ATOMIC_FLAG_INIT
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ATOMIC_FLAG_INIT
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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 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
Run this code
#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 whether ATOMIC_FLAG_INITcan be used with other initialization contexts |
other usages are not guaranteed |
| LWG 3659 | C++20 | ATOMIC_FLAG_INIT was deprecated, but needed in C on some platforms
|
it is undeprecated |
[edit] See also
C documentation for ATOMIC_FLAG_INIT