Ownership

systems memory security Ownership is Rust’s most unique feature. It enables memory safety without a garbage collector by enforcing a strict set of rules at compile time. If these rules are violated, the program will not compile.

The Three Rules

IMPORTANT

1. Each value in Rust has a variable that’s called its owner.
2. There can only be one owner at a time.
3. When the owner goes out of scope, the value will be dropped (memory freed).

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Stack vs. Heap

Understanding Stack(s) and Heap(s) is crucial for Ownership.

• Stack: Fast, fixed size. Stores primitives (int, bool, char) and fixed-size arrays. Values are “copied” cheaply.
• Heap: Slower, dynamic size. Stores complex types (String, Vector). Pointers to the heap data are stored on the stack.

The “Move” Semantics

In many languages, assigning variable a to b copies the value. In Rust, for heap data, it moves ownership.

let s1 = String::from("hello");
let s2 = s1; // s1 is MOVED to s2. s1 is now INVALID.
 
// println!("{}", s1); // Compile Error! "Value used after move"

Why?

To prevent Double Free errors. If both s1 and s2 owned the memory, both would try to free it when they go out of scope, causing memory corruption.

Clone vs. Copy

• Copy: Types stored entirely on the stack (integers, bools) implement the Copy trait. Assignment makes a bitwise copy. s1 remains valid.
• Clone: For heap data, if you want a deep copy (new heap allocation), you must call .clone().

  let s1 = String::from("hello");
  let s2 = s1.clone(); // Deep copy. Both s1 and s2 are valid.

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Ownership and Functions

Passing a variable to a function follows the same rules:

• Passing a heap variable moves ownership to the function.
• Passing a stack variable copies it.

fn main() {
    let s = String::from("hello");
    takes_ownership(s); 
    // s is no longer valid here!
}
 
fn takes_ownership(some_string: String) { // some_string comes into scope
    println!("{}", some_string);
} // some_string goes out of scope and `drop` is called. Memory freed.

To use s again, we would need to return it, or use Borrowing and References.

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