値の借用

前に説明したように、関数を呼び出すときに所有権を移動する代わりに、関数で値を借用できます。

#[derive(Debug)]
struct Point(i32, i32);

fn add(p1: &Point, p2: &Point) -> Point {
    Point(p1.0 + p2.0, p1.1 + p2.1)
}

fn main() {
    let p1 = Point(3, 4);
    let p2 = Point(10, 20);
    let p3 = add(&p1, &p2);
    println!("{p1:?} + {p2:?} = {p3:?}");
}
  • add 関数は 2 つのポイントを 借用 し、新しいポイントを返します。
  • 呼び出し元は入力の所有権を保持します。
This slide should take about 10 minutes.

このスライドでは、1 日目の参照に関する資料を振り返りですが、少し対象を広げ、関数の引数と戻り値も含めています。

その他

Notes on stack returns and inlining:

  • Demonstrate that the return from add is cheap because the compiler can eliminate the copy operation, by inlining the call to add into main. Change the above code to print stack addresses and run it on the Playground or look at the assembly in Godbolt. In the "DEBUG" optimization level, the addresses should change, while they stay the same when changing to the "RELEASE" setting:

    #[derive(Debug)]
struct Point(i32, i32);

fn add(p1: &Point, p2: &Point) -> Point {
    let p = Point(p1.0 + p2.0, p1.1 + p2.1);
    println!("&p.0: {:p}", &p.0);
    p
}

pub fn main() {
    let p1 = Point(3, 4);
    let p2 = Point(10, 20);
    let p3 = add(&p1, &p2);
    println!("&p3.0: {:p}", &p3.0);
    println!("{p1:?} + {p2:?} = {p3:?}");
}

  • The Rust compiler can do automatic inlining, that can be disabled on a function level with #[inline(never)].

  • Once disabled, the printed address will change on all optimization levels. Looking at Godbolt or Playground, one can see that in this case, the return of the value depends on the ABI, e.g. on amd64 the two i32 that is making up the point will be returned in 2 registers (eax and edx).