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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../../cpp/atomic/atomic_is_lock_free.html below:

std::atomic_is_lock_free, ATOMIC_xxx_LOCK_FREE - cppreference.com

template< class T > bool atomic_is_lock_free( const volatile std::atomic<T>* obj ) noexcept; (1) (since C++11) template< class T > bool atomic_is_lock_free( const std::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

â0â 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.

[edit] Parameters obj - pointer to the atomic object to examine [edit] Return value

true if *obj is a lock-free atomic, false otherwise.

[edit] Notes

All atomic types except for std::atomic_flag 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 CMPXCHG16B 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.

[edit] Example

#include <atomic>
#include <iostream>
#include <utility>
 
struct A { int a[4]; };
struct B { int x, y; };
 
int main()
{
    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

[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 3249 C++11 atomic_is_lock_free was specified via pointers, which
was ambiguous and might accept invalid pointer values specified via
atomic objects [edit] See also checks if the atomic object is lock-free
(public member function of std::atomic<T>) [edit] the lock-free boolean atomic type
(class) [edit] indicates that the type is always lock-free
(public static member constant of std::atomic<T>) [edit]

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