Module show_notes::e016

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RefCells and code smells

  • Date: July 23, 2016
  • Subject: Digging deeper on smart pointers and mutability with Cell and RefCell.
  • Audio


What are the Cell and RefCell types, and when should we use them?

Today, we follow up both the detailed discussion of smart pointers in e015 and the closely related discussion in Interview 2 with Raph Levien, and look at two types you need to have a good idea how to deal with if you want to use these smart pointer types more ergonomically—that is, how to use them without feeling like you’re beating your head against a wall!

The descriptions of the code below are minimal; see the inline comments in the source for the actual informative discussion.

§A comment on the code samples

Note that in several cases below we use &[] to borrow a reference to a slice, rather than requiring this to pass a reference to a Vec specifically. Making the first argument be of type &Vec<RefCell<SimpleNonCopyable>> would also work, but would be more restrictive in what it could and couldn’t accept. Since Vec implements Deref to automatically convert to slices, this works just fine, and is more general. This is how you should usually write function signatures which operate on reference to vectors (and likewise for other types which can dereference to slices). We’ll talk about this more in a future episode!


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  • A container showing a type where Cell<T> works with Vec<T>.
  • A container showing where Cell<T> doesn’t work and RefCell<T> does.