Why doesn't C/C++ have an atomic flag test_and_clear()?
The atomic_flag type in C/C++ provides a minimal set of lock-free atomic operations, including test_and_set() and clear(). However, there is no test_and_clear() operation defined for atomic_flag.
The reason for this is that test_and_clear() is not a necessary operation for lock-free synchronization. In fact, it can be implemented using test_and_set() and clear(), as follows:
bool test_and_clear(std::atomic_flag& flag) {
bool old_value = flag.test_and_set();
flag.clear();
return old_value;
}
Since test_and_clear() can be implemented using other atomic operations, there is no need to include it as a separate operation in the atomic_flag type.
Another reason why test_and_clear() is not included in atomic_flag is that it is not as efficient as test_and_set() and clear(). The test_and_set() operation only needs to read and write a single memory location, while the test_and_clear() operation needs to read and write two memory locations.
For these reasons, the C/C++ standard does not define a test_and_clear() operation for atomic_flag.