how can I create a throwaway mutable reference?

Question

I'm trying to wrap a vector to change its index behaviour, so that when an out of bounds access happens, instead of panicking it returns a reference to a dummy value, like so:

use std::ops::Index;

struct VecWrapper(Vec<()>);

impl Index<usize> for VecWrapper {
    type Output = ();
    fn index(&self, idx: usize) -> &() {
        if idx < self.0.len() {
            &self.0[idx]
        } else {
            &()
        }
    }
}

which works just fine for the implementation of Index, but trying to implement IndexMut the same way fails for obvious reasons. the type in my collection does not have a Drop implementation so no destructor needs to be called (other than freeing the memory).

the only solution I can think of is to have a static mutable array containing thousands of dummies, and distributing references to elements of this array, this is a horrible solution though it still leads to UB if there are ever more dummies borrowed than the size of the static array.

How will you debug the inevitable weirdness when out of bounds access happens? — Sebastian Redl, Apr 20 '22 at 08:09
Maybe you just want to get `.get` instead of indexing your original vector? It's more idiomatic, it's less boilerplate, and everyone will understand reading your code. — jthulhu, Apr 20 '22 at 10:08
I'm not afraid of that wierdness, the intended behaviour is that any data written out of bounds gets thrown away and doesn't influence anything else — terepy, Apr 21 '22 at 01:04
Related: [Implementing Index/overloading the `[]` operator for a sparse vector](https://stackoverflow.com/q/71835219/7884305). — Chayim Friedman, Apr 24 '22 at 06:06

score 6 · Accepted Answer · answered Apr 20 '22 at 08:08

6

Give the wrapper an additional field that's the dummy. It's got the same lifetime restrictions, and so can't be aliased.

answered Apr 20 '22 at 08:08

Sebastian Redl

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how can I create a throwaway mutable reference?

1 Answers1