The Copy trait - what does it actually copy?


#21

That’s not quite it either. In your setup you’re imagining that hello and hello1 point to an intermediate address which then points to "hello, world!", but in reality there is no intermediary. hello and hello1 are the addresses that do the pointing.


#22

Nope, there’s no pointers to pointers going on here. A “fat pointer” is not a pointer to another pointer. It’s just a pointer and some other value that the type system refuses to let you split apart (unless you go unsafe).

Lemme try this…

let hello = "Hello, world!";
let hello1 = hello;

does this:

Read-only memory:


Hello, World! // let’s say the ‘H’ is at address 0xABC

Stack:


0xABC // hello's pointer
13 // hello's length
0xABC // hello1's pointer
13 // hello1's length


#23

You may find this cheat sheet that someone created a while ago useful: https://i.redd.it/moxxoeir7iqz.png


#24

Thank you all so much!

As I now understand, &str and &i8 behave the same (maybe as all &T), so when the Rust Book says:

let x = 5;
let y = x;

We can probably guess what this is doing: “bind the value 5 to x; then make a copy of the value in x and bind it to y.” We now have two variables, x and y, and both equal 5. This is indeed what is happening, because integers are simple values with a known, fixed size, and these two 5 values are pushed onto the stack.

The value being Copy is the integer 5.


But if we were to instead create a reference, i.e. &T:

let x = &5;
let y = x;

The value being Copy the reference to a hardcoded 5 somewhere off in read-only memory, i.e. not the stack or heap.


#25

This is a small rewrite. Here, I’m just specifying that it is a (fat) pointer which is being copied, not the actual string:

The string literal, "Hello, World!", is stored somewhere in read-only memory, (neither in the stack nor heap), and a pointer to that string is stored on the stack. Because it’s a string literal, it usually shows up as a reference, meaning that we use a pointer to the string stored in permanent memory, (see Ownership in Rust, Part 2 for more on references), and it’s guaranteed to be valid for the duration of the entire program, (it has a static lifetime).

Here, the pointers stored in hello and hello1 are using the stack. When we use the = operator, Rust pushes a new copy of the pointer stored in hello onto the stack, and binds it to hello1. At the end of the scope, Rust adds a call to drop which pops the values from the stack in order to free up memory. These pointers can be stored and easily copied to the stack because their size is known at compile-time.


#26

It might also be useful to make another block for the string “Hello, World”, which is in a separate region titled “static data”, and add dotted arrows between the pointers and that block.


#27

That’s a good idea, thank you!


#28

More to the point, the two intermediary fat pointer blocks should be relatively small in size, with just one large block for the actual string. So I would add two small yellow “fat pointer” rectangles that are pointed to from the stacks, with both rectangles pointing to the large purple blob that represents the actual string literal. If you wanted to put content in the “fat pointer” rectangles, they should each contain the same two usize items: the byte address of the start of the string literal and the byte length of the string literal.


#29

The book also has a good explanation of ownership: https://doc.rust-lang.org/book/second-edition/ch04-01-what-is-ownership.html