Invalid use of non-static member function c++

Question

I am following this example. But when I compile, it returns an error:

Invalid use of non-static member function

at the line

void(Machine:: *ptrs[])() = 
  {
    Machine::off, Machine::on
  };

I tried to add static to void on(); at class

class Machine
{
  class State *current;
  public:
    Machine();
    void setCurrent(State *s)
    {
        current = s;
    }
    static void on(); // I add static here ...
    static void off(); // and here
};

But it complains that

Invalid use of member Machine::current in static member function

Can you help me fix this?

Answer 1

Unlike static member functions or free functions, non-static member functions won't implicitly convert to member function pointers.

(emphasis mine)

An lvalue of function type T can be implicitly converted to a prvalue pointer to that function. This does not apply to non-static member functions because lvalues that refer to non-static member functions do not exist.

So you need to use & explicitly to take the address of the non-static member functions (i.e. to get non-static member function pointers). e.g.

void(Machine:: *ptrs[])() = 
  {
    &Machine::off, &Machine::on
  };

If you declare them as static member function, you should change the type of ptrs (to array of non-member function pointers). Note that for static member function it's fine to not use & explicitly. e.g.

void(*ptrs[])() = 
  {
    Machine::off, Machine::on
  };