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In teaching the course last week, we broke here, partly due to time
constraints, but partly because this is a pretty mind-bending topic to
tackle at the end of an information-dense day. A break helps, and
spreading the content over a few slides helps as well.
By the timings in the course, this leaves day 2 looking like
*Fundamentals // Day 2 Morning*
_1 hour and 55 minutes: (1 hour and 10 minutes short)_
* Welcome - _3 minutes_
* Pattern Matching - _45 minutes_
* Methods and Traits - _50 minutes_
*Fundamentals // Day 2 Afternoon*
_3 hours and 30 minutes (⏰ *30 minutes too long*)_
* Welcome - _0 minutes_
* Generics - _45 minutes_
* Standard Library Types - _1 hour_
* Standard Library Traits - _1 hour_
* Closures - _20 minutes_
Maybe we should move generics to the morning session?
1.4 KiB
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| 5 |
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
}
};
println!(
"clamped values at {max_value}: {:?}",
(0..10).map(clamp).collect::<Vec<_>>()
);
}
-
By default, a closure captures values by reference. Here
max_valueis captured byclamp, but still available tomainfor printing. Try makingmax_valuemutable, 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 += 1toclamp. -
You can force a closure to move values instead of referencing them with the
movekeyword. 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 ifmaincan still accessmax_valueafter definingclamp. -
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
movekeyword forces capture by value.