How to access a field of a struct within a struct?

Question

I have a struct container of Children and a method pop() that removes the last added Child and returns it's a value:

struct Child {
    a: i32,
    b: String,
}

struct Container<'a> {
    vector: &'a mut Vec<Child>,
}

impl<'a> Container<'a> {
    fn pop(&mut self) -> i32 {
        return self.vector.pop().a;
    }
}

I get the error during compilation:

error: no field `a` on type `std::option::Option<Child>`
  --> src/main.rs:12:34
   |
12 |         return self.vector.pop().a;
   |                                  ^

Does the scope of Container's pop() not allow access to values of its Children's scope?

Shepmaster · Accepted Answer · 2017-03-09 16:01:27Z

8

Vec::pop returns an Option<Child>, not a Child. This allows it to have something reasonable to return in case there are no elements in the Vec to pop off. To get at the a that may be inside, you can convert from Option<Child> to Child using unwrap(), but that will cause your program to panic if the Vec was empty. The code for that would look like this:

fn pop(&mut self) -> i32 {
    return self.vector.pop().unwrap().a;
}

Another option would be to more closely copy Vec's behavior, and return None in case there are no elements. You could do that using Option's map method:

fn pop(&mut self) -> Option<i32> {
    return self.vector.pop().map(|child| child.a)
}

edited Mar 9, 2017 at 16:01

Shepmaster

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answered Jul 30, 2015 at 22:47

Peter Reid

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1 Comment

Aleksandra Zalcman Over a year ago

Great, I didn't pay attention to the Option type, I've just started playing with Rust. It's pretty cool! Thx.

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