Thread safe increment of static local variable

Question

void foo() {
    static int id = 0;
    const int local_id = id++;
    //do something with local_id;
}

Multiple threads can call foo in parallel mu

Answer 1

Your code is not thread safe because two threads may increment id at the same time.

Use mutual exclusion or std::atomic for the shared id variable.

Answer 2

Your code is not thread-safe, because multiple threads can read id concurrently, and producing the same value of local_id.

If you want a thread-safe version, use std::atomic_int, which is available in C++11:

void foo() {
    static std::atomic_int id;
    const int local_id = id++;
    //do something with local_id;
}