How can you access methods of an iterator while using it in a for loop? [duplicate]

Question

I have implemented a custom iterator for a graph traversal. The iterator contains state variables which I plan to access inside the body of the loop:

#[derive(Clone, Debug)]
pub struct NodeIterator<'a, T: 'a + Data> {
    graph: &'a DAG<T>,

    /// true if we should fetch the nodes recursively (default = true)
    pub recursive: bool,

    /// true if we explore depth first, and after that go for neighour nodes (default=false)
    pub depth_first: bool,

    /// maximum depth to iterate on lower levels
    /// recursive = true : we use recursion to iterate down the hierarchy
    /// recursive =false : we only iterate either through children or parents
    ///                                  (depending on follow_direction field)
    pub max_depth: NodeID,

    /// follow_direction = true : we iterate using 'parents' vec
    /// follow_direction =false : we iterate using 'children' vec
    pub follow_direction: bool,

    /// current node, that was already provided by the iterator
    pub cur_node_id: Option<NodeID>,

    /// index to the current children or parent (while iterating corresponding collection)
    pub cur_index: Option<NodeID>,

    /// current Edge being iterated
    pub cur_edge: Option<Edge>,

    /// current depth (starts from 0)
    cur_depth: u32,
}

impl<'a, T> NodeIterator<'a, T>
where
    T: 'a + Data,
{
    pub fn get_cur_depth(&self) -> u32 {
        self.cur_depth
    }
}

When I use it in a for loop, it takes ownership of the object:

let it = graph.iter();
for node_entry in it {
    println!(
        "node_id: {}, depth: {},",
        node_entry.node_id,
        it.get_cur_depth()
    );
}

If I try to use it inside the code block, I get this error:

     ^^ value used here after move

The error occurs when I am trying to acces to it.get_cur_depth() function. How would you get around this error to access internal state of the iterator through methods?

Your question is unclear, no minimal reproducible example.

Stargateur
– Stargateur

2019-06-04 23:35:25 +00:00
Commented Jun 4, 2019 at 23:35 — Stargateur
– Stargateur, Commented Jun 4, 2019 at 23:35

Shepmaster · Accepted Answer · 2019-06-04 23:16:08Z

3

When I use it in a for loop, it takes ownership of the object:

Then don't use a for loop:

fn main() {
    let s = "a b c";
    let mut it = s.chars();

    while let Some(_) = it.next() {
        println!("remaining: {},", it.as_str());
    }
}

1 Comment

Peter Varo Over a year ago

Moreover, to understand what for is actually doing under the hood consult the relevant chapters of The Rust Reference

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