IntoIterator

The Iterator trait tells you how to iterate once you have created an iterator. The related trait IntoIterator defines how to create an iterator for a type. It is used automatically by the for loop.

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Speaker Notes

This slide should take about 5 minutes.
  • IntoIterator is the trait that makes for loops work. It is implemented by collection types such as Vec<T> and references to them such as &Vec<T> and &[T]. Ranges also implement it. This is why you can iterate over a vector with for i in some_vec { .. } but some_vec.next() doesn’t exist.

Click through to the docs for IntoIterator. Every implementation of IntoIterator must declare two types:

  • Item: the type to iterate over, such as i8,
  • IntoIter: the Iterator type returned by the into_iter method.

Note that IntoIter and Item are linked: the iterator must have the same Item type, which means that it returns Option<Item>

The example iterates over all combinations of x and y coordinates.

Try iterating over the grid twice in main. Why does this fail? Note that IntoIterator::into_iter takes ownership of self.

Fix this issue by implementing IntoIterator for &Grid and creating a GridRefIter that iterates by reference. A version with both GridIter and GridRefIter is available in this playground.

The same problem can occur for standard library types: for e in some_vector will take ownership of some_vector and iterate over owned elements from that vector. Use for e in &some_vector instead, to iterate over references to elements of some_vector.