Copy Types

While move semantics are the default, certain types are copied by default:

fn main() {
    let x = 42;
    let y = x;
    println!("x: {x}"); // would not be accessible if not Copy
    println!("y: {y}");
}

These types implement the Copy trait.

You can opt-in your own types to use copy semantics:

#[derive(Copy, Clone, Debug)]
struct Point(i32, i32);

fn main() {
    let p1 = Point(3, 4);
    let p2 = p1;
    println!("p1: {p1:?}");
    println!("p2: {p2:?}");
}

After the assignment, both p1 and p2 own their own data.
We can also use p1.clone() to explicitly copy the data.

This slide should take about 5 minutes.

Copying and cloning are not the same thing:

Copying refers to bitwise copies of memory regions and does not work on arbitrary objects.
Copying does not allow for custom logic (unlike copy constructors in C++).
Cloning is a more general operation and also allows for custom behavior by implementing the Clone trait.
Copying does not work on types that implement the Drop trait.

In the above example, try the following:

Add a String field to struct Point. It will not compile because String is not a Copy type.
Remove Copy from the derive attribute. The compiler error is now in the println! for p1.
Show that it works if you clone p1 instead.

Comprehensive Rust 🦀

Copy Types