Defined in header <atomic>
#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 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.

(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)


#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

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 3659 C++20 ATOMIC_FLAG_INIT was deprecated, but needed in C on some platforms it is undeprecated

See also

the lock-free boolean atomic type
C documentation for ATOMIC_FLAG_INIT

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.