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 multiple times. I want each call of foo use "unique" value of local_id. Is it ok with the above code? I wonder if second thread assign the value of id to local_id before the value is increased by the first thread. If it is not safe, is there any standard solution for this?

Answer 1

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;
}