Ownership and functions in Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What happens to ownership when a variable is passed to a function in Rust?

When a variable is passed to a function, ownership is moved to that function unless the type implements the Copy trait or a reference is passed.

beginner

How can you allow a function to use a value without taking ownership in Rust?

You can pass a reference to the value using the & symbol, so the function borrows the value instead of taking ownership.

intermediate

What is the difference between passing a value and passing a reference to a function in Rust?

Passing a value moves ownership to the function, while passing a reference lets the function borrow the value without taking ownership.

intermediate

What happens to the ownership of a value when a function returns it?

The ownership of the returned value moves from the function back to the caller, allowing the caller to use it.

beginner

Why does Rust prevent using a value after its ownership has been moved to a function?

Rust prevents this to avoid bugs like double free or data races by ensuring only one owner exists at a time.

What happens when you pass a String to a function without using a reference?

A Ownership of the String moves to the function.

B The function borrows the String without ownership.

C The String is copied to the function.

D The String is shared between caller and function.

Passing a String without a reference moves ownership to the function.

How do you let a function use a value without taking ownership?

A Pass the value directly.

B Pass an immutable reference.

C Pass a mutable reference.

D Clone the value inside the function.

Passing an immutable reference (&) lets the function borrow the value without taking ownership.

What must you do to use a value after passing it to a function that takes ownership?

A Nothing, you can use it freely.

B Use the value inside the function only.

C Pass a reference instead of the value.

D Declare the variable as mutable.

To use a value after passing it, you should pass a reference so ownership is not moved.

When a function returns a value, what happens to ownership?

A Ownership is shared.

B Ownership is lost.

C Ownership stays inside the function.

D Ownership moves to the caller.

The returned value's ownership moves from the function back to the caller.

Why does Rust enforce ownership rules with functions?

A To prevent bugs like double free and data races.

B To make code run faster.

C To allow multiple owners of data.

D To simplify syntax.

Rust enforces ownership to ensure memory safety and prevent bugs like double free or data races.

Explain how ownership works when passing variables to and returning from functions in Rust.

Think about who owns the data at each step.

Describe why Rust uses ownership rules with functions and how it helps prevent bugs.

Consider what problems ownership solves.

Practice

1. What happens when a function in Rust takes ownership of a variable passed to it?

easy

A. The variable is shared between the function and the caller simultaneously.

B. The variable is copied and both original and function have ownership.

C. The function borrows the variable temporarily without taking ownership.

D. The original variable can no longer be used after the function call.

Solution

1. Step 1: Understand ownership transfer in Rust functions

   When a function takes ownership, it means the variable's ownership moves to the function.

2. Step 2: Effect on original variable after function call

   Since ownership moved, the original variable cannot be used anymore after the call.

3. Final Answer:

   The original variable can no longer be used after the function call. -> Option D

4. Quick Check:

   Ownership transfer = original variable unusable [OK]

Hint: Ownership moves to function, original variable becomes invalid [OK]

Common Mistakes:

• Thinking ownership is shared, not moved
• Assuming variable is copied automatically
• Confusing borrowing with ownership transfer

2. Which of the following function signatures correctly borrows a string slice without taking ownership?

easy

A. fn print_name(name: String) { println!("{}", name); }

B. fn print_name(name: &String) { println!("{}", name); }

C. fn print_name(name: &str) { println!("{}", name); }

D. fn print_name(name: String) -> String { name }

Solution

1. Step 1: Identify borrowing syntax for string slices

   Borrowing a string slice uses &str type, which is a reference to a string slice.

2. Step 2: Check each signature for borrowing without ownership

   fn print_name(name: &str) { println!("{}", name); } uses &str, which borrows without taking ownership. fn print_name(name: &String) { println!("{}", name); } borrows a String reference but &str is more idiomatic for slices.

3. Final Answer:

   fn print_name(name: &str) { println!("{}", name); } -> Option C

4. Quick Check:

   Borrow string slice = &str parameter [OK]

Hint: Use &str to borrow string slices without ownership [OK]

Common Mistakes:

• Using String parameter takes ownership
• Confusing &String with &str
• Returning ownership when not needed

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

fn take_ownership(s: String) {
    println!("Inside function: {}", s);
}

fn main() {
    let s = String::from("hello");
    take_ownership(s);
    println!("Outside function: {}", s);
}

medium

A. Inside function: hello
Compile error: use of moved value 's'

B. Inside function: hello
Outside function: hello

C. Inside function: hello
Outside function: <empty>

D. Runtime error: variable s not found

Solution

1. Step 1: Analyze ownership transfer in function call

   The function take_ownership takes ownership of s, so s is moved into the function.

2. Step 2: Check usage of s after function call

   After the call, s is no longer valid in main, so using it causes a compile error.

3. Final Answer:

   Inside function: hello Compile error: use of moved value 's' -> Option A

4. Quick Check:

   Ownership moved, use after move = compile error [OK]

Hint: Using moved variable after function call causes compile error [OK]

Common Mistakes:

• Assuming variable is copied automatically
• Expecting output after ownership moved
• Ignoring Rust's ownership rules

4. Identify the error in this Rust code and how to fix it:

fn calculate_length(s: String) -> usize {
    s.len()
}

fn main() {
    let s1 = String::from("hello");
    let len = calculate_length(s1);
    println!("The length of '{}' is {}.", s1, len);
}

medium

A. Error: s1 is moved into calculate_length; fix by passing &s1 instead.

B. Error: calculate_length returns wrong type; fix by returning &usize.

C. Error: println! macro syntax incorrect; fix by removing quotes.

D. No error; code runs and prints length correctly.

Solution

1. Step 1: Identify ownership issue in function call

   calculate_length takes ownership of s1, so s1 is moved and invalid after call.

2. Step 2: Fix by borrowing s1 instead of moving

   Change function parameter to &String and pass &s1 to avoid moving ownership.

3. Final Answer:

   Error: s1 is moved into calculate_length; fix by passing &s1 instead. -> Option A

4. Quick Check:

   Pass reference to avoid moving ownership [OK]

Hint: Pass references to keep ownership when calling functions [OK]

Common Mistakes:

• Trying to return reference to local variable
• Ignoring ownership move on function parameters
• Misunderstanding borrowing syntax

5. You want to write a function that takes ownership of a vector, doubles each element, and returns ownership back. Which function signature and body correctly implement this in Rust?

hard

A. fn double_vec(v: &Vec<i32>) -> Vec<i32> { v.iter().map(|x| x * 2).collect() }

B. fn double_vec(v: Vec<i32>) -> Vec<i32> { v.iter().map(|x| x * 2).collect() }

C. fn double_vec(v: Vec<i32>) { for x in &v { println!("{}", x * 2); } }

D. fn double_vec(v: &mut Vec<i32>) { for x in v { *x *= 2; } }

Solution

1. Step 1: Understand ownership and return requirements

   The function must take ownership of the vector and return ownership back after doubling elements.

2. Step 2: Analyze each option for ownership and functionality

   fn double_vec(v: Vec<i32>) -> Vec<i32> { v.iter().map(|x| x * 2).collect() } takes ownership (Vec<i32>), creates a new doubled vector, and returns it, satisfying requirements.

3. Final Answer:

   fn double_vec(v: Vec<i32>) -> Vec<i32> { v.iter().map(|x| x * 2).collect() } -> Option B

4. Quick Check:

   Take ownership, return new doubled vector [OK]

Hint: Take Vec ownership and return new Vec after processing [OK]

Common Mistakes:

• Borrowing vector but not returning ownership
• Not returning anything from function
• Modifying borrowed vector without ownership