ジェネリックトレイト
Traits can also be generic, just like types and functions. A trait's parameters get concrete types when it is used.
#[derive(Debug)] struct Foo(String); impl From<u32> for Foo { fn from(from: u32) -> Foo { Foo(format!("Converted from integer: {from}")) } } impl From<bool> for Foo { fn from(from: bool) -> Foo { Foo(format!("Converted from bool: {from}")) } } fn main() { let from_int = Foo::from(123); let from_bool = Foo::from(true); println!("{from_int:?}, {from_bool:?}"); }
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The
From
trait will be covered later in the course, but its definition in thestd
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 noFrom<&str>
implementation forFoo
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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.
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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.