Implement Iterator in Rust

Question

I'm making an iterator in rust. In the next() method I want to extract next_element as current_element which will be returned, and create new Term instance and set it as next_element:

pub struct Term { pub coefficient: f64, pub index: i32, pub exponent: f64, } pub struct SeriesIterator { next_element: Term, } impl Iterator for SeriesIterator { type Item = Term; fn next(&mut self) -> Option<Self::Item> { let current_element = self.next_element; self.next_element = Term { coefficient: 1.0, index: current_element.index + 1, exponent: f64::from(current_element.index + 1), }; Some(current_element) } }

How to move ownership of next_element into current_element which will finally be moved outside with the return of next()?

cannot move out of `self.next_element` which is behind a mutable reference | | let current_element = self.next_element; | ^^^^^^^^^^^^^^^^

wasmup · Accepted Answer · 2020-01-04 13:03:23Z

Using std::mem::replace, try this:

 fn next(&mut self) -> Option<Self::Item> { let index = self.next_element.index + 1; Some(std::mem::replace( &mut self.next_element, Term { coefficient: 1.0, index: index, exponent: f64::from(index), }, )) }

Using #[derive(Clone, Copy)]:

 fn next(&mut self) -> Option<Self::Item> { let current_element = self.next_element; // copy self.next_element.coefficient = 1.0; self.next_element.index += 1; self.next_element.exponent = f64::from(self.next_element.index); Some(current_element) }

Try this working example:

 #[derive(Clone, Copy)] pub struct Term { pub coefficient: f64, pub index: i32, pub exponent: f64, } pub struct SeriesIterator { next_element: Term, } impl Iterator for SeriesIterator { type Item = Term; fn next(&mut self) -> Option<Self::Item> { let current_element = self.next_element; // copy self.next_element.coefficient = 1.0; self.next_element.index += 1; self.next_element.exponent = f64::from(self.next_element.index); Some(current_element) } } fn main() { let mut t = SeriesIterator { next_element: Term { coefficient: 1.0, index: 0, exponent: 1.0, }, }; let next = t.next().unwrap(); println!("{}", next.index); // 0 println!("{}", t.next_element.index); // 1 let next = t.next().unwrap(); println!("{}", next.index); // 1 println!("{}", t.next_element.index); // 2 }

Output:

0 1 1 2

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