Reflection on C and C++ Memory Allocation Difference

When I was coding in C at uni, I was always confused about how to manage memory manually. Although I used malloc() and realloc(), I’m a bit rusty on how they work now. And you know, what’s learned from textbook and slides always feels shallow – even back then, I didn’t deeply and thoroughly understand those things.

So I’ve decided to work it out and strengthen my comprehension and memory by logging this brainstorming and learning process — and, by the way, augmenting it by implementing C++’s delete and new using lower-level C techniques.

What is malloc() ，calloc() and realloc()

Stack vs Heap in C and C++

Stack

• A region of memory that stores local variables and function call information.
• Managed automatically by the compiler.

Key properties:

• Fast allocation and deallocation (LIFO – last-in, first-out)
• Memory is freed automatically when the function returns
• Limited in size (usually a few megabytes)
• No need to call free() or delete

Examples:

C

void func() {
    int x = 10;  // 'x' is stored on the stack
}

C++

void func() {
    int x = 10;         // stack variable
    std::string name;   // still on the stack, but with constructor/destructor
}

Heap

• A region of memory used for dynamic allocation.
• Managed manually by the programmer.

Key properties:

• Allocated using functions like malloc()/calloc() (in C) or new (in C++)

• Must be manually deallocated using free() (in C) or delete (in C++)

• Memory stays allocated until explicitly freed

• Useful when size or lifetime of data is not known at compile time

• A region of memory used for dynamic allocation.

• Managed manually by the programmer.

Examples:

C

#include <stdlib.h>

void func() {
    int *p = (int *)malloc(sizeof(int));  // heap allocation
    *p = 42;
    free(p);                              // free the heap memory
}

C++

void func() {
    int *p = new int;     // allocate on heap
    *p = 42;
    delete p;             // deallocate memory

    std::string* name = new std::string("Alice");
    delete name;          // always match new with delete
}

Feature	Stack	Heap
Lifetime	Automatically managed	Manually managed
Allocation Speed	Fast	Slower
Memory Size	Limited (few MBs)	Much larger (system dependent)
Syntax in C	Just declare	malloc(), calloc(), free()
Syntax in C++	Just declare	new, delete
Use Case	Temporary local variables	Dynamic data (e.g. arrays, objects)

In C++: malloc()/ free() vs new/delete

My Implementation of new and delete