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Defined in header
template<classT>boolatomic_is_lock_free(constvolatilestd::atomic<T>*obj)noexcept; (1) (since C++11)template<classT>boolatomic_is_lock_free(conststd::atomic<T>*obj)noexcept; (2) (since C++11)#define ATOMIC_BOOL_LOCK_FREE /* unspecified */#define ATOMIC_CHAR_LOCK_FREE /* unspecified */#define ATOMIC_CHAR16_T_LOCK_FREE /* unspecified */#define ATOMIC_CHAR32_T_LOCK_FREE /* unspecified */#define ATOMIC_WCHAR_T_LOCK_FREE /* unspecified */#define ATOMIC_SHORT_LOCK_FREE /* unspecified */#define ATOMIC_INT_LOCK_FREE /* unspecified */#define ATOMIC_LONG_LOCK_FREE /* unspecified */#define ATOMIC_LLONG_LOCK_FREE /* unspecified */#define ATOMIC_POINTER_LOCK_FREE /* unspecified */ (3) (since C++11)#define ATOMIC_CHAR8_T_LOCK_FREE /* unspecified */ (4) (since C++20)1,2) Determines if the atomic object pointed to by obj is implemented lock-free, as if by calling obj->is_lock_free(). In any given program execution, the result of the lock-free query is the same for all atomic objects of the same type.
3,4) Expands to an integer constant expression with value
0 for the built-in atomic types that are never lock-free,
1 for the built-in atomic types that are sometimes lock-free,
2 for the built-in atomic types that are always lock-free.
Parameters
obj - pointer to the atomic object to examine Return value
true if *obj is a lock-free atomic, false otherwise.
Notes
All atomic types except for
may be implemented using mutexes or other locking operations, rather than using the lock-free atomic CPU instructions. Atomic types are also allowed to be sometimes lock-free: for example, if only some sub-architectures support lock-free atomic access for a given type (such as the
instruction on x86-64), whether atomics are lock-free may not be known until runtime.
The C++ standard recommends (but does not require) that lock-free atomic operations are also address-free, that is, suitable for communication between processes using shared memory.
Example
Run this code
#include<atomic>#include<iostream>#include<utility>structA{inta[4];};structB{intx,y;};intmain(){std::atomic<A>a;std::atomic<B>b;std::cout<<std::boolalpha<<"std::atomic<A> is lock free? "<<std::atomic_is_lock_free(&a)<<'\n'<<"std::atomic<B> is lock free? "<<std::atomic_is_lock_free(&b)<<'\n';}Possible output:
std::atomic<A> is lock free? false std::atomic<B> is lock free? true Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
C++11 atomic_is_lock_free was specified via pointers, which
was ambiguous and might accept invalid pointer values specified via
atomic objects See also
checks if the atomic object is lock-free
(public member function of std::atomic<T>)
(C++11)
the lock-free boolean atomic type
(class)
[static](C++17)
indicates that the type is always lock-free
(public static member constant of std::atomic<T>)