The Problem

A trait that is generic over its container type has type specification
requirements - users of the trait must specify all of its generic types.

In the example below, the Contains trait allows the use of the generic
types A and B. The trait is then implemented for the Container type,
specifying i32 for A and B so that it can be used with fn difference().

Because Contains is generic, we are forced to explicitly state all of the
generic types for fn difference(). In practice, we want a way to express that
A and B are determined by the input C. As you will see in the next
section, associated types provide exactly that capability.

  1. struct Container(i32, i32);
  3. // A trait which checks if 2 items are stored inside of container.
  4. // Also retrieves first or last value.
  5. trait Contains<A, B> {
  6.     fn contains(&self, &A, &B) -> bool; // Explicitly requires `A` and `B`.
  7.     fn first(&self) -> i32; // Doesn't explicitly require `A` or `B`.
  8.     fn last(&self) -> i32;  // Doesn't explicitly require `A` or `B`.
  9. }
  11. impl Contains<i32, i32> for Container {
  12.     // True if the numbers stored are equal.
  13.     fn contains(&self, number_1: &i32, number_2: &i32) -> bool {
  14.         (&self.0 == number_1) && (&self.1 == number_2)
  15.     }
  17.     // Grab the first number.
  18.     fn first(&self) -> i32 { self.0 }
  20.     // Grab the last number.
  21.     fn last(&self) -> i32 { self.1 }
  22. }
  24. // `C` contains `A` and `B`. In light of that, having to express `A` and
  25. // `B` again is a nuisance.
  26. fn difference<A, B, C>(container: &C) -> i32 where
  27.     C: Contains<A, B> {
  28.     container.last() - container.first()
  29. }
  31. fn main() {
  32.     let number_1 = 3;
  33.     let number_2 = 10;
  35.     let container = Container(number_1, number_2);
  37.     println!("Does container contain {} and {}: {}",
  38.         &number_1, &number_2,
  39.         container.contains(&number_1, &number_2));
  40.     println!("First number: {}", container.first());
  41.     println!("Last number: {}", container.last());
  43.     println!("The difference is: {}", difference(&container));
  44. }

See also:

structs, and traits