为什么我们不从Iterator实现所有函数来实现迭代器？

Question

To implement an iterator in Rust, we only need to implement the next method, as explained in the documentation . However, the Iterator trait has many more methods .

As far as I know, we need to implement all the methods of a trait. For instance, this does not compile ( playground link ):

struct SomeStruct {}

trait SomeTrait {
    fn foo(&self);
    fn bar(&self);
}

impl SomeTrait for SomeStruct {
    fn foo(&self) {
        unimplemented!()
    }
}

fn main() {}

The error is pretty clear:

error[E0046]: not all trait items implemented, missing: `bar`
 --> src/main.rs:8:1
  |
5 |     fn bar(&self);
  |     -------------- `bar` from trait
...
8 | impl SomeTrait for SomeStruct {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `bar` in implementation

Answer 1

Because every method on Iterator except next has a default implementation . These are methods implemented in the trait itself, and implementors of the trait gain them "for free":

struct SomeStruct {}

trait SomeTrait {
    fn foo(&self);

    fn bar(&self) {
        println!("default")
    }
}

impl SomeTrait for SomeStruct {
    fn foo(&self) {
        unimplemented!()
    }
}

fn main() {}

You can tell if a trait method has a default implementation or not through the documentation :

Required methods

 fn next(&mut self) -> Option<Self::Item> 

Provided methods

 fn size_hint(&self) -> (usize, Option<usize>) 

Note that size_hint is in the "provided methods" section — that's the indication that there's a default implementation.

If you can implement the method in a more efficient way, you are welcome to do so, but note that it is not possible to call the default implementation if you decide to override it .

Specifically for Iterator , it's a great idea to implement size_hint if you can, as that can help optimize methods like collect .