rust_ex3.rs

// without borrowing
fn print_sum1(v: Vec<i32>) -> Vec<i32> {
    println!("{}", v[0] + v[1]);
    // returning v as a means of transferring ownership back
    // by the way, there's no need to use "return" if it's the last line
    // because Rust is expression-based
    v
}

// with borrowing, explicit references
fn print_sum2(vr: &Vec<i32>) {
    println!("{}", (*vr)[0] + (*vr)[1]);
    // vr, the reference, is dropped here
    // thus the borrow ends
}

// this is how you should actually do it
fn print_sum3(v: &Vec<i32>) {
    println!("{}", v[0] + v[1]);
    // same as in print_sum2
}

fn main() {
    let mut v = Vec::new(); // creating the resource
    for i in 1..1000 {
        v.push(i);
    }
    // at this point, v is using
    // no less than 4000 bytes of memory

    // transfer ownership to print_sum and get it back after they're done
    v = print_sum1(v);
    // now we again own and control v
    println!("(1) We still have v: {}, {}, ...", v[0], v[1]);
    
    // take a reference to v (borrow it) and pass this reference to print_sum2
    print_sum2(&v);
    // v is still completely ours
    println!("(2) We still have v: {}, {}, ...", v[0], v[1]);
    
    // exacly the same here
    print_sum3(&v);
    println!("(3) We still have v: {}, {}, ...", v[0], v[1]);
    
    // v is dropped and deallocated here
}