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16 Traits

16.7 Clone

When dealing with resources, the default behavior is to transfer them during assignments or function calls. However, sometimes we need to make a copy of the resource as well.

The Clone trait helps us do exactly this. Most commonly, we can use the .clone() method defined by the Clone trait.

Copy: Implicit Cloning

The Copy trait allows a type to be duplicated simply by copying bits, with no additional logic required. When a type implements Copy, assignments and function calls will implicitly copy the value instead of moving it.

Important: Copy requires Clone - any type that implements Copy must also implement Clone. This is because Copy is defined as a subtrait: trait Copy: Clone {}. The Clone implementation for Copy types simply copies the bits.

Not all types can implement Copy. A type can only be Copy if:

// A unit struct without resources
// Note: Copy requires Clone, so we must derive both
#[derive(Debug, Clone, Copy)]
struct Unit;

// A tuple struct with resources that implements the `Clone` trait
// This CANNOT be Copy because Box<T> is not Copy
#[derive(Clone, Debug)]
struct Pair(Box<i32>, Box<i32>);

fn main() {
    // Instantiate `Unit`
    let unit = Unit;
    // Copy `Unit` - this is an implicit copy, not a move!
    // Because Unit implements Copy, the value is duplicated automatically
    let copied_unit = unit;

    // Both `Unit`s can be used independently
    println!("original: {:?}", unit);
    println!("copy: {:?}", copied_unit);

    // Instantiate `Pair`
    let pair = Pair(Box::new(1), Box::new(2));
    println!("original: {:?}", pair);

    // Move `pair` into `moved_pair`, moves resources
    // Pair does not implement Copy, so this is a move
    let moved_pair = pair;
    println!("moved: {:?}", moved_pair);

    // Error! `pair` has lost its resources
    //println!("original: {:?}", pair);
    // TODO ^ Try uncommenting this line

    // Clone `moved_pair` into `cloned_pair` (resources are included)
    // Unlike Copy, Clone is explicit - we must call .clone()
    let cloned_pair = moved_pair.clone();
    // Drop the moved original pair using std::mem::drop
    drop(moved_pair);

    // Error! `moved_pair` has been dropped
    //println!("moved and dropped: {:?}", moved_pair);
    // TODO ^ Try uncommenting this line

    // The result from .clone() can still be used!
    println!("clone: {:?}", cloned_pair);
}
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