Mutable references in Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a mutable reference in Rust?

A mutable reference in Rust allows you to change the value of the data it points to without taking ownership of the data. It is declared with &mut.

beginner

How do you declare a mutable reference to a variable x in Rust?

You declare it like this: <code>let r = &mut x;</code>. This means <code>r</code> can change the value of <code>x</code> through the reference.

intermediate

Why does Rust allow only one mutable reference to a value at a time?

Rust enforces only one mutable reference at a time to prevent data races and ensure safe memory access. This rule avoids conflicts when changing data.

intermediate

What happens if you try to have a mutable reference and an immutable reference to the same data at the same time?

Rust will give a compile-time error because it forbids having mutable and immutable references to the same data simultaneously to keep data safe and consistent.

beginner

Explain the difference between &T and &mut T in Rust.

&T is an immutable reference, meaning you can read but not change the data. &mut T is a mutable reference, allowing you to both read and change the data.

What keyword is used to create a mutable reference in Rust?

A mut

B &

C &mut

D *mut

The keyword &mut is used to create a mutable reference in Rust.

Why does Rust allow only one mutable reference to a value at a time?

A To prevent data races and ensure safe memory access

B To make the code run faster

C To allow multiple threads to write simultaneously

D Because mutable references are slower

Rust enforces one mutable reference at a time to prevent data races and keep memory safe.

What will happen if you try to have both a mutable and an immutable reference to the same data at the same time?

A Program runs normally

B Data is copied automatically

C Runtime error

D Compile-time error

Rust forbids mutable and immutable references to the same data simultaneously, causing a compile-time error.

Which of these is a valid mutable reference declaration in Rust?

A let r = &mut x;

B let r = &x;

C let r = mut &x;

D let r = *mut x;

The correct syntax for a mutable reference is let r = &mut x;.

What can you do with a mutable reference that you cannot do with an immutable reference?

A Copy the value it points to

B Modify the value it points to

C Read the value it points to

D Delete the value it points to

Mutable references allow you to change the value they point to, unlike immutable references.

Explain what a mutable reference is in Rust and why it is useful.

Think about how you can change a value without owning it.

Describe Rust's rules about mutable and immutable references and why these rules matter.

Consider how Rust keeps your data safe when multiple parts of code access it.

Practice

1. What does a mutable reference &mut allow you to do in Rust?

easy

A. Prevent any changes to the value it points to

B. Create multiple immutable references at the same time

C. Move ownership of the value to another variable

D. Change the value it points to without taking ownership

Solution

1. Step 1: Understand what &mut means

   The &mut symbol creates a mutable reference, which means you can change the value it points to without taking ownership.

2. Step 2: Compare with other options

   Options B and D describe immutable references or no changes allowed, and C talks about ownership transfer, which is different from mutable references.

3. Final Answer:

   Change the value it points to without taking ownership -> Option D

4. Quick Check:

   Mutable reference = change value without ownership [OK]

Hint: Mutable references let you change data without moving ownership [OK]

Common Mistakes:

• Confusing mutable references with ownership transfer
• Thinking multiple mutable references are allowed simultaneously
• Believing mutable references prevent changes

2. Which of the following is the correct syntax to create a mutable reference to a variable num in Rust?

easy

A. let ref = #

B. let ref = &mut num;

C. let ref = *num;

D. let ref = mut #

Solution

1. Step 1: Identify the syntax for mutable references

   In Rust, to create a mutable reference, you use &mut before the variable name.

2. Step 2: Check each option

   A creates an immutable reference, B dereferences the value, and D has incorrect syntax order.

3. Final Answer:

   let ref = &mut num; -> Option B

4. Quick Check:

   Mutable reference syntax = &mut variable [OK]

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

Common Mistakes:

• Forgetting the 'mut' keyword after &
• Placing 'mut' before & incorrectly
• Using * instead of &mut for references

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

fn main() {
    let mut x = 5;
    let r = &mut x;
    *r += 1;
    println!("{}", x);
}

medium

A. 6

B. Runtime panic

C. Compilation error

D. 5

Solution

1. Step 1: Understand the mutable reference usage

   The variable x is mutable. A mutable reference r is created to x. The value pointed by r is incremented by 1 using *r += 1;.

2. Step 2: Determine the final value of x

   Since r points to x, incrementing *r changes x from 5 to 6. The print statement outputs 6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Mutable reference modifies original value = 6 [OK]

Hint: Dereference mutable ref with * to change original value [OK]

Common Mistakes:

• Expecting original value unchanged
• Confusing reference with copy
• Thinking code causes compile error

4. What is wrong with this Rust code?

fn main() {
    let mut val = 10;
    let r1 = &mut val;
    let r2 = &mut val;
    *r1 += 1;
    *r2 += 1;
    println!("{}", val);
}

medium

A. Dereferencing mutable references is not allowed

B. Cannot mutate a variable declared as mutable

C. Cannot have two mutable references to the same variable at the same time

D. Missing semicolon after println!

Solution

1. Step 1: Identify the mutable references

   The code creates two mutable references r1 and r2 to the same variable val at the same time.

2. Step 2: Recall Rust's borrowing rules

   Rust allows only one mutable reference at a time to prevent data races. Having two mutable references simultaneously causes a compile-time error.

3. Final Answer:

   Cannot have two mutable references to the same variable at the same time -> Option C

4. Quick Check:

   Only one mutable reference allowed at once [OK]

Hint: Only one mutable reference allowed at a time [OK]

Common Mistakes:

• Trying to create multiple mutable references simultaneously
• Thinking mutable references can be copied freely
• Ignoring Rust's borrowing rules

5. Given the following code, what is the correct way to safely modify the value inside data using mutable references without causing a compile error?

fn main() {
    let mut data = 100;
    let r1 = &data;
    let r2 = &mut data;
    *r2 += 50;
    println!("{}", data);
}

hard

A. Remove the immutable reference r1 before creating r2

B. Change r2 to an immutable reference &data

C. Use two mutable references r1 and r2 simultaneously

D. Make data immutable by removing mut

Solution

1. Step 1: Understand Rust's borrowing rules

   Rust does not allow mutable and immutable references to the same variable at the same time. Here, r1 is immutable and r2 is mutable, causing a conflict.

2. Step 2: Fix the code by removing the immutable reference

   Removing r1 allows only the mutable reference r2 to exist, so modifying data through r2 is safe and allowed.

3. Final Answer:

   Remove the immutable reference r1 before creating r2 -> Option A

4. Quick Check:

   Mutable and immutable refs cannot coexist [OK]

Hint: Remove immutable refs before mutable refs to avoid errors [OK]

Common Mistakes:

• Trying to use mutable and immutable references simultaneously
• Changing mutable ref to immutable without reason
• Making variable immutable but still trying to mutate