#define ATOMIC_FLAG_INIT /* implementation-defined */
(since C++11)Defines the initializer which can be used to initialize std::atomic_flag to clear (false) state in the form std::atomic_flag v = ATOMIC_FLAG_INIT;. It is unspecified if it can be used with other initialization contexts.
If the flag has is a complete object with 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.
(until C++20)This macro is no longer needed since default constructor of std::atomic_flag initializes it to clear state. It is kept for the compatibility with C.
(since C++20) [edit] Example#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
}[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 2159 C++11 it was unclear whetherATOMIC_FLAG_INIT
ATOMIC_FLAG_INIT was deprecated, but needed in C on some platforms it is undeprecated [edit] See also
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