c++ - Using an enum as an array index

Question

I have this enum:

enum ButtonState {
    BUTTON_NORMAL = 0,
    BUTTON_PRESSED = 1,
    BUTTON_CLICKED = 2
};

const u8 NUM_BUTTON_STATES = 3;

In my Button class I have member variables ButtonState state; and ButtonColors colors[NUM_BUTTON_STATES];. When drawing the button, I use colors[state] to get the colours for whatever state the button is in.

My questions:

1. Is this good programming style? Is there a better way to do it? (I usually only use enums with switch statements... using an enum as an array index doesn't feel right.)
2. Do I have to specify the values of the enum? It seems to start from 0 by default and increment by 1 but is it guaranteed to work that way in all compilers?

Answer 1

Is this good programming style?

I think so. I do the same thing quite frequently.

Is there a better way to do it?

class Button
{
public:
    // Used for array indexes!  Don't change the numbers!
  enum State {
    NORMAL = 0,
    PRESSED,
    CLICKED,
    NUMBER_OF_BUTTON_STATES
  };
};

Drawback is that NUMBER_OF_BUTTON_STATES is now a valid Button::State value. Not a big issue if you are passing these values around as ints. But trouble if you are actually expecting a Button::State.

Using an enum as an array index doesn't feel right.

It's fine. Just DOCUMENT it, so the next guy knows what's going on! (That's what comments are for.)

Do I have to specify the values of the enum?

With no '=' assignment, enum's are supposed to start at zero and increment upwards.

If a enum entry has an '=' assigned value, subsequent non '=' enum entries continue counting from there.

Source: The Annotated C++ Reference Manual, pg 113

That said, I like to specify the initial value just to make the code that much clearer.