rust , Passing a component of a pointer by value


Passing a component of a pointer by value

Question:

Tag: rust

I've just started out with Rust, so some of my concepts may be wrong. I'd really appreciate corrections if this is the case.

I was following the lifetimes guide and trying some of the examples. I modified the rectangle example a little.

I changed the compute_distance function to accept Point by value in the first parameter. I then removed the & before on_the_stack.origin in the call to compute_distance.

This gives me the following error:

cannot move out of dereference of &-pointer

If instead I add the & before the on_the_stack.origin call as well as accept Point by reference in the compute_distance function, it works without a hitch.

The second method working makes sense to me, but why does my original approach throw an error?

use std::num::Float;

struct Point {
    x : f64,
    y : f64
}

struct Size {w: f64, h: f64}
struct Rectangle {origin: Point, size: Size}

#[cfg(not(test))]
fn main() {
    let on_the_stack = &Rectangle{origin: Point {x: 1.0, y: 2.0},
                                  size: Size {w: 3.0, h: 4.0}};

    let on_the_heap = box Rectangle {origin: Point {x: 5.0, y: 6.0},
                                  size: Size {w: 3.0, h: 4.0}};

    println!("Distance: {}", compute_distance(on_the_stack.origin,&on_the_heap.origin));
}

fn compute_distance ( p1:Point,p2:&Point) -> f64 {
    let x_d = p1.x - p2.x;
    let y_d = p1.y - p2.y;
    Float::sqrt(x_d * x_d + y_d * y_d)
}

Answer:

As you said, your changed compute_distance wants a value as the first parameter. This means that the function is going to take ownership of the argument.

However, you are attempting to pass a smaller piece of a structure (origin is embedded in Rectangle). To do this, Rust would have to give ownership of part of Rectangle to the method, but exactly what state would Rectangle be left in? A very broken one, so Rust prevents you from doing that. Yay, you didn't shoot yourself in the foot!

"But wait", you say, "it's totally cool if the function gets a copy of origin". You, as a human, can recognize that and tell the compiler that it's "totally cool":

#[deriving(Copy)]
struct Point { /* ... */ }

Now Rust knows that it is safe to make a bit-for-bit copy of a Point if it would like to. Huzzah!

"But wait", you say, "what if it's not cool to make a bit-for-bit copy? My structure needs more love than being treated like a bag of bits!". Again, as an intelligent human, you can express that:

#[deriving(Clone)]
struct Point { /* ... */ }

// later...

on_the_stack.origin.clone()

Now you can make an explicit call (clone()) to get a semantic duplicate of the option. Because this could be an expensive operation, the compiler won't do it for you.

A note on deriving

As I write this answer, deriving is transitioning to derive. Adjust as needed for your code.


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