Copy types in Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean for a type to implement the Copy trait in Rust?

A type that implements the Copy trait can be duplicated simply by copying bits, without needing to run any special code. This means values are copied automatically when assigned or passed to functions, instead of moved.

beginner

Which of these Rust types usually implement the Copy trait?
A) String
B) i32
C) Vec<T>
D) &str

The correct answers are i32 and &str. These types are simple and stored on the stack, so they implement Copy. Types like String and Vec<T> manage heap data and do not implement Copy.

beginner

What happens if you try to use a value after it has been moved in Rust?

Rust will give a compile-time error because the value is no longer valid after being moved. However, if the type implements Copy, the value is copied instead of moved, so you can still use the original.

intermediate

How do you make a custom struct implement the Copy trait?

You add #[derive(Copy, Clone)] above the struct definition. The Clone trait is required because Copy is a marker trait that depends on Clone.

intermediate

Why can't types that own heap data implement the Copy trait?

Because copying heap data requires more than just copying bits; it needs to allocate new memory and copy the contents safely. Rust requires explicit cloning for these types to avoid accidental shallow copies that could cause bugs.

Which trait must a Rust type implement to be copied automatically instead of moved?

A Default

B Clone

C Drop

D Copy

The Copy trait allows values to be duplicated by simple bitwise copy without moving.

What happens when you assign a Copy type to a new variable?

A The value is copied and both variables are valid.

B The value is moved and the old variable is invalidated.

C The program crashes.

D The compiler asks for a clone method.

For Copy types, assignment copies the value, so both variables remain valid.

Which of these types does NOT implement Copy by default?

A String

B f64

C bool

D char

String owns heap data and does not implement Copy.

To implement Copy for your struct, you must also implement which trait?

A Default

B Clone

C Debug

D PartialEq

Copy requires Clone because copying is a special kind of cloning.

Why is Copy trait useful in Rust?

A It automatically frees memory.

B It prevents any copying of values.

C It allows automatic duplication of simple values without ownership transfer.

D It makes types mutable.

Copy lets simple types be duplicated automatically, avoiding ownership moves.

Explain what the Copy trait does in Rust and give examples of types that implement it.

Think about how simple values like numbers behave when assigned.

Describe how to make a custom struct implement the Copy trait and why you might want to do that.

Remember that Copy is a marker trait and depends on Clone.

Practice

1. Which of the following types in Rust is a Copy type by default?

easy

A. A String

B. An integer like i32

C. A vector like Vec<i32>

D. A struct with a String field

Solution

1. Step 1: Identify primitive types with Copy trait

   Primitive scalar types like integers (i32) implement Copy by default.

2. Step 2: Understand ownership of heap data

   Types like String and Vec own heap data and do not implement Copy automatically.

3. Final Answer:

   An integer like i32 -> Option B

4. Quick Check:

   Primitive types = Copy [OK]

Hint: Primitive scalars like i32 are Copy by default [OK]

Common Mistakes:

• Thinking String is Copy because it looks simple
• Assuming all structs are Copy without checking fields
• Confusing heap-allocated types with Copy types

2. Which syntax correctly derives the Copy trait for a struct in Rust?

easy

A. struct Point { x: i32, y: i32 } // Copy is automatic

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

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

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

Solution

1. Step 1: Recall trait derivation rules

   To make a struct Copy, you must derive both Copy and Clone traits.

2. Step 2: Check each option

   #[derive(Copy, Clone)] struct Point { x: i32, y: i32 } correctly derives both traits. #[derive(Clone)] struct Point { x: i32, y: i32 } misses Copy. struct Point { x: i32, y: i32 } // Copy is automatic is wrong because Copy is not automatic for structs. #[derive(Copy)] struct Point { x: i32, y: i32 } misses Clone.

3. Final Answer:

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

4. Quick Check:

   Copy requires Copy + Clone traits [OK]

Hint: Always derive both Copy and Clone for Copy types [OK]

Common Mistakes:

• Deriving only Copy without Clone
• Assuming Copy is automatic for structs
• Forgetting to derive Clone along with Copy

3. What will be the output of this Rust code?

#[derive(Copy, Clone)]
struct Color(u8, u8, u8);

fn main() {
    let c1 = Color(255, 0, 0);
    let c2 = c1;
    println!("{} {} {}", c1.0, c1.1, c1.2);
}

medium

A. 255 0 0

B. Compile error: use of moved value

C. 0 0 0

D. Runtime panic

Solution

1. Step 1: Understand Copy trait effect on assignment

   Since Color derives Copy, assigning c1 to c2 copies the data, not moves it.

2. Step 2: Check usage after assignment

   Because c1 is still valid, printing its fields outputs the original values.

3. Final Answer:

   255 0 0 -> Option A

4. Quick Check:

   Copy means no move, data duplicated [OK]

Hint: Copy types allow use after assignment without move errors [OK]

Common Mistakes:

• Expecting a move error after assignment
• Thinking c1 is invalid after c2 = c1
• Confusing Copy with Clone behavior

4. Identify the error in this Rust code snippet:

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

fn main() {
    let p1 = Point { x: 5, y: 10 };
    let p2 = p1;
    println!("{} {}", p1.x, p1.y);
}

medium

A. Assignment moves ownership, so p1 is invalid

B. Cannot print struct fields directly

C. No error, code runs fine

D. Missing Clone trait derivation causes compile error

Solution

1. Step 1: Check trait derivations

   The struct derives only Copy, but Copy requires Clone to be implemented too.

2. Step 2: Understand compiler requirements

   Rust compiler will error because Clone is missing, even if Copy is derived.

3. Final Answer:

   Missing Clone trait derivation causes compile error -> Option D

4. Quick Check:

   Copy needs Clone too [OK]

Hint: Derive Clone along with Copy to avoid errors [OK]

Common Mistakes:

• Assuming Copy alone is enough
• Thinking printing fields causes error
• Confusing move error with trait error

5. You want to create a struct Pixel with fields x: u32, y: u32, and color: (u8, u8, u8). You want Pixel to be a Copy type. Which of the following is the correct approach?

hard

A. Derive Copy and Clone for Pixel since all fields are Copy types

B. Implement Copy manually without Clone

C. Use String for color and derive Copy

D. Derive only Clone for Pixel

Solution

1. Step 1: Check field types for Copy trait

   Fields u32 and tuple of u8 are all Copy types.

2. Step 2: Derive Copy and Clone for struct

   Since all fields are Copy, deriving both Copy and Clone for Pixel makes it a Copy type.

3. Final Answer:

   Derive Copy and Clone for Pixel since all fields are Copy types -> Option A

4. Quick Check:

   All fields Copy + derive Copy & Clone = Copy type [OK]

Hint: All fields Copy? Derive Copy and Clone for struct [OK]

Common Mistakes:

• Using String for color which is not Copy
• Deriving only Clone without Copy
• Trying to implement Copy manually