How can I swap in a new value for a field in a mutable reference to a structure?

Question

I have a struct with a field:

struct A {
    field: SomeType,
}

Given a &mut A , how can I move the value of field and swap in a new value?

fn foo(a: &mut A) {
    let mut my_local_var = a.field;
    a.field = SomeType::new();

    // ...
    // do things with my_local_var
    // some operations may modify the NEW field's value as well.
}

The end goal would be the equivalent of a get_and_set() operation. I'm not worried about concurrency in this case.

Answer 1

Use std::mem::swap() .

fn foo(a: &mut A) {
    let mut my_local_var = SomeType::new();
    mem::swap(&mut a.field, &mut my_local_var);
}

Or std::mem::replace() .

fn foo(a: &mut A) {
    let mut my_local_var = mem::replace(&mut a.field, SomeType::new());
}    

Answer 2

If your type implements Default , you can use std::mem::take :

#[derive(Default)]
struct SomeType;

fn foo(a: &mut A) {
    let mut my_local_var = std::mem::take(&mut a.field);
}

If your field happens to be an Option , there's a specific method you can use — Option::take :

struct A {
    field: Option<SomeType>,
}

fn foo(a: &mut A) {
    let old = a.field.take();
    // a.field is now None, old is whatever a.field used to be
}

The implementation of Option::take uses mem::take , just like the more generic answer above shows, but it is wrapped up nicely for you:

pub fn take(&mut self) -> Option<T> {
    mem::take(self)
}

See also:

• Temporarily move out of borrowed content
• Change enum variant while moving the field to the new variant