Is using the result of an std::atomic fetch_add to index an array still atomic?

  atomicity, c++, multithreading

So I wanted to try implementing a fixed sized, wait free stack with the fetch_add and fetch_sub atomic instructions. Say that I have a basic stack, with two operations, push and pop.

struct WaitFreeStack {
    std::atomic<size_t> cur;
    std::atomic<int>* buf;

    void push(int i) {
        buf[cur.fetch_add(1)].store(i);
    }

    int pop() {
        return buf[cur.fetch_sub(1) - 1].load();
    }
};

My question is, are operations in the form of B[X], where B is an array and X is an integer atomic ? In my example for instance, is it possible that after a fetch_add call for the push() method is executed, and before the B[X] is executed, that a whole pop and push in separate threads could be executed, causing a push to overwrite another push ?

Source: Windows Questions C++

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