Good Brothers: Copy and Clone in Rust

In Rust, the Copy and Clone traits control the copying behavior of types. They allow you to define how values of a type are copied and under what circumstances copying is allowed. This article will introduce the purpose and usage of these two traits in detail, along with code examples demonstrating their usage.

The Copy Trait

The Copy trait signifies that a type can be copied bit by bit. When a type implements the Copy trait, its values are automatically duplicated when assigned, passed as arguments, or returned.

What is the Copy Trait?

The Copy trait is a marker trait, meaning it does not define any methods. It simply marks a type as being eligible for bitwise copying.

#[derive(Copy)]
struct Point {
    x: i32,
    y: i32,
}

How to Implement the Copy Trait?

To implement the Copy trait, you need to add the #[derive(Copy)] attribute to the type definition. Additionally, the type must also implement the Clone trait because all types that implement Copy must also implement Clone.

#[derive(Copy, Clone)]
struct Point {
    x: i32,
    y: i32,
}

If you attempt to implement Copy without also implementing Clone, the compiler will throw an error:

#[derive(Copy)]
struct Point {
    x: i32,
    y: i32,
}

// error[E0277]: the trait bound `Point: std::clone::Clone` is not satisfied

The error message indicates that the Point type does not implement the Clone trait, and therefore it cannot implement Copy.

This requirement exists because all Copy types must also implement Clone. When you explicitly call the clone method, Rust assumes you intend to create a copy and wants to ensure that copying behavior is well-defined. Thus, if you want to implement Copy, you must also implement Clone.

Which Types Can Implement Copy?

Not all types can implement Copy. Only types that meet the following criteria are eligible:

• The type itself is a Plain Old Data (POD) type, meaning it does not contain any pointers or references.
• All fields of the type must also implement Copy.

For example, the following type cannot implement Copy because it contains a reference field:

struct Foo<'a> {
    x: &'a i32,
}

// error[E0204]: the trait `Copy` may not be implemented for this type
impl Copy for Foo<'_> {}

Why Do We Need the Copy Trait?

The Copy trait allows control over a type’s copying behavior. When a type implements Copy, its values are automatically duplicated upon assignment, function parameter passing, and returning. This eliminates the need to explicitly call clone() to copy values.

Moreover, since Copy types always undergo bitwise copying, the performance overhead is minimal. This is particularly useful for optimizing performance in Rust programs.

The Clone Trait

Unlike Copy, the Clone trait allows explicit copying of a type’s value. When a type implements Clone, you can call its clone() method to create a new instance.

What is the Clone Trait?

Unlike Copy, Clone is a regular trait that contains a method: clone(). This method is responsible for creating a new copy of the value.

#[derive(Clone)]
struct Point {
    x: i32,
    y: i32,
}

How to Implement the Clone Trait?

To implement the Clone trait, you can either add the #[derive(Clone)] attribute or manually implement the clone() method.

#[derive(Clone)]
struct Point {
    x: i32,
    y: i32,
}

// Manually implementing the `clone()` method
impl Clone for Point {
    fn clone(&self) -> Self {
        Self { x: self.x, y: self.y }
    }
}

Which Types Can Implement Clone?

Almost all types can implement Clone. As long as you can define how to create a new copy of a value, you can implement Clone.

Why Do We Need the Clone Trait?

The Clone trait allows explicit duplication of values. This is particularly useful for types that cannot be bitwise copied, such as those containing pointers or references.

Additionally, Clone allows customization of the copying process. You can add any necessary logic inside the clone() method to perform specific actions during copying.

Differences and Relationship Between Copy and Clone

Both Copy and Clone control how types are copied, but they have key differences:

• Copy is a marker trait, indicating that a type supports bitwise copying. When a type implements Copy, its values are automatically duplicated upon assignment, passing as function arguments, and returning.
• Clone is a regular trait that contains a method: clone(). When a type implements Clone, you can explicitly call clone() to create a new copy.

Additionally, all Copy types must also implement Clone. This ensures that when you explicitly call clone(), Rust assumes you know what you are doing and allows bitwise copying.

Example Analysis

Below is an example demonstrating the usage of Copy and Clone:

#[derive(Copy, Clone)]
struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let p1 = Point { x: 1, y: 2 };
    let p2 = p1; // Automatically copied
    let p3 = p1.clone(); // Explicitly copied
}

In this example, we define a Point type and implement both the Copy and Clone traits. In the main function, we create a Point value and assign it to another variable. Since Point implements Copy, the assignment operation automatically duplicates the value. Additionally, we explicitly call clone() to create another copy of the value.

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