Chapter 4.3 : memory diagram errors

[dhmoclex]

• I have searched open and closed issues and pull requests for duplicates, using these search terms: "4.3"
• I have checked the latest main branch to see if this has already been fixed, in this file: I was not able to find where the memory diagrams are...

URL to the section(s) of the book with this problem: https://rust-book.cs.brown.edu/ch04-03-fixing-ownership-errors.html

• Section Fixing an Unsafe Program: Not Enough Permissions
• Section Fixing an Unsafe Program: Copying vs. Moving Out of a Collection

Description of the problem: Concerning the examples:

fn stringify_name_with_title(name: &Vec<String>) -> String {
    name.push(String::from("Esq."));`{}`
    let full = name.join(" ");
    full
}
fn main() {
    let name = vec![String::from("Ferris")];
    let first = &name[0];
    stringify_name_with_title(&name);
    println!("{}", first);
}

and

fn main() {
let v: Vec<String> = 
  vec![String::from("Hello world")];
let s_ref: &String = &v[0];
let s: String = *s_ref;

// These drops are normally implicit, but we've added them for clarity.
drop(s);
drop(v);
}

The memory diagram shows that the reference to the first string in the vector point to the vector, not the string. According to the following test code, it seems to be false :

fn main() {
    let name = vec![String::from("A"), String::from("B")];
    println!("name: {:p}", &name);
    println!("name_0: {:p}", &name[0]);
    let first = &name[0];
    println!("first: {:p}", first);
    println!("&first: {:p}", &first);
}

Output :

name: 0xc955b6f930 name_0: 0x2b29b410ab0 first: 0x2b29b410ab0 &first: 0xc955b6fa38

Suggested fix: Correct the diagrams so that the reference to the first string in the vector points to the string not the vec.

[willcrichton]

I think you have to be careful in your tests with auto-dereferencing. The &-operator will return a different address depending on the expected type of the &-expression:

fn main() {
    let name = vec![String::from("A"), String::from("B")];
    let a: &Vec<String> = &name;
    let b: &[String] = &name;
    println!("a: {a:p}"); // a: 0x7ffda85297a8
    println!("b: {b:p}"); // b: 0x55a8163c49b0
    println!("name_0: {:p}", &name[0]); // name_0: 0x55a8163c49b0
}

[dhmoclex]

Ok, I'm a bit at a loss here...

use std::mem;

fn main() {
    let name = vec![String::from("A"), String::from("B")];
    let a: &Vec<String> = &name;
    let b: &[String] = &name;
    let c: &String = &name[0];
    let d: &str = &name[0];
    println!("a: {a:p} {}", mem::size_of_val(a));  // a: 0x842330f2a0 24
    println!("b: {b:p} {}", mem::size_of_val(b));  // b: 0x22ca8fbfed0 48
    println!("c: {c:p} {}", mem::size_of_val(c));  // c: 0x22ca8fbfed0 24
    println!("d: {d:p} {}", mem::size_of_val(d));  // d: 0x22ca8fbaf70 1
}

[dhmoclex]

Hi again,

I've done another test :

use std::mem;

fn main() {
    let name = vec![String::from("A"), String::from("B")];
    let a: &Vec<String> = &name;
    let b: &[String] = &name;
    let c: &String = &name[0];
    let d: &String = &name[1];
    let e: &str = &name[0];
    let f: &str = &name[1];
    println!("a: {a:p} {}", mem::size_of_val(a)); // a: 0xe37d4ff5c8 24
    println!("b: {b:p} {}", mem::size_of_val(b)); // b: 0x276383303b0 48
    println!("c: {c:p} {}", mem::size_of_val(c)); // c: 0x276383303b0 24
    println!("d: {d:p} {}", mem::size_of_val(d)); // d: 0x276383303c8 24
    println!("e: {e:p} {}", mem::size_of_val(e)); // e: 0x2763832af70 1
    println!("f: {f:p} {}", mem::size_of_val(f)); // f: 0x2763832af90 1
}

It seems that &name[1] do not point to name but the actual String "B", that would says that the diagrams are indeed erroneous, no ? (Not by much, but still...)