如何使用Iterator特征构建通用API

Question

I may not see the forest for the trees but I wonder how do I actually design my methods to not work against hard collection types but against Iterators instead. Consider this method.

pub fn print_strings(strings: Vec<String>) {
    for val in strings.iter() {
        println!("{}", val);
    }
}

Obviously this falls short if I want to use that with an HashSet or HashMap .

So, I tried this:

use std::collections::*;

fn main () {
    let strings = vec!("Foo", "Bar");

    let mut more_strings = HashMap::new();
    more_strings.insert("foo", "bar");
    more_strings.insert("bar", "foo");

    print_strings(&strings.iter());
    print_strings(&more_strings.values())
}

fn print_strings(strings: &Iterator<Item=&str>) {
    for val in strings {
        println!("{}", val);
    }
}

Playpen (also to view lengthy compiler error)

http://is.gd/EYIK11

Unfortunately, this doesn't seem to do the trick either. What am I missing?

Answer 1

Even better, you can do

fn print_strings<Iterable>(strings: Iterable)
    where Iterable: IntoIterator,
          Iterable::Item: AsRef<str>
{
    for val in strings {
        println!("{}", val.as_ref());
    }
}

^{(Kudos Shepmaster for the improvement.)}

This means that you can call this with &mut Iterator s for dynamic dispatch or concrete iterator or collection types for static dispatch. Further, the iterator type can be anything that can be simply converted to &str , which includes but is not limited to &str , &&str and even String .

print_strings(&strings);
print_strings(strings.iter().map(|s| s.to_owned()));
print_strings(vec![&&&&"xyz"]);
print_strings(strings);
print_strings(more_strings.values());

Answer 2

When you call .iter() on a Vec<T> , you get an Iterator<Item=&T> . So when you call .iter() on a Vec<&str> , you get an Iterator<Item=&&str> , not a Iterator<Item=&str> . You should look at the .cloned() method for Iterator , it should help solve your problem.

Also, note that in order to iterate through an iterator, you must be able to mutate it (either own the iterator or have a mutable reference to it). So just having an immutable reference to it is sorta useless. I would recommend moving the iterator value into print_strings rather than passing it by reference. If you want to use trait objects for this, you can do that by using Box , but it might be easier to just make print_strings a generic function.

Answer 3

playpen
So the first thing is that you're expecting Iterator<Item=&str> , but it's Iterator<Item=&&str> actually.
Then, you're trying to call .iter() , but Iterator doesn't have this method. You can simply remove .iter() call and receive (and send ofc) &mut Iterator<...> in order to get for loop working ( for loop needs something, that implements IntoIterator , and &mut Iterator is that thing).
Add lifetimes and you're all set! :)
Also, I'm recommending to use static dispatch. You can see it in the example I provided.