Can a non-generic struct implement a generic function in rust?

Question

Can a non-generic struct implement a generic function in rust, which works like:


struct S;

impl <T> S {
    fn do_something(value : T) {
        //do something
    }
}

fn main() {
    let a = /*a value*/;
    S::do_something(a);
}

If not, assuming S implement a generic trait Tt:


struct S;

impl <T> Tt<T> for S {
    fn a_func(value : T) {
        //do something
    }
}

impl <T> S {
    fn do_something(value : T) {
        //do something
    }
}

fn main() {
    let a = /*a value*/;
    S::do_something(a);
}

Is there any way to make it works?

how about defining generic parameter for fn instead of struct ? — Ömer Erden
– Ömer Erden, Commented Jul 4, 2019 at 15:41

Peter Hall · Accepted Answer · 2019-07-04 15:49:21Z

1

You can only declare a type variable for an impl block if it is used in the type itself.

However, you can introduce new type variables on individual methods too:

impl S {
    fn do_something<T>(value: T) {
        //do something
    }
}

This is particularly common for arguments that are closures, because every closure has a different type, so this is necessary for you to be able to call the method with a different closure each time. For example, Iterator::map is defined like this:

fn map<B, F>(self, f: F) -> Map<Self, F>
where
    F: FnMut(Self::Item) -> B,

If the F was declared for the type instead of the method then you would have to use the same closure every time you call map - not very useful.

answered Jul 4, 2019 at 15:49

Peter Hall

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