Get Box Pointer from reference in rust

Question

Say I have a function like this:

fn foo(item: &FooType) -> Box<FooType>{
    Box::new(item)
}

I get the following error:

mismatched types: expected enum FooType, found &FooType

I'm assuming I need to do some reference/boxing and lifetime magic, but as a rust beginner, I'm not 100% sure how to get this working.

This is impossible in general, unless the FooType is clonable. The reason is simple: Box<FooType> owns the FooType, but &FooType doesn't. — Cerberus

prog-fh prog-fh · Accepted Answer · 2021-04-24T08:19:49

I think there is no definitive answer since it depends on what you expect.

If you want to obtain a heap-allocated clone of something you want to keep usable, then cloning in foo1() can help.

A better solution could be to pass a value not a reference, as in foo2(), because this way the call site can decide if a clone is necessary or if moving the original value into the box is enough.

You may also want the box to hold the reference instead of the value, as in foo3(), but this is like a code-smell to me (many indirections that make reasoning about lifetimes very complicated).

#[derive(Debug, Clone)]
struct FooType {
    value: usize,
}

fn foo1(item: &FooType) -> Box<FooType> {
    Box::new(item.clone())
}

fn foo2(item: FooType) -> Box<FooType> {
    Box::new(item)
}

fn foo3(item: &FooType) -> Box<&FooType> {
    Box::new(item)
}

fn main() {
    let ft_a = FooType { value: 12 };
    let bft_a = foo1(&ft_a);
    println!("bft_a: {:?}", bft_a);
    println!("ft_a: {:?}", ft_a);
    println!("~~~~~~~~~~~~~~~~~~");
    let ft_b = FooType { value: 34 };
    let bft_b = foo2(ft_b);
    println!("bft_b: {:?}", bft_b);
    //  println!("ft_b: {:?}", ft_b); // borrow of moved value: `ft_b`
    println!("ft_b is consumed");
    println!("~~~~~~~~~~~~~~~~~~");
    let ft_c = FooType { value: 56 };
    let bft_c = foo2(ft_c.clone());
    println!("bft_c: {:?}", bft_c);
    println!("ft_c: {:?}", ft_c);
    println!("~~~~~~~~~~~~~~~~~~");
    let ft_d = FooType { value: 78 };
    let bft_d = foo3(&ft_d);
    println!("bft_d: {:?}", bft_d);
    println!("ft_d: {:?}", ft_d);
}
/*
bft_a: FooType { value: 12 }
ft_a: FooType { value: 12 }
~~~~~~~~~~~~~~~~~~
bft_b: FooType { value: 34 }
ft_b is consumed
~~~~~~~~~~~~~~~~~~
bft_c: FooType { value: 56 }
ft_c: FooType { value: 56 }
~~~~~~~~~~~~~~~~~~
bft_d: FooType { value: 78 }
ft_d: FooType { value: 78 }
*/

Get Box Pointer from reference in rust

2 Answers