泛型

Traits can also be generic, just like types and functions. A trait’s parameters get concrete types when it is used.

1. #[derive(Debug)]
2. struct Foo(String);

4. impl From<u32> for Foo {
5. fn from(from: u32) -> Foo {
6. Foo(format!("Converted from integer: {from}"))
7. }
8. }

10. impl From<bool> for Foo {
11. fn from(from: bool) -> Foo {
12. Foo(format!("Converted from bool: {from}"))
13. }
14. }

16. fn main() {
17. let from_int = Foo::from(123);
18. let from_bool = Foo::from(true);
19. println!("{from_int:?}, {from_bool:?}");
20. }

• The From trait will be covered later in the course, but its definition in the std docs is simple.
• Implementations of the trait do not need to cover all possible type parameters. Here, Foo::From("hello") would not compile because there is no From<&str> implementation for Foo.
• Generic traits take types as “input”, while associated types are a kind of “output” type. A trait can have multiple implementations for different input types.
• In fact, Rust requires that at most one implementation of a trait match for any type T. Unlike some other languages, Rust has no heuristic for choosing the “most specific” match. There is work on adding this support, called specialization.