Ownership and functions in Rust

Introduction

Ownership helps Rust keep track of who owns data. Functions can take or give ownership to keep programs safe and fast.

When you want a function to use data without making a copy.

When you want a function to change data and keep the changes.

When you want to give data to a function and not use it afterward.

When you want to borrow data temporarily inside a function.

When you want to return ownership of data from a function.

Syntax

Rust

fn function_name(parameter: Type) -> ReturnType {
    // function body
}

Parameters can take ownership or borrow data using references.

Functions can return ownership by returning values.

Examples

This function takes ownership of the string. After calling it, the original variable cannot be used.

Rust

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

fn main() {
    let s = String::from("hello");
    take_ownership(s);
    // s is no longer valid here
}

This function borrows the string using a reference. The original string stays valid after the call.

Rust

fn borrow_string(s: &String) {
    println!("{}", s);
}

fn main() {
    let s = String::from("hello");
    borrow_string(&s);
    // s can still be used here
    println!("Still can use s: {}", s);
}

This function creates a string and returns it, giving ownership to the caller.

Rust

fn give_ownership() -> String {
    let s = String::from("hello");
    s
}

fn main() {
    let s1 = give_ownership();
    println!("{}", s1);
}

Sample Program

This program shows two functions: one takes ownership of a string, the other borrows it. After ownership is taken, the original variable cannot be used. After borrowing, it can still be used.

Rust

fn main() {
    let s = String::from("Rust");
    print_string(s);
    // s is no longer valid here

    let s2 = String::from("Ownership");
    print_string_borrow(&s2);
    println!("After borrowing: {}", s2);
}

fn print_string(s: String) {
    println!("Owned string: {}", s);
}

fn print_string_borrow(s: &String) {
    println!("Borrowed string: {}", s);
}

Important Notes

When a function takes ownership, the original variable can no longer be used after the call.

Borrowing with references lets you use data without giving up ownership.

Returning a value from a function transfers ownership to the caller.

Summary

Functions can take or borrow ownership of data.

Ownership rules help prevent bugs and keep programs safe.

Understanding ownership with functions is key to writing Rust code.

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