with as constructor

with as a constructor sets one value among a type while using default values for the rest.

with as in “<Type> with specific setting.”

#![allow(unused)]
fn main() {
impl<T> Vec<T> {
    // Initializes memory for at least N elements, len is still 0.
    fn with_capacity(capacity: usize) -> Vec<T>;
}
}

  • with can appear as a constructor prefix, most commonly when initializing heap memory for container types.

    In this case, it’s distinct from new constructors because it specifies the value for something that is not usually cared about by API users.

  • Ask the class: Why not from_capacity?

    Answer: Vec::with_capacity as a method call scans well as creating a “Vec with capacity”. Consider how Vec::new_capacity or Vec::from_capacity scan when written down, they do not communicate what’s going on well.