ATOMIC_FLAG_INIT

From Cppreference

Defines the expression which can be used to initialize std::atomic_flag to clear state. If the flag has static storage duration, this initialization is static.

[edit] Example

#include <atomic>
 
std::atomic_flag static_flag = ATOMIC_FLAG_INIT;
 
int main()
{
    std::atomic_flag automatic_flag = ATOMIC_FLAG_INIT;
    std::atomic_flag another_flag(ATOMIC_FLAG_INIT);
}

[edit] See also