Immutable references in Rust

Introduction

Immutable references let you look at data without changing it. This helps keep your program safe and clear.

When you want to read data without changing it.

When multiple parts of your program need to see the same data at the same time.

When you want to avoid copying large data but still use it safely.

When you want to prevent accidental changes to important information.

When you want to share data between functions without ownership transfer.

Syntax

Rust

&variable_name

The & symbol creates an immutable reference to a value.

You cannot change the value through an immutable reference.

Examples

This creates an immutable reference y to the value x.

Rust

let x = 5;
let y = &x;

Passing an immutable reference to a function to read the value without taking ownership.

Rust

fn print_number(num: &i32) {
    println!("Number is {}", num);
}

let a = 10;
print_number(&a);

You can have multiple immutable references at the same time.

Rust

let s = String::from("hello");
let r1 = &s;
let r2 = &s;
println!("{} and {}", r1, r2);

Sample Program

This program shows how to create and use an immutable reference to a string.

Rust

fn main() {
    let name = String::from("Alice");
    let name_ref = &name; // immutable reference
    println!("Name is: {}", name_ref);
    // name_ref cannot change the value of name
}

Important Notes

You cannot have mutable and immutable references to the same data at the same time.

Immutable references help prevent bugs by making sure data is not changed unexpectedly.

Summary

Immutable references let you read data without changing it.

Use & to create an immutable reference.

Multiple immutable references can exist at once, but no mutable references at the same time.

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