Capturing

A closure can capture variables from the environment where it was defined.

fn main() {
    let max_value = 5;
    let clamp = |v| {
        if v > max_value { max_value } else { v }
    };

    dbg!(clamp(1));
    dbg!(clamp(3));
    dbg!(clamp(5));
    dbg!(clamp(7));
    dbg!(clamp(10));
}
This slide should take about 5 minutes.

  • By default, a closure captures values by reference. Here max_value is captured by clamp, but still available to main for printing. Try making max_value mutable, changing it, and printing the clamped values again. Why doesn’t this work?

  • If a closure mutates values, it will capture them by mutable reference. Try adding max_value += 1 to clamp.

  • You can force a closure to move values instead of referencing them with the move keyword. This can help with lifetimes, for example if the closure must outlive the captured values (more on lifetimes later).

    This looks like move |v| ... Try adding this keyword and see if main can still access max_value after defining clamp.

  • By default, closures will capture each variable from an outer scope by the least demanding form of access they can (by shared reference if possible, then exclusive reference, then by move). The move keyword forces capture by value.