CODEWITHSUNDEEP
rustreferencepass-by-referencepass-by-value
Andrew Moffat
Andrew Moffat

Reputation: 425

Is it more conventional to pass-by-value or pass-by-reference when the method needs ownership of the value?

When I'm passing a object by reference to a struct's new() method, and the struct will own the object, is it more conventional to:

I can think of pros and cons of each in terms of clarity and separation-of-concerns.

#[derive(Clone)]
struct MyState;

struct MyStruct {
    state: MyState,
}

impl MyStruct {
    pub fn new_by_ref(state: &MyState) -> Self {
        MyStruct {
            state: state.to_owned(),
        }
    }

    pub fn new_by_val(state: MyState) -> Self {
        MyStruct { state }
    }
}

fn main() {
    let state1 = MyState;
    let struct1 = MyStruct::new_by_ref(&state1);

    let state2 = MyState;
    let struct2 = MyStruct::new_by_val(state2.clone());
}

Upvotes: 17

Views: 3836

Answers (1)

Shepmaster
Shepmaster

Reputation: 430801

Pass by value.

This way, the program can avoid unnecessarily doubly-allocating the value if the caller no longer needs it.

In many cases, I recommend accepting anything that can be made into the owned type. This is easily demonstrated with String:

struct MyStruct {
    state: String,
}

impl MyStruct {
    fn new(state: impl Into<String>) -> Self {
        let state = state.into();
        MyStruct { state }
    }
}

fn main() {
    let struct1 = MyStruct::new("foo");
    let struct2 = MyStruct::new(String::from("bar"));
}

See also:

Upvotes: 14

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