_ptr<T> atomic_load_explicit( const shared_ptr<T>* p,
+
std::shared_ptr<T> atomic_load_explicit
−
                                        std::memory_order mo );
+
    ( const std::shared_ptr<T>* p, std::memory_order mo );
 
}}
 
}}
−
{{ddcl list item | num=4 | notes={{mark since c++11}} | 1=  
+
{{dcl|num=4|since=c++11|deprecated=c++20|until=c++26|1=  
 
template< class T >
 
template< class T >
−
void atomic_store( std::shared_ptr<T>* p,
+
void atomic_store( std::shared_ptr<T>* p, std::shared_ptr<T> r );
−
                  std::shared_ptr<T> r );
+
 
}}
 
}}
−
{{ddcl list item | num=5 | notes={{mark since c++11}} | 1=  
+
{{dcl|num=5|since=c++11|deprecated=c++20|until=c++26|1=  
 
template< class T >
 
template< class T >
−
void atomic_store_explicit( std::shared_ptr<T>* p,
+
void atomic_store_explicit
−
                            shared_ptr<T> r,
+
    ( std::shared_ptr<T>* p, std::shared_ptr<T> r,
−
                            std::memory_order mo);
+
      std::memory_order mo );
 
}}
 
}}
−
{{ddcl list item | num=6 | notes={{mark since c++11}} | 1=  
+
{{dcl|num=6|since=c++11|deprecated=c++20|until=c++26|1=  
 
template< class T >
 
template< class T >
−
std::shared_ptr<T> atomic_exchange( std::shared_ptr<T>* p,
+
std::shared_ptr<T> atomic_exchange
−
                                    std::shared_ptr<T> r);
+
    ( std::shared_ptr<T>* p, std::shared_ptr<T> r );
 
}}
 
}}
−
{{ddcl list item | num=7 | notes={{mark since c++11}} | 1=  
+
{{dcl|num=7|since=c++11|deprecated=c++20|until=c++26|1=  
−
template<class T>
+
template< class T >
−
std::shared_ptr<T> atomic_exchange_explicit( std::shared_ptr<T>* p,
+
std::shared_ptr<T> atomic_exchange_explicit
−
                                            std::shared_ptr<T> r,
+
    ( std::shared_ptr<T>* p, std::shared_ptr<T> r,
−
                                            std::memory_order mo);
+
      std::memory_order mo );
 
}}
 
}}
−
{{ddcl list item | num=8 | notes={{mark since c++11}} | 1=  
+
{{dcl|num=8|since=c++11|deprecated=c++20|until=c++26|1=  
 
template< class T >
 
template< class T >
−
bool atomic_compare_exchange_weak( std::shared_ptr<T>* p,
+
bool atomic_compare_exchange_weak
−
                                  std::shared_ptr<T>* v,
+
    ( std::shared_ptr<T>* p, std::shared_ptr<T>* expected,
−
                                  std::shared_ptr<T> w);
+
      std::shared_ptr<T> desired );
 
}}
 
}}
−
{{ddcl list item | num=9 | notes={{mark since c++11}} | 1=  
+
{{dcl|num=9|since=c++11|deprecated=c++20|until=c++26|1=  
−
template<class T>
+
template< class T >
−
bool atomic_compare_exchange_strong( std::shared_ptr<T>* p,
+
bool atomic_compare_exchange_strong
−
                                    std::shared_ptr<T>* v,
+
    ( std::shared_ptr<T>* p, std::shared_ptr<T>* expected,
−
                                    std::shared_ptr<T> w);
+
      std::shared_ptr<T> desired );
 
}}
 
}}
−
{{ddcl list item | num=11 | notes={{mark since c++11}} | 1=  
+
{{dcl|num=10|since=c++11|deprecated=c++20|until=c++26|1=  
−
bool atomic_compare_exchange_strong_explicit( std::shared_ptr<T>* p,
+
template< class T >
−
                                              std::shared_ptr<T>* v,
+
bool atomic_compare_exchange_strong_explicit
−
                                              std::shared_ptr<T> w,
+
    ( std::shared_ptr<T>* p, std::shared_ptr<T>* expected,
−
                                              std::memory_order success,
+
      std::shared_ptr<T> desired,
−
                                              std::memory_order failure);
+
      std::memory_order success, std::memory_order failure );
 
}}
 
}}
−
{{ddcl list item | num=10 | notes={{mark since c++11}} | 1=  
+
{{dcl|num=11|since=c++11|deprecated=c++20|until=c++26|1=  
−
template<class T>
+
template< class T >
−
bool atomic_compare_exchange_weak_explicit( std::shared_ptr<T>* p,
+
bool atomic_compare_exchange_weak_explicit
−
                                            std::shared_ptr<T>* v,
+
    ( std::shared_ptr<T>* p, std::shared_ptr<T>* expected,
−
                                            std::shared_ptr<T> w,
+
      std::shared_ptr<T> desired,
−
                                            std::memory_order success,
+
      std::memory_order success, std::memory_order failure );
−
                                            std::memory_order failure);
+
−
template<class T>
+
 
}}
 
}}
−
{{ddcl list end}}
+
{{dcl end}}
  
−
If multiple threads of execution access the object referenced by the same {{c|std::shared_ptr}} simultaneously, a data race occurs, unless all such access is performed through these functions, which are partial specializations of the corresponding atomic access functions ({{c|std::atomic_load}}, {[c|std::atomic_store}}, etc)
+
If multiple threads of execution access the same {{lc|std::shared_ptr}} object without synchronization and any of those accesses uses a non-const member function of {{tt|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 ({{lc|std::atomic_load}}, {{lc|std::atomic_store}}, etc.).
  
−
Note that the control block of a shared_ptr is thread-safe: different {{c|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.
+
Note that the control block of a {{tt|shared_ptr}} is thread-safe: different {{lc|std::shared_ptr}} objects can be accessed using mutable operations, such as {{c/core|1=operator=}} or {{tt|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 {{tt|p}} is lock-free.
+
@1@ Determines whether atomic access to the shared pointer pointed-to by {{c|p}} is lock-free.
−
@2@ Equivalent to {{c|atomic_load_explicit(p, std::memory_order_seq_cst)}}
+
−
@3@ Returns the shared pointer pointed-to by {{tt|p}}. As with the non-specialized {{c|std::atomic_load_explicit}}, {{tt|mo}} cannot be {{c|std::memory_order_release}} or {{c|std::memory_order_acq_rel}}
+
−
@4@ Equivalent to {{c|atomic_store_explicit(p, r, memory_order_seq_cst)}}
+
−
@5@ Swaps the shared pointers {{tt|p}} and {{tt|r}}, effectively executing {{c|p->swap(r)}}. As with the non-specialized {{c|std::atomic_store_explicit}}, {{tt|mo}} cannot be {{c|std::memory_order_acquire}} or {{c|std::memory_order_acq_rel}}
+
−
@6@ Equivalent to {{c|atomic_exchange_explicit(p, r, memory_order_seq_cst)}}
+
−
@7@ Swaps the shared pointers {{tt|p}} and {{tt|r}}, effectively executing {{c|p->swap(r)}} and returns a copy of the shared pointer formerly pointed-to by {{tt|p}}
+
−
@8@ Equivalent to {{c|atomic_compare_exchange_weak_explicit(p, v, w, std::memory_order_seq_cst, std::memory_order_seq_cst)}}
+
−
@9@ Equivalent to {{c|atomic_compare_exchange_strong_explicit(p, v, w, std::memory_order_seq_cst, std::memory_order_seq_cst)}}
+
−
@10@ Compares the shared pointers pointed-to by {{tt|p}} and {{tt|v}}. If they are equivalent (share ownership of the same pointer and refer to the same pointer), assigns {{tt|w}} into {{tt|*p}} using the memory ordering constraints specified by {{tt|success}} and returns {{c|true}}. If they are not equivalent, assigns {{tt|*p}} into {{tt|*v}} using the memory ordering constraints specified by {{tt|failure}} and returns {{c|false}}.
+
−
@11@ Same as 10), but may fail spuriously.
+
  
−
All these functions invoke undefined behavior if {{tt|p}} is a null pointer.
+
@2@ Equivalent to {{c|atomic_load_explicit(p, std::memory_order_seq_cst)}}.
 +
 
 +
@3@ Returns the shared pointer pointed-to by {{c|p}}.
 +
@@ As with the non-specialized {{lc|std::atomic_load_explicit}}, if {{c|mo}} is {{c|std::memory_order_release}} or {{c|std::memory_order_acq_rel}}, the behavior is undefined.
 +
 
 +
@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, {{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.
 +
 
 +
@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, {{lt|cpp/language/as if}} by {{c|p->swap(r)}} and returns a copy of {{c|r}} after the swap.
 +
 
 +
@8@ Equivalent to
 +
@@ {{c multi
 +
|atomic_compare_exchange_weak_explicit
 +
|    (p, expected, desired, std::memory_order_seq_cst,
 +
|                          std::memory_order_seq_cst)}}.
 +
 
 +
@9@ Equivalent to
 +
@@ {{c multi
 +
|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 {{c|p}} and {{c|expected}}.
 +
* If they are equivalent (store the same pointer value, and either share ownership of the same object or are both empty), assigns {{c|desired}} into {{c|*p}} using the memory ordering constraints specified by {{c|success}} and returns {{c|true}}.
 +
* If they are not equivalent, assigns {{c|*p}} into {{c|*expected}} using the memory ordering constraints specified by {{c|failure}} and returns {{c|false}}.
 +
@@ {{tt|atomic_compare_exchange_weak_explicit}} may fail spuriously.
 +
@@ If {{c|expected}} is a null pointer, or {{c|failure}} is {{c|std::memory_order_release}} or {{c|std::memory_order_acq_rel}}, the behavior is undefined.
 +
 
 +
If {{c|p}} is a null pointer, the behaviors of these functions are all undefined.
  
 
===Parameters===
 
===Parameters===
−
{{param list begin}}
+
{{par begin}}
−
{{param list item | p, v | a pointer to a {{c|std::shared_ptr}} }}
+
{{par|p, expected|a pointer to a {{lc|std::shared_ptr}}}}
−
{{param list item | r, w | a {{c|std::shared_ptr}} }}
+
{{par|r, desired|a {{lc|std::shared_ptr}}}}
−
{{param list item | mo, success, failure | memory ordering selectors of type {{c|std::memory_order}} }}
+
{{par|mo, success, failure|memory ordering selectors of type {{lc|std::memory_order}}}}
−
{{param list end}}
+
{{par end}}
  
 
===Exceptions===
 
===Exceptions===
 
These functions do not throw exceptions.
 
These functions do not throw exceptions.
  
−
===Return===
+
===Return value===
−
@1@ {{c|true}} if atomic access is implemented using lock-free instructions
+
@1@ {{c|true}} if atomic access is implemented using lock-free instructions.
 
@2,3@ A copy of the pointed-to shared pointer.
 
@2,3@ A copy of the pointed-to shared pointer.
−
@4,5@ nothing
+
@4,5@ (none)
−
@6,7@ A copy of the formerly pointed-to shared pointer
+
@6,7@ A copy of the formerly pointed-to shared pointer.
−
@8,9,10,11@ {{c|true}} if the shared pointers were equivalent and the exchange was performed, {{c|false}} otherwise.
+
@8-11@ {{c|true}} if the shared pointers were equivalent and the exchange was performed, {{c|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 [[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 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>}}.
 +
|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>}}.
 +
}}
 +
 
 +
===Example===
 +
{{example}} <!-- adapt from http://stackoverflow.com/a/16617935/273767 -->
 +
 
 +
===Defect reports===
 +
{{dr list begin}}
 +
{{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}}
  
 
===See also===
 
===See also===
−
{{dcl list begin}}
+
{{dsc begin}}
−
{{dcl list template | cpp/atomic/dcl list atomic_is_lock_free}}
+
{{dsc inc|cpp/atomic/dsc atomic_is_lock_free}}
−
{{dcl list template | cpp/atomic/dcl list atomic_store}}
+
{{dsc inc|cpp/atomic/dsc atomic_store}}
−
{{dcl list template | cpp/atomic/dcl list atomic_load}}
+
{{dsc inc|cpp/atomic/dsc atomic_load}}
−
{{dcl list template | cpp/atomic/dcl list atomic_exchange}}
+
{{dsc inc|cpp/atomic/dsc atomic_exchange}}
−
{{dcl list template | cpp/atomic/dcl list atomic_compare_exchange}}
+
{{dsc inc|cpp/atomic/dsc atomic_compare_exchange}}
−
{{dcl list end}}
+
{{dsc end}}
 +
 
 +
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

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]
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/memory/shared_ptr/atomic&oldid=177196"