Box pointer in Rust - Step-by-Step Execution

Concept Flow - Box pointer

Create value on stack

↓

Allocate heap memory

↓

Move value to heap

↓

Store heap address in Box pointer

↓

Use Box pointer to access heap value

↓

Box goes out of scope -> heap memory freed

This flow shows how a Box pointer moves a value from stack to heap, stores the heap address, and frees memory when done.

Execution Sample

Rust

fn main() {
    let b = Box::new(5);
    println!("Value in box: {}", b);
}

Creates a Box pointer holding the value 5 on the heap and prints it.

Execution Table

Step	Action	Stack	Heap	Box Pointer	Output
1	Declare b with Box::new(5)	b (Box pointer)	5 (heap)	Points to heap address of 5
2	Print value inside b	b (Box pointer)	5 (heap)	Points to heap address of 5	Value in box: 5
3	End of main, b goes out of scope	No b	Heap memory freed	No pointer

💡 b goes out of scope, Box pointer drops and heap memory is freed

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3 (End)
b	uninitialized	Box pointer to heap 5	Box pointer to heap 5	dropped (no longer exists)
heap memory	empty	5 allocated	5 allocated	freed

Key Moments - 3 Insights

Why does the value 5 move to the heap instead of staying on the stack?

Because Box::new allocates memory on the heap and stores the value there, as shown in execution_table step 1 where heap has 5 allocated.

What happens to the heap memory when the Box pointer goes out of scope?

The heap memory is automatically freed when the Box pointer is dropped, as shown in execution_table step 3.

How do we access the value inside the Box pointer?

We use the Box pointer like a smart pointer; println! dereferences it to get the value, shown in execution_table step 2 output.

Visual Quiz - 3 Questions

Look at the execution_table at step 1, what does the Box pointer 'b' point to?

A The value 5 on the stack

B An uninitialized memory

C The value 5 on the heap

D The value 0

💡 Hint

Check the 'Heap' and 'Box Pointer' columns in execution_table step 1

At which step does the heap memory get freed?

A Step 2

B Step 3

C Step 1

D Never

💡 Hint

Look at the 'Heap' column in execution_table step 3

If we print the value inside the Box pointer, what output do we get at step 2?

A "Value in box: 5"

B "Value in box: b"

C "Value in box: 0"

D No output

💡 Hint

Check the 'Output' column in execution_table step 2

Concept Snapshot

Box pointer in Rust:
- Use Box::new(value) to allocate value on heap
- Box stores heap address on stack
- Access value via dereferencing Box
- Memory freed automatically when Box drops
- Useful for heap allocation and recursive types

Full Transcript

This example shows how Rust's Box pointer works. First, a value 5 is created on the heap using Box::new. The Box pointer 'b' stores the address of this heap memory on the stack. When we print 'b', Rust dereferences the Box to get the value 5. Finally, when 'b' goes out of scope, Rust automatically frees the heap memory. This process helps manage heap allocation safely and efficiently.