Rc pointer in Rust - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new Rc pointer.

Rust

let value = Rc::[1](5);

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

Anew

Bclone

Cstrong_count

Ddowngrade

Attempts:

3 left

💡 Hint

Common Mistakes

Using clone instead of new to create the Rc.

Using downgrade which is for Weak pointers.

Use Rc::new to create a new reference-counted pointer.

2fill in blank

medium

Complete the code to clone an existing Rc pointer.

Rust

let rc2 = [1](&rc1);

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

ARc::clone

BRc::new

CRc::downgrade

DRc::strong_count

Attempts:

3 left

💡 Hint

Common Mistakes

Using Rc::new which creates a new pointer with new data.

Using Rc::downgrade which creates a Weak pointer.

Use Rc::clone to create a new pointer to the same data, increasing the reference count.

3fill in blank

hard

Fix the error in the code to get the strong count of an Rc pointer.

Rust

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

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

Aclone

Bdowngrade

Cstrong_count

Dnew

Attempts:

3 left

💡 Hint

Common Mistakes

Using clone which copies the pointer.

Using downgrade which creates a Weak pointer.

The method strong_count returns how many Rc pointers share ownership.

4fill in blank

hard

Fill both blanks to create an Rc pointer and clone it.

Rust

let rc1 = Rc::[1](10);
let rc2 = Rc::[2](&rc1);

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

Anew

Bclone

Cdowngrade

Dstrong_count

Attempts:

3 left

💡 Hint

Common Mistakes

Using downgrade instead of clone.

Using strong_count instead of clone.

First create a new Rc pointer with new, then clone it with clone.

5fill in blank

hard

Fill all three blanks to create an Rc pointer, clone it, and get the strong count.

Rust

let rc1 = Rc::[1](20);
let rc2 = Rc::[2](&rc1);
let count = Rc::[3](&rc1);

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

Anew

Bclone

Cstrong_count

Ddowngrade

Attempts:

3 left

💡 Hint

Common Mistakes

Using downgrade instead of clone or strong_count.

Mixing up the order of methods.

Create a new Rc pointer with new, clone it with clone, and get the count with strong_count.

Practice

1. What is the main purpose of using Rc in Rust?

easy

A. To automatically convert data types

B. To enable mutable access to data from multiple threads

C. To allow multiple owners to share read-only data safely in one thread

D. To create a new thread for parallel execution

Solution

1. Step 1: Understand Rc usage

   Rc stands for Reference Counted pointer, used to share ownership of data safely within a single thread.

2. Step 2: Identify the correct purpose

   Rc allows multiple owners to read the same data without mutable access or threading concerns.

3. Final Answer:

   To allow multiple owners to share read-only data safely in one thread -> Option C

4. Quick Check:

   Rc = shared read-only ownership [OK]

Hint: Rc is for shared ownership, not mutability or threads [OK]

Common Mistakes:

• Confusing Rc with Arc for multi-threading
• Thinking Rc allows mutable access
• Assuming Rc creates new threads

2. Which of the following is the correct way to create a new Rc pointer to an integer value 10?

easy

A. let x = Rc::new(10);

B. let x = Rc::clone(10);

C. let x = Rc::new(&10);

D. let x = Rc::create(10);

Solution

1. Step 1: Recall Rc creation syntax

   The correct way to create an Rc pointer is using Rc::new(value).

2. Step 2: Check each option

   let x = Rc::new(10); uses Rc::new(10) which is correct. Options B, C, and D use invalid methods or references.

3. Final Answer:

   let x = Rc::new(10); -> Option A

4. Quick Check:

   Rc::new(value) creates pointer [OK]

Hint: Use Rc::new(value) to create an Rc pointer [OK]

Common Mistakes:

• Using Rc::clone to create new Rc
• Passing reference instead of value to Rc::new
• Using non-existent Rc::create method

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

use std::rc::Rc;

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

medium

A. 1

B. 2

C. 0

D. Compilation error

Solution

1. Step 1: Understand Rc cloning effect

   Creating b as Rc::clone(&a) increases the reference count by 1.

2. Step 2: Check the strong count

   Initially, a has count 1. After cloning, count becomes 2. So, Rc::strong_count(&a) returns 2.

3. Final Answer:

   2 -> Option B

4. Quick Check:

   Cloning Rc increases count = 2 [OK]

Hint: Rc::clone increases count; check strong_count for owners [OK]

Common Mistakes:

• Assuming clone creates a new independent Rc
• Expecting count to remain 1 after clone
• Confusing strong_count with weak_count

4. Identify the error in the following Rust code using Rc:

use std::rc::Rc;

fn main() {
    let x = Rc::new(10);
    let y = x;
    println!("{}", Rc::strong_count(&x));
}

medium

A. Cannot move Rc pointer; should use Rc::clone(&x) instead

B. Rc::strong_count requires mutable reference

C. Rc::new cannot hold integers

D. No error; code runs fine

Solution

1. Step 1: Analyze ownership transfer

   Assigning y = x moves ownership of the Rc pointer, invalidating x.

2. Step 2: Identify correct way to share ownership

   To share ownership, use Rc::clone(&x) instead of move. Otherwise, x cannot be used after move.

3. Final Answer:

   Cannot move Rc pointer; should use Rc::clone(&x) instead -> Option A

4. Quick Check:

   Rc must be cloned, not moved for shared ownership [OK]

Hint: Use Rc::clone to share ownership, not assignment [OK]

Common Mistakes:

• Moving Rc instead of cloning
• Thinking Rc::strong_count needs mutable reference
• Believing Rc cannot hold primitive types

5. Given the following code, what will be printed?

use std::rc::Rc;

fn main() {
    let data = Rc::new(vec![1, 2, 3]);
    let a = Rc::clone(&data);
    let b = Rc::clone(&data);
    drop(data);
    println!("{}", Rc::strong_count(&a));
}

hard

A. 1

B. Compilation error

C. 3

D. 2

Solution

1. Step 1: Count Rc owners before drop

   Initially, data has 1 owner. Cloning twice adds two more owners, total 3.

2. Step 2: Drop original owner and check count

   Calling drop(data) removes one owner, leaving 2 owners: a and b.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   Dropping one Rc reduces count by one [OK]

Hint: Dropping one Rc reduces count; clones keep count higher [OK]

Common Mistakes:

• Assuming drop removes all owners
• Forgetting clones increase count
• Expecting compilation error due to drop