Review of Program Memory

Programs allocate memory in two ways:

  • Stack: Continuous area of memory for local variables.

    • Values have fixed sizes known at compile time.
    • Extremely fast: just move a stack pointer.
    • Easy to manage: follows function calls.
    • Great memory locality.
  • Heap: Storage of values outside of function calls.

    • Values have dynamic sizes determined at runtime.
    • Slower than the stack: some book-keeping needed.
    • No guarantee of memory locality.


Creating a String puts fixed-sized metadata on the stack and dynamically sized data, the actual string, on the heap:

fn main() {
    let s1 = String::from("Hello");
This slide should take about 5 minutes.
  • 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