Immutable references in Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an immutable reference in Rust?

An immutable reference in Rust allows you to read data without changing it. It is created using & before a variable, and you cannot modify the data through this reference.

beginner

How do you create an immutable reference to a variable x in Rust?

You create it by writing &x. This means you borrow the value of x without taking ownership and without allowing changes.

intermediate

Can you have multiple immutable references to the same data at the same time in Rust?

Yes, Rust allows many immutable references to the same data simultaneously because none of them can change the data, so it is safe.

intermediate

What happens if you try to create a mutable reference while immutable references exist?

Rust will give a compile-time error because mutable references require exclusive access, and immutable references mean shared access. You cannot have both at the same time.

advanced

Why does Rust enforce rules about immutable and mutable references?

Rust enforces these rules to prevent bugs like data races and unexpected changes. Immutable references guarantee safe, read-only access, while mutable references allow safe changes without conflicts.

What symbol is used to create an immutable reference in Rust?

A ->

B *

C &&

D &

The ampersand (&) is used to create an immutable reference in Rust.

Can you modify data through an immutable reference?

A Yes, always

B No, never

C Only if the data is mutable

D Only inside unsafe blocks

Immutable references do not allow modifying the data they point to.

How many immutable references to the same data can exist at once?

A Only one

B None

C Multiple

D Exactly two

Rust allows multiple immutable references to the same data simultaneously.

What error occurs if you try to create a mutable reference while immutable references exist?

A Compile-time error

B Logical error

C Runtime panic

D No error

Rust prevents this with a compile-time error to ensure safety.

Why does Rust restrict mutable and immutable references from coexisting?

A To prevent data races and bugs

B To improve performance

C To make syntax simpler

D To allow unsafe code

Rust's rules prevent data races and unexpected bugs by controlling access.

Explain what an immutable reference is in Rust and why it is useful.

Think about how you can look at data without changing it.

Describe the rules Rust enforces about mutable and immutable references and the reasons behind them.

Consider how Rust keeps your program safe from bugs.

Practice

1. What does an immutable reference in Rust allow you to do?

easy

A. Prevent any access to the data

B. Read data without changing it

C. Create multiple mutable references

D. Change data while reading it

Solution

1. Step 1: Understand the purpose of immutable references

   Immutable references allow you to look at data without changing it.

2. Step 2: Compare options with this understanding

   Only Read data without changing it correctly states that immutable references let you read data without changing it.

3. Final Answer:

   Read data without changing it -> Option B

4. Quick Check:

   Immutable reference = read-only access [OK]

Hint: Immutable references only allow reading data [OK]

Common Mistakes:

• Thinking immutable references allow data modification
• Confusing mutable and immutable references
• Believing immutable references block all access

2. Which of the following is the correct syntax to create an immutable reference to a variable x in Rust?

easy

A. let ref = x&;

B. let ref = *x;

C. let ref = &mut x;

D. let ref = &x;

Solution

1. Step 1: Recall syntax for immutable references

   In Rust, an immutable reference is created using the ampersand before the variable: &x.

2. Step 2: Check each option

   let ref = &x; uses &x correctly. let ref = *x; uses dereference operator incorrectly. let ref = &mut x; creates a mutable reference, not immutable. let ref = x&; is invalid syntax.

3. Final Answer:

   let ref = &x; -> Option D

4. Quick Check:

   Immutable reference syntax = &variable [OK]

Hint: Use & before variable name for immutable reference [OK]

Common Mistakes:

• Using *x instead of &x
• Confusing mutable (&mut) with immutable (&)
• Writing invalid syntax like x&

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

let x = 10;
let r1 = &x;
let r2 = &x;
println!("{} and {}", r1, r2);

medium

A. Compilation error due to multiple references

B. Runtime error

C. 10 and 10

D. 10 and 0

Solution

1. Step 1: Understand multiple immutable references

   Rust allows multiple immutable references to the same data at the same time without error.

2. Step 2: Check the print statement

   Both r1 and r2 point to x which is 10, so printing them shows "10 and 10".

3. Final Answer:

   10 and 10 -> Option C

4. Quick Check:

   Multiple immutable refs print same value [OK]

Hint: Multiple immutable references can coexist safely [OK]

Common Mistakes:

• Thinking multiple immutable references cause errors
• Expecting runtime errors from references
• Confusing mutable and immutable references rules

4. Identify the error in this Rust code snippet:

let mut x = 5;
let r1 = &x;
let r2 = &mut x;
println!("{} and {}", r1, r2);

medium

A. Cannot have mutable and immutable references at the same time

B. Missing semicolon after let r1

C. Cannot print references directly

D. Variable x should not be mutable

Solution

1. Step 1: Check reference rules in Rust

   Rust forbids having a mutable reference while immutable references exist to the same variable.

2. Step 2: Analyze the code

   Here, r1 is an immutable reference and r2 is a mutable reference to x at the same time, causing a compile error.

3. Final Answer:

   Cannot have mutable and immutable references at the same time -> Option A

4. Quick Check:

   Mutable + immutable refs together = error [OK]

Hint: Mutable and immutable refs can't coexist simultaneously [OK]

Common Mistakes:

• Ignoring Rust's borrowing rules
• Thinking printing references causes error
• Assuming mutability of variable is the problem

5. Given this Rust function:

fn print_value(val: &i32) {
    println!("Value is {}", val);
}

fn main() {
    let mut num = 20;
    let r1 = #
    print_value(r1);
    let r2 = &mut num;
    *r2 += 5;
    print_value(&num);
}

What is the output when this program runs?

hard

A. Value is 20\nValue is 25

B. Compilation error due to mutable and immutable references

C. Value is 25\nValue is 25

D. Runtime error

Solution

1. Step 1: Analyze references in main function

   First, an immutable reference r1 to num is created and passed to print_value. Then a mutable reference r2 is created after r1 is no longer used.

2. Step 2: Check Rust borrowing rules

   Rust allows mutable references after immutable references are no longer used. Here, r1 is used only in print_value, so r2 creation is valid.

3. Step 3: Determine output

   First print_value prints 20, then after incrementing via r2, second print_value prints 25.

4. Final Answer:

   Value is 20 Value is 25 -> Option A

5. Quick Check:

   Immutable ref used before mutable ref creation [OK]

Hint: Mutable ref can be created after immutable ref is no longer used [OK]

Common Mistakes:

• Assuming code runs into compile error
• Ignoring Rust's borrowing rules about scopes
• Thinking mutable ref can coexist with immutable ref simultaneously