Reference to struct wrapped in Option<>

help
1

I would like to have a 'load on demand' Vec and make a function that returns a reference to MyStruct if already loaded.

pub data: Vec<Option<MyStruct>> 

fn get_element(index: usize) -> &MyStruct
{
    // if self.data[index] != None
    // return ??
}
2

Hint: Start by doing a pattern match on self.data.

3

What does the function / method return if the element is None?

4

self.data.get(idx).and_then(Option::as_ref)

5

What does the function / method return if the element is None?

if the element is None I want to load / create it

6

Yes, but I want to get rid of Option when returning from the get_element

7

Then what should this method do on out-of-bounds access (i.e. when the index is too large)? Panic? Resize the Vec up to this index (and then create the struct at it, as if it was in bounds)?

8

I have a 'ensure_prepared' function which fills the vector with None's and the correct number of entries.
(it is reading some stuff from a directory)
The number of entries is known to caller.

9 
    pub fn get_element(&mut self, idx: usize) -> &MyStruct {
        match &mut self.data[idx] {
            Some(existing) => existing,
            opt @ None => {
                *opt = Some(MyStruct);
                opt.as_ref().unwrap()
            }
        }
    }
10

Or:

fn get_element(&mut self, index: usize) -> &MyStruct
{
    self.data[index].get_or_insert_with(|| MyStruct {})
}
3 Likes
11

Thanks jongiddy and quinedot

12

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