Is a reference to String a pointer to a String variable or a pointer to its ptr member?

Question

I saw this image while reading "the book" (https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html#references-and-borrowing):

reference representation in Rust

I have two questions：

Does reference s point to s1 itself (address of struct s1) or the member ptr of s1?
Is s (the reference) itself a Struct that contains a public member ptr or a pointer (string* s) like in C++?

For easier understanding:

fn main() {
    // String
    let s1 = String::from("hello");

    // reference
    let s = &s1;

    println!("s:{:p}, s1:{:p}", s, &s1);
    // OUTPUT: s:0x63373afae0, s1:0x63373afae0

}

Is this a visualization of let s = &s1 where let s1 = "hello".to_string()? — tadman
– tadman, Commented Jan 4, 2023 at 16:39
In Rust, references tend to behave like pointers but with all sorts of other checks, like borrow checking, no ability to create "null" references, etc. — tadman
– tadman, Commented Jan 4, 2023 at 16:40
You should include the context of how s and s1 are defined in their example. — Alexander
– Alexander, Commented Jan 4, 2023 at 16:49
@tadman I saw it in the Rust book, I have added the link, Thank you. — Lorenzo Yang
– Lorenzo Yang, Commented Jan 4, 2023 at 17:35

Chayim Friedman · Accepted Answer · 2023-01-04 18:05:55Z

3

Does reference s point to s1 itself (address of struct s1) or the member ptr of s1?

It is pointing on the whole struct.

Is s (the reference) itself a Struct that contains a public member ptr or a pointer (string* s) like in C++?

A pointer; references are a primitive.

edited Jan 4, 2023 at 18:05

answered Jan 4, 2023 at 17:39

Chayim Friedman

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7 Comments

E_net4 Over a year ago

"both have the same address" How can you be sure of this?

Chayim Friedman Over a year ago

@E_net4thecommentflagger Because that's what the figure shows :) Not because I know the layout of String of course.

E_net4 Over a year ago

The figure merely shows a possible arrangement of fields within struct String. s and s1 are part of a code snippet. The way they are arranged and padded in memory is unspecified, so they might not always "have the same address".

Chayim Friedman Over a year ago

@E_net4thecommentflagger Ok, got your opinion. Will edit.

Lorenzo Yang Over a year ago

But the Slice type is also a pointer, it should be a Struct, right? Because it has two public fileds: ptr and length: Struct fatPointer {pub ptr, pub length},

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