< cpp‎ | atomic
Defined in header <atomic>
#define ATOMIC_FLAG_INIT /* implementation-defined */

Defines the expression which can be used to initialize std::atomic_flag to clear (false) state with the statement std::atomic_flag v = ATOMIC_FLAG_INIT;. It is unspecified if it can be used with other initialization contexts.

If the flag has static storage duration, this initialization is static.

This is the only way to initialize std::atomic_flag to a definite value: the value held after any other initialization is unspecified.


#include <atomic>
std::atomic_flag static_flag = ATOMIC_FLAG_INIT; // static initialization,
// guaranteed to be available during dynamic initialization of static objects.
int main()
    std::atomic_flag automatic_flag = ATOMIC_FLAG_INIT; // guaranteed to work
//    std::atomic_flag another_flag(ATOMIC_FLAG_INIT); // unspecified

See also

the lock-free boolean atomic type