Memory addresses of variable and of data

help
1 
    let temp = String::from("The black cat");
    println!("p: {:p} ptr: {:?} ", &temp, temp.as_ptr());

Do I understand correctly that in the above code {:p} will print the address of the variable and as_ptr() returns the address of the payload, the actual content of this variable?

2

Yes.

fn main() {
    let temp = String::from("The black cat");
    let x = &temp;
    let addr_of_temp = std::ptr::addr_of!(temp);
    let y = temp.as_ptr();
    let x_deref = &**x as *const str;
    
    println!("p: {x:p} addr_of_temp: {addr_of_temp:?} ptr: {y:?} x_deref: {x_deref:?}");
}

Playground.

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3

What are the differences between the two (or in this example 4) different pointers?

4

String is just a vector with valid utf-8 bytes. The bytes are stored on the heap in some contiguous slice of memory. The String instance is a fat pointer consisting of a pointer to the bytes, together with the length and capacity. When we call String::as_ptr, we get the address of the bytes on the heap. When we print the address of a reference to the String instance, we get the address of the fat pointer.

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5

Here's a diagram. The two different values printed in the playground correspond to the two bottom Pointer values in the diagram.

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6

Did you put in the love or do you use a tool to create your ASCII diagrams? Also, if anyone has an idea how boats makes the diagrams in their blog, I'd love to know, they are super fancy.

7

Just by hand (80% copy-pasting).

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8

You can try this site: https://asciiflow.com/

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9

Well yes, one could use a graphical editor for that, but I can't. They make me anxious :sweat_smile:. I was hoping for something like mermaid.js or tikz, some sort of declarative markup language with which I can define the graph that is rendered automatically as an ASCII diagram.

10

Sounds a bit like graph-easy or adia (sequence diagrams only) might be more suitable for you.

Edit: I just found that there's an ASCII renderer for graphviz, too: ggerganov/dot-to-ascii: Graphviz to ASCII converter using Graph::Easy (github.com)

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