Generic Functions

Rust supports generics, which lets you abstract algorithms or data structures (such as sorting or a binary tree) over the types used or stored.

  1. /// Pick `even` or `odd` depending on the value of `n`.
  2. fn pick<T>(n: i32, even: T, odd: T) -> T {
  3.     if n % 2 == 0 {
  4.         even
  5.     } else {
  6.         odd
  7.     }
  8. }
  10. fn main() {
  11.     println!("picked a number: {:?}", pick(97, 222, 333));
  12.     println!("picked a string: {:?}", pick(28, "dog", "cat"));
  13. }

This slide should take about 5 minutes.

  • Rust infers a type for T based on the types of the arguments and return value.

  • In this example we only use the primitive types i32 and &str for T, but we can use any type here, including user-defined types:

    1. struct Foo {
    2.     val: u8,
    3. }
    5. pick(123, Foo { val: 7 }, Foo { val: 456 });

  • This is similar to C++ templates, but Rust partially compiles the generic function immediately, so that function must be valid for all types matching the constraints. For example, try modifying pick to return even + odd if n == 0. Even if only the pick instantiation with integers is used, Rust still considers it invalid. C++ would let you do this.

  • Generic code is turned into non-generic code based on the call sites. This is a zero-cost abstraction: you get exactly the same result as if you had hand-coded the data structures without the abstraction.