Defined in header <atomic> | ||
---|---|---|
(1) | (since C++11) | |
template< class T > bool atomic_is_lock_free( const volatile std::atomic<T>* obj ) noexcept; | ||
template< class T > bool atomic_is_lock_free( const std::atomic<T>* obj ) noexcept; | ||
#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 */ | (2) | (since C++11) |
#define ATOMIC_CHAR8_T_LOCK_FREE /* unspecified */ | (3) | (since C++20) |
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 pointers of the same type. 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.obj | - | pointer to the atomic object to examine |
true
if *obj
is a lock-free atomic, false
otherwise.
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 subarchitectures 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.
#include <iostream> #include <utility> #include <atomic> struct A { int a[100]; }; 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
checks if the atomic object is lock-free (public member function of std::atomic<T> ) |
|
specializes atomic operations for std::shared_ptr (function template) |
|
(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> ) |
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