Module show_notes::e019

Let’s Clone a Cow!

• Date: February 28, 2017
• Subject: The final pieces of the story for (single-threaded) memory management in Rust.
• Audio

Notes

Sometimes, we actually do need to copy types. Wouldn’t it be nice if Rust gave us a convenient way to do that when it’s convenient, or when the cost is low enough that the ergonomic tradeoffs are worth it? Well, perhaps unsurprisingly, it does! The Copy and Clone traits, plus the Cow type, give us everything we need!

Links

• underhanded.rs

• The typess

  • std::marker::Copy

    • “Copy types” in the book

    • “Stack-Only Data: Copy” in the new book

    • 7.2.0.2 Moved and copied types:

      When a local variable is used as an rvalue, the variable will be copied if its type implements Copy. All others are moved.

    • Extended example in “Traits” section of new book

  • std::clone::Clone

  • std::borrow::Cow

• Default implementations

  • discussion in the current book

  • discussion in the new book

• Supertraits

  • from the discussion in the reference (6.1.9 Traits):

    Traits may inherit from other traits…. The syntax Circle : Shape means that types that implement Circle must also have an implementation for Shape. Multiple supertraits are separated by +, trait Circle : Shape + PartialEq { }. In an implementation of Circle for a given type T, methods can refer to Shape methods, since the typechecker checks that any type with an implementation of Circle also has an implementation of Shape…

  • discussion of trait “inheritance” in the book

  • discussion of trait super- and subtyping in the new book (note: still to-be-written at the time this episode was published)

• Marker traits

  • std::marker

  • in the reference: 9 Special Traits

• Previous episodes on traits:

  • e008: Just like something else

  • e009: Composing a Rustic tune

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Contact

• New Rustacean:
  • Twitter: @newrustacean
  • Email: hello@newrustacean.com
• Chris Krycho
  • GitHub: chriskrycho
  • Twitter: @chriskrycho

Structs

• ANoCopyOrClonePoint
  A non-copyable point type
• BJustClonePoint
  A struct which implements Clone but not Copy.
• CCopyPoint
  A struct with identical behavior to ANoCopyOrClonePoint, except with Copy.

Functions

• demonstrate_cow
  The Cow type can wrap around other types and make them “reusable”.
• demonstrate_mut_cow
  What if we need a mutable reference to the wrapped type?