Box pointer in Rust - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a Box pointer holding the value 5.

Rust

let b = [1]::new(5);

Drag options to blanks, or click blank then click option'

AArc

BRc

CBox

DVec

Attempts:

3 left

💡 Hint

Common Mistakes

Using Vec instead of Box, which creates a vector, not a pointer.

Using Rc or Arc which are reference counted pointers, not simple boxes.

The Box::new function creates a Box pointer that owns the value on the heap.

2fill in blank

medium

Complete the code to dereference the Box pointer and print its value.

Rust

let b = Box::new(10);
println!("{}", *[1]);

Drag options to blanks, or click blank then click option'

ABox

Bb

C&b

Db.as_ref()

Attempts:

3 left

💡 Hint

Common Mistakes

Using &b which is a reference to the Box, not the value.

Using b.as_ref() which returns a reference, not the value itself.

Dereferencing the Box pointer b with *b accesses the value inside.

3fill in blank

hard

Fix the error in the code by completing the blank to create a Box pointer to a string.

Rust

let s = [1]::new(String::from("hello"));

Drag options to blanks, or click blank then click option'

AArc

BVec

CRc

DBox

Attempts:

3 left

💡 Hint

Common Mistakes

Using Vec which is a growable array, not a pointer.

Using Rc or Arc which are reference counted pointers.

Box::new correctly creates a Box pointer owning the String on the heap.

4fill in blank

hard

Fill both blanks to create a Box pointer to an integer and then dereference it to get the value.

Rust

let b = [1]::new(42);
let val = *[2];

Drag options to blanks, or click blank then click option'

ABox

Bb

CRc

DArc

Attempts:

3 left

💡 Hint

Common Mistakes

Using Rc or Arc instead of Box for simple ownership.

Dereferencing the type name instead of the variable.

First, Box::new creates the Box pointer. Then, dereferencing b with *b gets the value.

5fill in blank

hard

Fill all three blanks to create a Box pointer to a vector, push a value, and print the length.

Rust

let mut v = [1]::new(vec![1, 2, 3]);
v.[2](4);
println!("Length: {}", v.[3]());

Drag options to blanks, or click blank then click option'

ABox

Bpush

Clen

DVec

Attempts:

3 left

💡 Hint

Common Mistakes

Using Vec::new instead of Box::new to create the pointer.

Calling push or len on the Box pointer without dereferencing (but in Rust, Box auto-derefs).

Box::new creates the pointer to the vector. Then push adds an element, and len gets the vector length.

Practice

1. What is the main purpose of a Box pointer in Rust?

easy

A. To create multiple mutable references to the same data

B. To automatically clone data when assigned

C. To store data on the heap and keep a pointer on the stack

D. To replace all references with raw pointers

Solution

1. Step 1: Understand what Box does

   A Box stores data on the heap, while the pointer itself lives on the stack.

2. Step 2: Compare options with Box behavior

   Only To store data on the heap and keep a pointer on the stack correctly describes this behavior. Other options describe different concepts like cloning or raw pointers.

3. Final Answer:

   To store data on the heap and keep a pointer on the stack -> Option C

4. Quick Check:

   Box = heap data + stack pointer [OK]

Hint: Box always stores data on heap, pointer on stack [OK]

Common Mistakes:

• Thinking Box allows multiple mutable references
• Confusing Box with cloning or copying
• Assuming Box replaces raw pointers

2. Which of the following is the correct syntax to create a Box containing the integer 10 in Rust?

easy

A. let b = Box::new(10);

B. let b = box 10;

C. let b = Box(10);

D. let b = new Box(10);

Solution

1. Step 1: Recall Box creation syntax

   In Rust, Box::new(value) is the correct way to create a Box pointer.

2. Step 2: Check each option

   let b = Box::new(10); uses Box::new(10), which is correct. let b = box 10; uses deprecated syntax. let b = Box(10); and let b = new Box(10); are invalid.

3. Final Answer:

   let b = Box::new(10); -> Option A

4. Quick Check:

   Box::new(value) creates a Box [OK]

Hint: Use Box::new(value) to create a Box pointer [OK]

Common Mistakes:

• Using deprecated 'box' keyword
• Trying to call Box like a function
• Using 'new Box' which is invalid

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

let b = Box::new(5);
println!("{}", *b + 3);

medium

A. Compilation error

B. 53

C. 5

D. 8

Solution

1. Step 1: Understand dereferencing Box

   The *b dereferences the Box to get the value 5 stored on the heap.

2. Step 2: Calculate the expression

   Adding 3 to 5 gives 8, so the printed output is 8.

3. Final Answer:

   8 -> Option D

4. Quick Check:

   *b + 3 = 8 [OK]

Hint: Dereference Box with * to access value [OK]

Common Mistakes:

• Printing Box pointer without dereference
• Concatenating instead of adding
• Expecting compilation error due to Box

4. Identify the error in this Rust code using Box:

let b = Box::new(10);
let r1 = &b;
let r2 = &mut b;
println!("{} {}", r1, r2);

medium

A. Box::new requires a mutable variable

B. Cannot borrow `b` as mutable because it is already borrowed as immutable

C. Box cannot be printed with println!

D. No error, code runs fine

Solution

1. Step 1: Analyze borrowing rules

   Rust does not allow mutable and immutable borrows at the same time. Here, r1 is an immutable borrow, and r2 is a mutable borrow of b.

2. Step 2: Identify the error

   This causes a compile-time error: cannot borrow as mutable because already borrowed as immutable.

3. Final Answer:

   Cannot borrow `b` as mutable because it is already borrowed as immutable -> Option B

4. Quick Check:

   Mutable + immutable borrow conflict = error [OK]

Hint: Cannot mix mutable and immutable borrows simultaneously [OK]

Common Mistakes:

• Thinking Box cannot be printed
• Assuming Box::new needs mut
• Ignoring Rust's borrowing rules

5. Given this recursive enum using Box:

enum List {
    Cons(i32, Box<List>),
    Nil,
}

fn main() {
    let list = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
}

Why is Box necessary in this definition?

hard

A. To allocate recursive data on the heap and avoid infinite size

B. To allow multiple ownership of the list elements

C. To automatically clone the list elements

D. To make the enum implement Copy trait

Solution

1. Step 1: Understand recursive enum size problem

   Recursive enums like List have infinite size if they directly contain themselves.

2. Step 2: Role of Box in recursion

   Using Box stores the recursive part on the heap, giving a fixed pointer size on the stack, avoiding infinite size.

3. Final Answer:

   To allocate recursive data on the heap and avoid infinite size -> Option A

4. Quick Check:

   Box breaks infinite size recursion by heap allocation [OK]

Hint: Use Box to break infinite size in recursive types [OK]

Common Mistakes:

• Thinking Box allows multiple ownership
• Assuming Box clones data automatically
• Believing Box makes enum Copy