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Applied Intermediate

16 Smart pointers

Box gives a value a fixed-size home on the heap -- which is what makes recursive types possible. Rc adds shared ownership: several owners of one allocation, freed when the last one drops.

Step 1 / 2

Sum the recursive List in sum. Box is what lets the type nest.

Hints

Recurse on the tail: a node's total is its value plus the rest's total.
match list { List::Node(v, rest) => v + sum(rest), List::End => 0 } -- &Box<List> coerces to &List.

16_smart_pointers.rs

// Without the Box this type would be infinitely large.
enum List {
    Node(i32, Box<List>),
    End,
}

// sum every value in the list
fn sum(list: &List) -> i32 {
    todo!()
}

// Without the Box this type would be infinitely large.
enum List {
    Node(i32, Box<List>),
    End,
}

// sum every value in the list
fn sum(list: &List) -> i32 {
    todo!()
}

Reset to starterCompiled in a sandbox; hidden checks must pass.

Reveal solution

enum List {
    Node(i32, Box<List>),
    End,
}

fn sum(list: &List) -> i32 {
    match list {
        List::Node(v, rest) => v + sum(rest),
        List::End => 0,
    }
}

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16 Smart pointers