Memory allocation and deallocation in C++ - Step-by-Step Execution

Concept Flow - Memory allocation and deallocation

Start Program

↓

Request Memory Allocation

↓

Memory Allocated

↓

Use Allocated Memory

↓

Request Memory Deallocation

↓

Memory Freed

↓

End Program

The program starts, requests memory from the system, uses it, then frees it before ending.

Execution Sample

C++

#include <iostream>
int main() {
  int* ptr = new int(10);
  std::cout << *ptr << std::endl;
  delete ptr;
  return 0;
}

This code allocates memory for an int, stores 10, prints it, then frees the memory.

Execution Table

Step	Action	Pointer Value	Memory State	Output
1	Allocate memory with new int(10)	ptr -> address1	Memory block allocated at address1 with value 10
2	Dereference ptr to get value	ptr -> address1	Memory block unchanged	10
3	Delete ptr to free memory	ptr -> address1 (dangling)	Memory at address1 freed
4	Program ends	ptr -> address1 (dangling)	No allocated memory

💡 Program ends after memory is freed to avoid leaks

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
ptr	uninitialized	address1	address1	dangling pointer	dangling pointer

Key Moments - 3 Insights

Why does ptr become a dangling pointer after delete?

After delete, the memory is freed but ptr still holds the old address (see execution_table step 3), so it points to invalid memory.

What happens if we forget to delete the allocated memory?

Memory remains allocated and is not returned to the system, causing a memory leak (not shown in execution_table but implied if step 3 is skipped).

Can we use ptr after deleting it?

No, using ptr after delete leads to undefined behavior because the memory is freed (execution_table step 3).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of ptr after step 3?

A A dangling pointer to freed memory

B Null pointer

C Uninitialized pointer

D Pointer to valid allocated memory

💡 Hint

Check the 'Pointer Value' column at step 3 in execution_table

At which step is the memory actually freed?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Memory State' column for when memory changes from allocated to freed

If we remove the delete statement, what changes in the execution_table?

A Output changes to 0

B ptr becomes null after step 3

C Memory remains allocated after step 3

D Program crashes immediately

💡 Hint

Consider what happens if memory is not freed as shown in step 3

Concept Snapshot

Memory allocation uses 'new' to reserve space and returns a pointer.
Use the pointer to access or modify the data.
Free memory with 'delete' to avoid leaks.
After delete, pointer becomes dangling and should not be used.
Always pair new with delete to manage memory safely.

Full Transcript

This example shows how a C++ program allocates memory dynamically using 'new'. The pointer 'ptr' receives the address of the allocated memory holding the value 10. The program prints this value by dereferencing 'ptr'. Then, it frees the memory with 'delete', making 'ptr' a dangling pointer because it still holds the old address but the memory is no longer valid. The program ends after freeing memory to prevent leaks. Beginners often confuse the pointer's state after deletion and the importance of freeing memory to avoid leaks. The visual quiz reinforces understanding of pointer states and memory lifecycle.