Use function pointer parameter inside closure Rust

Question

I am looking to create a function, agg, which takes, as a parameter, another function, get_id, and returns an FnMut closure that uses the get_id function.

Concrete example:

struct CowRow {
    pub id : i32,
}
impl CowRow {
    fn get_id(&self) -> i32       { self.id }
}

pub fn agg<F>(col: F) -> Box<FnMut(&CowRow) -> i32>
    where F: Fn(&CowRow) -> i32 {
    let mut res = 0;
    Box::new(move |r| { res += col(&r); return res })
}

fn main() {
    let mut cow = CowRow { id: 0 };
    let a = agg(CowRow::get_id);
    a(&cow);

Which produces the error:

the parameter type `F` may not live long enough [E0310]

run `rustc --explain E0310` to see a detailed explanation

consider adding an explicit lifetime bound `F: 'static`...

...so that the type `[[email protected]:23:14: 23:53 col:F, res:i32]` will meet its required lifetime bounds

The idea here is that I want a generic function that allows for creating closures which operate on different fields in the struct. So, my thought was to pass a function in that is a getter for the struct and use this in the closure to extract the appropriate field.

I've tried various combinations of adding 'static to the agg signature but I'm not sure what that actually means and where it would need to go syntactically. Additionally, I've tried a number of techniques from: https://github.com/nrc/r4cppp/blob/master/closures.md such as adding the get_id method as a trait but have been unable to get that working either.

Djzin Djzin · Accepted Answer · 2016-04-09T19:08:58

The type parameter F to your function has an associated lifetime (just like every other type). But implicitly, the return value of your function, Box<FnMut(&CowRow) -> i32>, is really Box<FnMut(&CowRow) -> i32 + 'static>. That is, unless you specify a lifetime for a box, it assumes its contents can live forever. Of course if F only lives for 'a, then the borrow checker is going to complain. To fix this, either

Force F to have a static lifetime so that it can live forever inside the box (playpen):

fn agg<F>(col: F) -> Box<FnMut(&CowRow) -> i32>
    where F: Fn(&CowRow) -> i32 + 'static
{
    ...
}

Explicitly state that F has lifetime 'a and so does the Box (playpen):

fn agg<'a, F>(col: F) -> Box<FnMut(&CowRow) -> i32 + 'a>
    where F: Fn(&CowRow) -> i32 + 'a
{
    ...
}

The second version is more general than the first and will accept more closures as arguments.

Use function pointer parameter inside closure Rust

1 Answers