State machine with callback functions

I have a class contain multiple callbacks and a state variable s.

class foo
{
    public:
    void callback_1();
    void callback_2();
    ...
    private:
    State m_s;
}

Each callback function is connected to an external event through something similar to the boost or Qt's signal-slot.

Whether a callback shall be executed depends on the current state m_s, and each callback function has the right to modify the m_s according to a state machine.

What is the best coding practice/pattern that can be adopted to write clean, modular code for this kind of state machine?