Using static function variable vs class variable to store some state

Lets say I have function like this:

void processElement() {
    doSomething(someArray[lastProcessedElement + 1]);
}

The thing is, every time this function called, I need to the store the last element that I called doSomething on. So in here I have two choices:

• I can create a private class variable named lastProcessedElement and increment it's value every time that function is called. This approach seems most common. So the code can be something like this:

  class Foo {
      int lastProcessedElement = 0;     
  
      public:  
      void processElement() {
          doSomething(someArray[++lastProcessedElement]);
      }
  }
  

• As second option, I can create a static variable in function and increment it every time:

  // Class is not important here, so the function is:
  void processElement() {
      static int lastProcessedElement = 0;
      doSomething(someArray[++lastProcessedElement]);
  }
  

The first solution adds a little bit complexity which I don't want. I like to keep things in-place.

I know the second solution only works if that class have only one instance.

So using static variable method is a good solution? And is there any in-line solution to multi-instance class? (There can be a solution to this particular array element index thing, but I just made that up, I'm talking about storing some value for the next call of the function)