[−][src]Module show_notes::e016

RefCells and code smells

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

Notes

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!

Links

Rust 1.10
- blog post
- full release notes
- cdylib
  - RFC: text | discussion
  - implementation
rustup 0.3.0 release
Integer32 (Carol Nichols' and Jake Goulding's new Rust consultancy)
- announcement blog post
- reddit discussion
IntelliJ Rust
- reddit discussion
Tango: source | talk
Cell and RefCell:
- Cell Types in The Rust Programming Language
- Standard library:
  - std::cell module docs (detailed explanation, complementary to the book)
  - Cell<T> docs (at std::cell::Cell)
  - RefCell<T> docs (at stc::cell::RefCell)

Contact

New Rustacean:
- Twitter: @newrustacean
- Email: [email protected]
Chris Krycho
- GitHub: chriskrycho
- Twitter: @chriskrycho

Structs

SimpleContainer	A container showing a type where `Cell<T>` works with `Vec<T>`.
SimpleNonCopyable	A container showing where `Cell<T>` doesn't work and `RefCell<T>` does.

Functions

add_to_each_string	Demonstrate interior mutability with `Rc` and `RefCell`.
demonstrate_need_for_cell	Demonstrate how you need `Cell<T>` even just with a `Vec<T>`;
double_cell	Operate mutably on the contenets of an immutable reference to a `Vec`.