Rc pointer in Rust - Step-by-Step Execution

Concept Flow - Rc pointer

Create Rc pointer

↓

Reference count = 1

↓

Clone Rc pointer

↓

Reference count += 1

↓

Use Rc pointers

↓

Drop one Rc pointer

↓

Reference count -= 1

↓

Reference count == 0?

No→Keep data alive

Yes↓

Free data

This flow shows how an Rc pointer is created, cloned to increase reference count, used, and dropped to decrease count until data is freed.

Execution Sample

Rust

use std::rc::Rc;

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

Creates an Rc pointer to 5, clones it, and prints both values.

Execution Table

Step	Action	Rc Pointer	Reference Count	Output
1	Create Rc pointer a with value 5	a	1
2	Clone Rc pointer a to b	a, b	2
3	Print values of a and b	a, b	2	5 5
4	End of scope, drop b	a	1
5	End of scope, drop a, count 0, free data	-	0

💡 All Rc pointers dropped, reference count is 0, data is freed

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	Final
a	None	Rc(5), count=1	Rc(5), count=2	Rc(5), count=1	Dropped
b	None	None	Rc(5), count=2	Dropped	Dropped

Key Moments - 3 Insights

Why does cloning an Rc pointer not copy the data?

Cloning an Rc pointer only increases the reference count (see Step 2 in execution_table), it does not duplicate the actual data to save memory.

What happens when one Rc pointer is dropped but others still exist?

The reference count decreases by one (Step 4), but the data stays alive until all Rc pointers are dropped (Step 5).

Why do both a and b print the same value?

Because both Rc pointers point to the same data (Step 3), so printing either shows the same value.

Visual Quiz - 3 Questions

Look at the execution_table, what is the reference count after cloning Rc pointer a to b?

A 2

B 0

C 1

D 3

💡 Hint

Check Step 2 in the execution_table where cloning increases the count.

At which step does the data get freed?

A Step 4

B Step 5

C Step 3

D Never

💡 Hint

Look at the exit_note and Step 5 in the execution_table.

If we did not clone a to b, what would be the reference count after Step 1?

A 0

B 2

C 1

D Undefined

💡 Hint

Check Step 1 in the execution_table where only a exists.

Concept Snapshot

Rc pointer in Rust:
- Use Rc::new(value) to create a reference-counted pointer.
- Cloning Rc increases reference count, not data copy.
- Data is freed when reference count reaches zero.
- Useful for shared ownership in single-threaded contexts.

Full Transcript

This visual execution traces how Rust's Rc pointer works. First, an Rc pointer 'a' is created with value 5 and reference count 1. Then, cloning 'a' to 'b' increases the count to 2. Both pointers print the same value 5 because they share ownership. When 'b' is dropped, count decreases to 1, and finally dropping 'a' reduces count to 0, freeing the data. This shows how Rc manages shared ownership by counting references and freeing data only when none remain.