How can I take ownership of a Vec element and repl

Question

I am writing a function of the following format:

fn pop(data: &mut Vec<Option<T>>) -> Option<T> {
  // Let the item be the current element at head
  let item = data[0];

  // and "remove" it.
  data[0] = None;

  item
}

When I try to do this, I get a "cannot move out of indexed content" error, which makes sense. When I try to change it such that item is a reference, I get an error when I try to set data[0] to None, which also makes sense.

Is there some way I can do what I want to do? It seems to me that, whether I want to return a reference or not, I'm going to have to take ownership of the element from the Vec.

I noticed that Vec has a swap_remove method, which does almost exactly what I want, except that it swaps with an element already in the Vec, not with any arbitrary value as I would like. I know that I could just append None to the end of the Vec and use swap_remove, but I'm interested in seeing if there's another way.

Answer 1

Use std::mem::replace:

use std::mem;

fn pop<T>(data: &mut Vec<Option<T>>) -> Option<T> {
    mem::replace(&mut data[0], None)
}

replace essentially replaces the value in a particular location with another one and returns the previous value.