Question

Can I expect to see a performance hit for the casting in this . .

enum class myEnum {A,B,C};
myArray[(int)myEnum::A] = 123;

Compared to this?

enum myEnum {A,B,C};
myArray[A] = 123;

I’m leaning towards the new style enum classes for the type safety, but don’t want to do it at the expense of performance.

Answer 1

It depends whether the enum value used as an index is known at compile time or passed in a variable.

That is myArray[(int)myEnum::A] shall not incur any penalty but myArray[(int)e] might, depending on the physical representation of e (ie, it might be necessary to “extend” it).

On the other hand, a simple extension is a trivial operation that is unlikely to ever show up as a performance issues: things like branch prediction (in conditionals) and caching are much more important in most applications (for low-level), and at a higher level algorithms matter.

Note: to avoid the extension issue in the runtime scenario, you could define the base type of myEnum to be the natural type that is expected for compiler arithmetic, I believe a ptrdiff_t would be most appropriate here. It is a big integer though.