How do I specify type parameters as being function arguments in Rust?

Question

I'm trying to make a list that will hold Box<dyn Fn(&E)> where E is specified as part of the type. This works until E contains a reference, at which point it starts asking for lifetimes that aren't relevant.

A simpler example:

pub struct CallbackTest<E> {
    pub cb: Box<dyn Fn(&E)>,
}

impl<E> CallbackTest<E> {
    pub fn new<F>(cb: F) -> Self
    where
        F: Fn(&E)
    {
        Self { cb: Box::new(cb) }
    }
}

pub struct GameData { /* ... */ }

pub type TestRef = CallbackTest<(u32, &GameData)>;

This gives me a missing lifetime specifier error. I could put a lifetime parameter on TestRef to make it work, but that's not the correct lifetime. I don't want the &GameData to have to live for the entire lifetime of the CallbackTest, just during the function call.

EDIT: The &GameData is intentional. It's not a mistake. I hope my changes have made the goal behind this more obvious.

Any advice?

I don't know why, it does work with this way(play.rust-lang.org/…) , but i doubt it is trustworthy — Ömer Erden
– Ömer Erden, Commented Feb 6, 2020 at 7:53
As said by @Stargateur, I think that's due to the fact that you added mistakenly another reference layer to u32. In this situation, the compiler is right to ask for a lifetime. — Boiethios
– Boiethios, Commented Feb 6, 2020 at 9:55
The &u32 was intentional. I've changed it to be a bit more explicit that I'm trying to pass a tuple with multiple values, one of which is a reference. — BinderNews
– BinderNews, Commented Feb 6, 2020 at 14:36
I'm afraid your edits have left this sentence somewhat ambiguous: I don't want the &u32 to have to live for the entire lifetime of the CallbackTest, just during the function call. Does @ÖmerErden's suggestion work for you? — trent
– trent, Commented Feb 6, 2020 at 14:55

MaxV · Accepted Answer · 2020-02-06 19:21:44Z

1

Here we are rust.playground

use std::marker::PhantomData;

pub struct CallbackTest<'a, E, Fa: 'a + Fn(&E)> {
    pub cb: Box<Fa>,
    _e: &'a PhantomData<E>,
}

impl<'a, E, Fa: 'a + Fn(&E)> CallbackTest<'a, E, Fa> {
    pub fn new(cb: Fa) -> Self
    {
        Self { 
            cb: Box::new(cb), 
            _e: &PhantomData
        }
    }
}

pub struct GameData { 
    pub field: i32,
}

pub type TestRef<'a, 'b, Fb> = CallbackTest<'b, (u32, &'a GameData, ), Fb>;

fn main() {
    let game_data = GameData{ field: 42};
    let test_ref: TestRef<_> = CallbackTest::new(|(val, gd): &(u32, &GameData)| { println!("val:{}, field:{}", val, (*gd).field)});
    (test_ref.cb)(&(24, &game_data));
}

answered Feb 6, 2020 at 19:21

MaxV

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