Copies, moves, and references · Coding Class

Advanced · Take control← Read the Learn chapter

Apply the chapter in three steps: an exercise, a program project, and a debugging task. Implement first, then consult the hints and completion criteria. Use a separate file or working directory for each task.

1. Exercise · Compare copies and originals

Run append_copy and append_shared from the chapter, printing the vector before and after each call. Identify which operation preserves or changes the original.

Completion: The original is [1, 2] after changing a copy and [1, 2, 3] after changing through a reference.

Hint: Passing Vec<Int> by value uses an independent copy.

2. Build a program · Build an in-place update

Write append_marker(values: &mut Vec<Int>, marker: Int) to append to the original. Add 7 to an empty vector, then 9 to the result.

Completion: The original becomes [7, 9]. Enable explc in the function and caller; make the owner in main mutable.

Hint: Call push through the parameter and pass &mut from the caller.

3. Debug · Fix mutation during a borrow

Fix the code below by reordering statements without creating a copy. Identify the reference’s last use.

Start with this deliberately broken program.

class_borrow.xen

#![explc]
fn main() {
    let mut value = 1;
    let shared = &value;
    value = 2;
    println(*shared);
}

xen check class_borrow.xen

Diagnostic excerpt:

class_borrow.stderr

while it is borrowed

Completion: Read 1 through the reference, then change the original to 2; checking succeeds.

Hint: Move println(*shared) before the assignment.

Verify and record

Use xen check file.xen followed by xen run file.xen for ordinary programs; use xen test file.xen for test-only files. Record the original error, its cause, and the corrected result, then rerun every input in the completion criteria.