Associated Type and generic diff in rust
type outside impl
a type Foo = Bar outside is just type alias. most used in generic type.
like: type Thunk = Box<dyn Fn() + Send + 'static>;
type inside impl
type in an impl defines an associated type. associated type可以理解为一个类型占位符,在trait的方法声明中使用.
pub trait Iterator {
type Item; // or type T: Display;
fn next(&mut self) -> Option<Self::Item>;
}
这里Iterator的Implementors将会指定Item的具体类型.例如：
impl Iterator for Counter {
type Item = u32;
fn next(&mut self) -> Option<Self::Item> {
// --snip--
}
}
diff in associated type and generic
直接将上面的Iterator声明为如下泛型不是更简单么？
pub trait Iterator<T> {
fn next(&mut self) -> Option<T>;
}
// with generice, you can set default type:
/// trait Generic<T = String>
/// where T: Display,
主要的区别就是generic可是有任意多个实现,因为Add<Foo>和Add<Bar>是两个不同的类型.
而associated type只能有一个实现,因为Iterator只有一个类型,所以associated type可以用于限制类型.
when use
The quick and dirty answer to when to use generics and when to use associated types is:
Use generics if it makes sense to have multiple implementations of a trait for a specific type (such as the From<T> trait).
Otherwise, use associated types (like Iterator and Deref).
假设我们实现一个redis 客户端,那么比较适合使用associated types:
trait RedisCommand{
type Response;
fn receive(&self, message: String) -> Result<Self::Response>;
}
impl RedisCommand for PingCommand {
type Response = String
fn receive(&self, message: String) -> Result<Self::Response>{
// -- snip --
}
}