Implementing a trait for type vs type's reference

help
1

what is the difference between implementing a triat for type's
reference

impl<'a> BitOr for &'a Json {
    type Output=Json;

    fn bitor(self, rhs: &Json) -> Json {
    }
}

and for the type itself.

impl BitOr for Json {
    type Output=Json;

    fn bitor(&self, rhs: &Json) -> Json {
    }
}

Thanks,

2

BitOr::bitor is defined to only take self, so you can't write it this way at all. Your implementation signature must match the trait. So if you want to enable BitOr on a &Json, your first way is the only option.

If you control the trait too, then you have a design decision whether to take self or &self. This will depend in part whether you think it ever makes sense to call the method with self by value, then it should be defined that way. If the method never needs the value, then &self is fine.

Note also that method calls usually will auto-ref/deref, so a call on a value can automatically turn into a call on its reference. With operator traits like BitOr though, you don't get this behavior when it is used with the | operator. That is, using direct values json_a | json_b won't work if the trait is defined on references.

2 Likes
3

Thanks for the really good description.

4

Did an inventory of all operations that are defined as trait.

Traits on Arithmetic and bitwise-operations are defined over Self. Listed:
Neg, Not, Mul, Div, Rem, Add, Sub, Shr, Shl, BitAnd, BitXor, BitOr

Assign-variants of arithmetic and bitwise operation take mutable-reference.

Index and IndexMut are correspondingly defined over &Self and &mut Self

Meanwhile all comparision operation < <= > >= == != take
operands by shared borrow.

Deref and DerefMut use shared borrow and mutable borrow respectively.
Index and IndexMut use shared borrow and mutable borrow respectively.

1 Like