Should I use enums or boxed trait objects to emulate polymorphism?

Question

Using the enum Axes to confine Coordinate and Quaternion:

#[derive(Clone)]
pub enum Axes {
    Coordinate {
        x:          


        

Answer 1

Another difference not mentioned in @Kwarrtz's answer is memory related.

• enums can be stored directly on the stack, while a boxed trait will always require the heap. That is, enums are cheap to create, but boxed traits are not.

• an enum instance will always be as big as its biggest variant (plus a discriminant in most cases), even if you store mostly small variants. This would be a problem in a case like this:

  enum Foo {
      SmallVariant(bool),
      BigVariant([u64; 100]),
  }
  

  If you were to store N instances of this type in an vector, the vector would always need N*(100*sizeof:: + sizeOfDiscriminant) bytes of memory, even when the vector only contains SmallVariants.

  If you were using a boxed trait, the vector would use N * sizeOfFatPointer == N * 2 * sizeof::.