comprehensive-rust/src/memory-management/stack.md

Stack Memory

Creating a String puts fixed-sized data on the stack and dynamically sized data on the heap:

fn main() {
    let s1 = String::from("Hello");
}

 Stack                             Heap
.- - - - - - - - - - - - - -.     .- - - - - - - - - - - - - - - -.
:                           :     :                               :
:    s1                     :     :                               :
:   +-----------+-------+   :     :   +----+----+----+----+----+  :
:   | ptr       |   o---+---+-----+-->| H  | e  | l  | l  | o  |  :
:   | len       |     5 |   :     :   +----+----+----+----+----+  :
:   | capacity  |     5 |   :     :                               :
:   +-----------+-------+   :     :                               :
:                           :     `- - - - - - - - - - - - - - - -'
`- - - - - - - - - - - - - -'

• Mention that a String is backed by a Vec, so it has a capacity and length and can grow if mutable via reallocation on the heap.

• If students ask about it, you can mention that the underlying memory is heap allocated using the System Allocator and custom allocators can be implemented using the Allocator API

• We can inspect the memory layout with unsafe code. However, you should point out that this is rightfully unsafe!

  fn main() {
      let mut s1 = String::from("Hello");
      s1.push(' ');
      s1.push_str("world");
      // DON'T DO THIS AT HOME! For educational purposes only.
      // String provides no guarantees about its layout, so this could lead to
      // undefined behavior.
      unsafe {
          let (capacity, ptr, len): (usize, usize, usize) = std::mem::transmute(s1);
          println!("ptr = {ptr:#x}, len = {len}, capacity = {capacity}");
      }
  }