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How do I change an attribute on a struct to a new value which contains the old value?

July 17, 2023

struct Container {
    more: Box<Container>
}

struct ContainerBox {
    container: Option<Container>
}

impl ContainerBox {
    pub fn ingest(&mut self, container: Container) {
        match &self.container {
            Some(container) => {
                self.container = Some(Container { more: (*container).clone() }); // this doesn't look right.
            },
            None => self.container = Some(container)
        };
    }
}

Very silly example but it gets the point across. I have a struct that owns some data that may be something or nothing, when its something, I need to replace it with a new instance of the same type which is composed of the old value in that structs attribute.

I think I understand why the borrow checker is unhappy, but I also can’t see another way to write this.

>Solution :

You can use Option::take():

impl ContainerBox {
    pub fn ingest(&mut self, container: Container) {
        match self.container.take() {
            Some(container) => {
                self.container = Some(Container { more: Box::new(container) });
            },
            None => self.container = Some(container),
        };
    }
}