Ownership with smart pointers in Rust - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

Rust

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

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

Anew

Bcreate

Cmake

Dbuild

Attempts:

3 left

💡 Hint

Common Mistakes

Using functions like create or make which do not exist for Box.

Trying to call Box as a function directly.

The Box::new function creates a new Box smart pointer that owns the value inside it.

2fill in blank

medium

Complete the code to clone a Rc smart pointer.

Rust

let rc2 = Rc::[1](&rc1);

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

Acopy

Bclone

Cduplicate

Dreplicate

Attempts:

3 left

💡 Hint

Common Mistakes

Using copy which is not implemented for Rc.

Trying to use duplicate or replicate which do not exist.

The clone method on Rc increases the reference count and returns a new pointer to the same data.

3fill in blank

hard

Fix the error in the code by completing the blank to get a mutable reference from a RefCell.

Rust

let mut_ref = refcell.[1]().unwrap();

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Aborrow_mut

Bborrow

Cget_mut

Dtake_mut

Attempts:

3 left

💡 Hint

Common Mistakes

Using borrow which only gives an immutable reference.

Trying to use get_mut which requires exclusive ownership.

The borrow_mut method on RefCell returns a mutable reference wrapped in a RefMut, which you can unwrap safely if no other borrows exist.

4fill in blank

hard

Fill both blanks to create a HashMap from strings to their lengths, only including words longer than 3 characters.

Rust

let lengths = words.iter().filter(|&word| word.len() [1] 3).map(|word| (word.to_string(), word.[2]())).collect::<HashMap<_, _>>();

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

A>

Blen

C<

Dis_empty

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of > in the filter condition.

Using is_empty which returns a boolean, not length.

The filter keeps words with length greater than 3, and the map creates pairs of the word and its length.

5fill in blank

hard

Fill both blanks to create a HashMap from uppercase keys to values, including only entries with positive values.

Rust

let result = data.iter().map(|(k, v)| (k.[1](), *v)).filter(|(_, v)| *v [2] 0).collect::<HashMap<_, _>>();

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

Ato_uppercase

B>

C==

Dto_lowercase

Attempts:

3 left

💡 Hint

Common Mistakes

Using to_lowercase instead of uppercase.

Filtering values equal to zero instead of greater than zero.

The map converts keys to uppercase strings, the filter keeps only positive values, and the collect gathers them into a HashMap.

Practice

1. Which smart pointer in Rust allows multiple parts of your program to own the same data safely?

easy

A. RefCell

B. Box

C. Rc

D. Arc

Solution

1. Step 1: Understand ownership types

   Box provides unique ownership, Rc allows shared ownership in single-threaded contexts, RefCell allows interior mutability, and Arc is for shared ownership across threads.

2. Step 2: Identify shared ownership in single-threaded code

   Rc is the smart pointer designed for multiple owners in single-threaded programs.

3. Final Answer:

   Rc -> Option C

4. Quick Check:

   Shared ownership = Rc [OK]

Hint: Shared ownership in single-threaded Rust uses Rc [OK]

Common Mistakes:

• Confusing Box with shared ownership
• Thinking RefCell shares ownership
• Mixing Arc with single-threaded Rc

2. Which of the following is the correct way to create a Box smart pointer holding the integer 10?

easy

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

B. let b = Box(10);

C. let b = new Box(10);

D. let b = Box::create(10);

Solution

1. Step 1: Recall Box creation syntax

   The correct way to create a Box is using Box::new(value).

2. Step 2: Check each option

   let b = Box::new(10); uses Box::new(10), which is correct. Options A, C, and D use invalid syntax.

3. Final Answer:

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

4. Quick Check:

   Box creation = Box::new(value) [OK]

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

Common Mistakes:

• Using Box(10) instead of Box::new(10)
• Trying new Box(10) like other languages
• Using non-existent Box::create method

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

use std::rc::Rc;

let a = Rc::new(5);
let b = Rc::clone(&a);
println!("{}", Rc::strong_count(&a));

medium

A. 1

B. 2

C. 5

D. Compilation error

Solution

1. Step 1: Understand Rc cloning

   Cloning an Rc pointer increases the reference count but does not clone the data.

2. Step 2: Count strong references

   Initially, a has a count of 1. After b = Rc::clone(&a), the count becomes 2.

3. Final Answer:

   2 -> Option B

4. Quick Check:

   Rc clone increments count = 2 [OK]

Hint: Rc::clone increases strong_count by 1 [OK]

Common Mistakes:

• Thinking Rc::clone copies data
• Expecting count to remain 1
• Confusing Rc with Box ownership

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

use std::cell::RefCell;

let data = RefCell::new(10);
let r1 = data.borrow_mut();
let r2 = data.borrow_mut();
println!("{} {}", r1, r2);

medium

A. Cannot borrow mutably twice at the same time

B. RefCell cannot hold integers

C. Missing semicolon after borrow_mut()

D. Cannot print RefCell values directly

Solution

1. Step 1: Understand RefCell borrowing rules

   RefCell allows mutable borrowing at runtime but only one mutable borrow at a time.

2. Step 2: Analyze the code

   The code tries to create two mutable borrows r1 and r2 simultaneously, which causes a runtime panic.

3. Final Answer:

   Cannot borrow mutably twice at the same time -> Option A

4. Quick Check:

   RefCell mutable borrow rule = one at a time [OK]

Hint: Only one mutable borrow allowed with RefCell at once [OK]

Common Mistakes:

• Thinking RefCell forbids integers
• Assuming multiple mutable borrows are allowed
• Confusing compile-time and runtime borrow checks

5. You want to create a tree structure where each node can have multiple children and each child can access its parent. Which combination of smart pointers is best to avoid ownership errors?

hard

A. Use Weak for children and Box for parent references

B. Use Box for both children and parent references

C. Use RefCell for children and Rc for parent references

D. Use Rc for children and Weak for parent references

Solution

1. Step 1: Understand ownership in tree structures

   Children own their data, so Rc is used for shared ownership of children nodes.

2. Step 2: Avoid reference cycles

   Parents are referenced by children, but to avoid cycles, parents should be referenced with Weak pointers.

3. Step 3: Combine smart pointers correctly

   Using Rc for children and Weak for parents prevents memory leaks and ownership errors.

4. Final Answer:

   Use Rc for children and Weak for parent references -> Option D

5. Quick Check:

   Tree ownership = Rc children + Weak parents [OK]

Hint: Use Weak for back references to avoid cycles [OK]

Common Mistakes:

• Using Box for parent causes ownership conflicts
• Using Rc for parents causes reference cycles
• Ignoring Weak pointers to break cycles