Bounding an impl Iterator<> so I can borrow self inside a Map closure

help
#1

Hello,

Sorry in advance for the dumb question. This feels like it should be possible but I can't figure out the right way to bound the returned object so it can compile.

Any help is very much appreciated.

struct MyContainer {
    records : Vec<String>
}

struct MyElementPointer<'a> {
    container : &'a MyContainer,
    record : &'a String
}

impl MyContainer {
    pub fn iter<'incoming>(&'incoming self) -> impl Iterator<Item=MyElementPointer<'incoming>> + 'incoming {
        self.records.iter().map(|record| MyElementPointer{container : &self, record : record})
    }
}

fn main() {
    let my_container = MyContainer{records : vec!["one".to_string(), "two".to_string(), "three".to_string()]};
    
    for pointer in my_container.iter() {
        println!("{} of {}", pointer.record, pointer.container.records.len());
    }
}

Thank you in advance.

#2

FIxed it by replacing .map(|record| with .map(move|record|
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=9bb0de51d1ccb7fd839a53bba1dbc4cd

1 Like
#3

Thanks for the reply.

I thought, however, that this would make an owned copy of the container, effectively copying the whole container and every element in it, for every single iteration?

Am I misunderstanding the way the move directive works?

#4

move moves the variables it uses, it just copies the self reference here(not the value it points to, the reference itself).
Without the move it stores a &&'incoming MyContainer in the closure, which is only valid inside the method.

1 Like
#5

Ahhh. That was my misunderstanding. Thanks!