The difference btween std::atomic and std::mutex

Question

how to use std::atomic<>

In the question above, obviously we can just use std::mutex to keep thread safety. I want to know when to use which one.

classs A
{
    std::atomic<int> x;

public:
    A()
    {
        x=0;
    }

    void Add()
    {
        x++;
    }

    void Sub()
    {
        x--;
    }     
};

and

std::mutex mtx;
classs A
{
    int x;

public:
    A()
    {
        x=0;
    }

    void Add()
    {
        std::lock_guard<std::mutex> guard(mtx);
        x++;
    }

    void Sub()
    {
        std::lock_guard<std::mutex> guard(mtx);
        x--;
    }     
};

Answer 1

As a rule of thumb, use std::atomic for POD types where the underlying specialisation will be able to use something clever like a bus lock on the CPU (which will give you no more overhead than a pipeline dump), or even a spin lock. On some systems, an int might already be atomic, so std::atomic<int> will specialise out effectively to an int.

Use std::mutex for non-POD types, bearing in mind that acquiring a mutex is at least an order of magnitude slower than a bus lock.

If you're still unsure, measure the performance.