How to make the following code run?(somthing about lifetime)

help
1 
fn my_add<'a, T: Add<Output = T>>(a: &'a T, b: &'a T) -> T
where
    T: Add<&'a T, Output = T>,
    &'a T: Add<&'a T, Output = T>,
{
    let c = a + b;
    let d = &c + b;
    d + c
}

T doesn't implement Clone and Copy trait.

2

What is stopping it from running? Can you show us the full error you get from the compiler and what you've tried to make it work?

3 
fn my_add<'a, T: Add<Output = T>>(a: &'a T, b: &'a T) -> T
    |           -- lifetime `'a` defined here
...
150 |     let c = a + b;
    |         - binding `c` declared here
151 |     let d = &c + b;
    |             ^^----
    |             |
    |             borrowed value does not live long enough
    |             requires that `c` is borrowed for `'a`
152 |     d + c
153 | }
    | - `c` dropped here while still borrowed
4

Lifetime parameters of a function mean a lifetime the caller gets to pick, which will necessarily outlive the entire function. You can never refer to a local variable with a reference of that lifetime, which is what your program is trying to do with &c.

The solution is to use HRTBs to ask for Add independent of the exact lifetime:

use std::ops::Add;

fn my_add<T>(a: &T, b: &T) -> T
where
    T: Add<Output = T>,
    for<'a> &'a T: Add<Output = T>,
{
    let c = a + b;
    let d = &c + b;
    d + c
}

This bound says: “For any possible lifetime, which we will call 'a, it should be possible to Add together &'a Ts and get a T.”

4 Likes
5

Thank you!