Collection of trait instances - size cannot be known

I think I may need to do something with ?Sized to get this to work, but I haven't landed on the specific syntax yet. I have a feeling the solution isn't going to be pretty.

Why is that line necessary at all?

for item in self.items {
1 Like

why use a text variable?

        for item in self.items {
            writeln!(f, "{} ${}", item.get_description(), item.get_price())?;
1 Like

The items are all in a Box don't I have to dereference them in order to get the values out of them?

Good point! Also, I see that I needed to change the loop to for item in &self.items {.

I guess the unboxing happens automatically.

You don't need to get the values "out" of the Box at all, you can call object-safe trait methods on a reference to the trait object type. In this case auto-dereferencing kicks in, but you could do it "manually" as well:

        for boxed_item in self.items {
-            let item = *boxed_item;
+            let item = &*boxed_item;

Either way you do it the item doesn't need to be moved out of the Box.


Sidenote: If you switch to for item in &self.items, the type of item will now be &Box<dyn Priced>, but since auto-dereferencing happens as many times as necessary to find a valid receiver type, you still don't need to do anything special to handle the extra reference.


This topic was automatically closed 90 days after the last reply. We invite you to open a new topic if you have further questions or comments.