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# std::atomic_is_lock_free, ATOMIC_xxx_LOCK_FREE

From cppreference.com
< cpp‎ | atomic

Defined in header `<atomic>` |  |
| (1) | (since C++11)
template< class T > bool atomic_is_lock_free( const volatile std::atomic<T>* obj ); |  |
template< class T > bool atomic_is_lock_free( const std::atomic<T>* obj ); |  |
#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)

1) 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 pointers of the same type.
2) 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.

### [edit] Parameters

obj | - | pointer to the atomic object to examine

### [edit] Return value

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

### [edit] Exceptions

`noexcept` specification:
noexcept

### [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, e.g. if only aligned memory accesses are naturally atomic on a given architecture, misaligned objects of the same type have to use locks.

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

Run this code

    #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

### [edit] See also

is_lock_free | checks if the atomic object is lock-free (public member function of `std::atomic`) [edit]
std::atomic_is_lock_free(std::shared_ptr) | specializes atomic operations for std::shared_ptr (function template)
atomic_flag (C++11) | the lock-free boolean atomic type (class) [edit]
is_always_lock_free [static] (C++17) | indicates that the type is always lock-free (public static member constant of `std::atomic`) [edit]
C documentation for atomic_is_lock_free
C documentation for ATOMIC_*_LOCK_FREE

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