Master Pointers In C & C++

By Ahmed Mohamed Yousry (AKA Ahmed Arafat)

Main Topics To Be Discussed:

1. What Are Pointers ?
2. Let's Play A Little With pointers
3. Why Pointers are strong types?
4. Void Pointer
5. Pointer To Pointer
6. Pointers as function arguments (Call By Reference)
7. Pointers And Arrays
8. Array As Function Arguments
9. Character Arrays & Pointers
10. Pointers & Multi-Dimensional Arrays
11. Pointers & Dynamic Memory Allocation
12. Pointers As Function Return
13. Function Pointers
14. Function Pointers & Callbacks
15. What Is Memory Leak ?
16. Reference

---

1. Pointers : They are variables that store the address of another variables

int a;
int *p;
p = &a;
a = 5;
printf("%d\n",&a); // 204 (address of a)
printf("%d\n",p); // 204 (address of a stored in pointer p)
printf("%d\n",&p); // 64 (address of pointer p itself)
printf("%d\n",*p); // 5 (value stored in a)
*p = 8;
printf("%d\n",a); // 8 (value stored in a)

& means address of

Both *p = 8; & printf("%d\n",*p); are called dereferencing which means that I want the value which is stored inside the address that is stored inside pointer p

---

int a; // integer
int *p; // pointer to integer


char c; // character 
char *p1; // pointer to character

double d; // double
double *p1; //pointer to double

• Remember the memory occupied by each data type

- int : 4 bytes
- long : 8 bytes
- char : 1 byte
- float : 4 bytes

2. Let's play a little with pointers

#include <bits/stdc++.h>
using namespace std;
int main()
{
    int a = 10;
    int *p;
    p = &a;
    printf("Address of a is : %d\n",&a);
    printf("Address of p is : %d\n",p); // same as a
    printf("Value at p is : %d\n",*p); // 10
    int b = 20;
    *p = b;
    printf("Value at p is : %d\n",*p); // 20
}

int main()
{
    int a = 10;
    int *p;
    p = &a;
    printf("Address of p is : %d\n",p); // address : 200
    printf("Address of p+1 is : %d\n",p+1); // address : 204
}

3. Why Pointers are strong types (must specify the data type of the variable the pointer will refer to) ?

Why not generic type for Pointers?

- Why specifying the data type of Pointers while its job is to just store the address of another variable

Let's see, p+1 means that I want to increase the address by 1, but since this pointer points to an integer then it will increase by 4 bytes (1*4 bytes) , that's why we have to specify the data type on which the pointer refers to because when the pointer refers to an integer, it then knows that each time i will use p+1 it will increase by 4 bytes not like char data type for example, it will only increase by 1 byte

The second reason for specify the data type of pointers that it is important in derefercing , let's assume that int a occupies address 200,201,202,203, then a pointer refering to variable a will only store first address (200), now when I type printf("%d\n",*p) this means as we said before go to address 200 then print the value stored in it, but in this case the compiler should know the data type of that pointer so that if it is an integer for example the compiler will know that from address 200 till 203 (Look at 4 bytes starting from address 200) are all reserved for that variable so, it is important to know the data type of the pointer to be able to calculate number of reserved bytes for that variable while both reading or writing (Access/Modify) from that pointer

• Let's see the following example, to understand it well

int main()
{
    int a = 1025;// 00000000 00000000 00000100 00000001
    int *p;
    p = &a;
    printf("Size of integer %d\n", sizeof(int)); // 4
    printf("Address of p : %d , value of p: %d \n",p,*p); // 200 1025
    char *p0;
    p0 = (char *)p;
    printf("Size of character %d\n", sizeof(char)); // 1
    printf("Address of p0 : %d , value of p0: %d \n",p0,*p0); // 200 1
}

As we can see the value of char pointer p0 is 1, but why ?
Let's observe the binary representation of number 125
00000000 00000000 00000100 00000001 when we was printing *p it was referring to an integer so that the 4 bytes are all related to that variable, but when we cast the int pointer to char one p0 = (char *)p; the pointer p0 will again store the address of the first byte (same as p), but in this time it will only print it while dereferining (access/modify) as it is a char pointer (it takes only 1 byte) so that its corresponding value now is 00000001

Note: the output is a number even when I accessed a char pointer beacuse of printf("%d") that prints only numbers

int main()
{
    int a = 1025;
    int *p;
    p = &a;
    printf("Address of a %d\n", &a); //200
    printf("Address of p : %d , value of p: %d \n",p+1,*(p+1)); // 204 garbage
    char *p0;
    p0 = (char *)p; // Typecasting
    printf("Size of character %d\n", sizeof(char)); // 1
    printf("Address of c : %d , value of c: %d \n",p0+1,*(p0+1)); // 201 4
}

The value of *(p0+1) is 4 because I have moved 1 byte so now p0 refers to 00000100 byte the second byte in
00000000 00000000 00000100 00000001

printf("Size of int %d\n",sizeof(int));

int main()
{
    int a = 10;
    int *p;
    p = &a;
    printf("Address of p is : %d\n",p); // address : 200
    printf("Address of p+1 is : %d\n",p+1); // address : 204
    printf("Value at p+1 is : %d\n",*(p+1)); // garbage value
}

---

---

4. Void Pointer : it is used to just store the first address of a variable

Without being able to dereference or perform any arithmetic operation (+)

int main()
{
    int a = 1025;
    int *p;
    p = &a;
    void *p0;
    p0 = p; // like (void*) p
    printf("Address %d\n",p);
    printf("Address %d\n",p0); // we cannot dereference this pointer
    //printf("Value = %d\n",*p0); // we cannot dereference this pointer
    //printf("Address of next %d\n",p0+1); // we cannot perform any arithmetic operation on this pointer
}

void pointers have some use cases will be discussed later

5. Pointer To Pointer

• Let's see the following example:

int main()
{
    int x = 5; // address 225
    int *p = &x;
    *p = 6;
    int **q = &p;
    int ***r = &q;
    printf("%d\n",*p); // 6
    printf("%d\n",*q); // 225 (address stored in p)
    printf("%d\n",*(*q)); // 6
    printf("%d\n",*(*r)); //2525
    printf("%d\n",*(*(*r))); //6
    ***r = 10;
    printf("%d\n",x); //10
    **q = *p + 2; // 10 + 2 = 12
    printf("%d\n",x); //12
}

6. Pointers as function arguments (Call By Reference)

• Following code is an example of Call By Value

#include <bits/stdc++.h>
using namespace std;
void Increment(int a)
{
    a++;
    
}
int main()
{
    int a;
    a = 10;
    Increment(a);
    printf("a = %d\n",a); // 10
}

This is because a variable in Increment() function is different from a variable in main(), as a variable in Increment() function is created in the stack frame of this function while the other a variable in main() function is created in the stack frame of main()

• To prove that try to print the address of a in both functions, the address will be different

void Increment(int a)// Called Formal Argument
{
    a++;
    printf("%d" , &a); // 200
}
int main()
{
    int a;
    a = 10;
    Increment(a); // Called Actual Argument
    printf("%d" , &a); // 500
    //printf("a = %d\n",a);
}

• To Pass By Reference, just write this :

void Increment(int *a)
{
    *a= *a + 1;
}
int main()
{
    int a;
    a = 10;
    Increment(&a);
    printf("a = %d\n",a); // 11
}

7. Pointers And Arrays

• Observe the following code:

int main()
{
    int arr[5] = {1,2,3,4,5}; //Address 200 204 208 212 216
    int *p;
    p = &arr[0]; // store address of 200
    printf("%d\n",&arr[4]); // 216
    printf("%d\n",arr); // 200
    printf("%d\n",*arr); // 1
    printf("%d\n",arr+1); // 204
    printf("%d\n",*(arr+1)); // 2
    printf("%d\n",p); // 200
    printf("%d\n",*p); // 1
    printf("%d\n",(p+2)); // 200 + 2*4 = 208
    printf("%d\n",*(p+2)); // 3
    p++; // Allowed
    // arr++; // Error
}

Address At index i - &arr[i] or (arr+i)
Value At index i - arr[i] or *(arr+i)
Note: p++ is allowed, while arr++ is not allowed

8. Array As Function Arguments

• Observe the following code:

#include <bits/stdc++.h>
using namespace std;
void Double(int arr[], int sz) {
    int SZ = sizeof(arr)/sizeof(arr[0]); // 2
    printf("Size of arr %d & Size of arr[0] %d \n",sizeof(arr),sizeof(arr[0]));// 8 4
    for (int i = 0; i < sz; i++) {
        *(arr + i) = *(arr + i) * 2;
        // Or
        // arr[i] = arr[i] * 2;
    }
}
int main() {
    int arr[5] = {1, 2, 3, 4, 5}; // Address 200 204 208 212 216
    int sz = sizeof(arr) / sizeof(arr[0]);
    Double(arr, sz);
    for (int i = 0; i < sz; i++) {
        printf("%d\n", *(arr + i));
    }
}

• Some Important points from above code :
  1. When an array is passed as a parameter, it is passed by reference not by value this means that any change in Double() function to array's element will affect the array in main() , when you type int arr[] as a parameter in Double() function you actually pass the address of first element, so arrays will be passed in a function as a parameter is treated like pointers this is done to prevent memory wasting if you are passing an array with large number of elements, instead of creating another array having the same elements, you just can access the same array
  2. int SZ = sizeof(arr)/sizeof(arr[0]); // 2, the reason that SZ = 2 is that sizeof(arr) will be 8, as this is the size of a pointer in modern compilers, so don't ever calculate the size of a passed array in a function with sizeof(), always send the size in another parameter like this Double(int arr[], int sz)
  3. Referring to point 2, you can pass int *arr instead of int arr[] & still having the same result, but this time you are using a pointer that refers to the first byte of first element in the array, but in this case you will pass in the actual parameter the address of the array (base address) like this Double(arr, sz);
  4. Lines *(arr + i) = *(arr + i) * 2; & arr[i] = arr[i] * 2; have the same meaning

9. Character Arrays & Pointers

• In C language, size of array must be >= number of characters in a string +1
• This happens because if you defined an array of character like this

int main() {
    char C[10];
    C[0] = 'A';
    C[1] = 'r';
    C[2] = 'a';
    C[3] = 'f';
    C[4] = 'a';
    C[5] = 't';
}

Then we have to till the compiler that character t is the last character in that array (as you can see the array is with size 10, but I only occupied 6 characters), so that we want to add a null terminator after the last character like this C[6] = '\0';

• All functions for string manipulation in C expects that strings will null terminated (using \0)

A Rule: string in C has to be ended by Null character

• Let's see an example

int main() {
    char C[6];
    C[0] = 'A';
    C[1] = 'r';
    C[2] = 'a';
    C[3] = 'f';
    C[4] = 'a';
    C[5] = 't';
    printf("%s",C);
}

O/P: Arafat0����
As you can there is a garbage characters after Arafat because we did not specify the end of that character array

• To solve this type

    char C[7];
C[0] = 'A';
C[1] = 'r';
C[2] = 'a';
C[3] = 'f';
C[4] = 'a';
C[5] = 't';
C[6] = '\0'; // Add null character
printf("%s", C);

• Functions like strlen() will only count the number of characters in the array despite the size of that array (\0 will not be counted), let's see

int main() {
    char C[100];
    C[0] = 'A';
    C[1] = 'r';
    C[2] = 'a';
    C[3] = 'f';
    C[4] = 'a';
    C[5] = 't';
    C[6] = '\0';
    int len = strlen(C);
    printf("Length: %d",len); // 6
}

Note : \0is not counted in th length of that array of strings + it counts only the characters not the size of that array

• You can also initialize an array of characters like this

int main() {
    char C[100] = "Arafat";
    int len = strlen(C);
    printf("Length: %d",len);// 6
}

VIP Note : in the above code, \0 is implicitly exists by the compiler, this means that in this case you don't have to write it

• You cannot initialize an array of character after it's declaration, something like this

int main() {
    char C[100];
    C = "Arafat"; // Error
    // Rather
    C[0] = 'A';
    C[1] = 'l';
    C[2] = 'i';
}

• We can write it also like this

int main() {
    char C[] = "Arafat"; // Without adding a size
    int len = strlen(C);
    printf("Size In Bytes : %d\n", sizeof(C)); // 7 Bytes
    printf("Length : %d\n",len); // 6
}

Note : the size in bytes of the above array will be 7 , this because as we said Arafat word is 6 Characters * 1 Byte = 6 Bytes + 1 Byte (the implicit \0) character, so then the Total will be 7

• Observe the following

int main() {
    char C[6] = "Arafat";// Compilation Error
}

Because the compiler will force the size of that array with minimum size 7 (to be able to add \0), so that it will give an error

• Another way to initialize an array of characters

int main() {
    char C[7] = {'A','r','a','f','a','t','\0'};
}

In this way we have to add the null character (\0) as it is stored explicitly (must be writen by the developer) not implicitly (automatically by the compiler)

• Now let's play a litter with character arrays & pointers

int main() {
    char C[] = "Arafat"; // Address of 200 (A) 201 (r) 202 (a) 203 (f) 204 (a) 205 (t)
    char *c1 = C;// c1 pointer will store the address of 200
    printf("%c\n",c1[1]); // like saying *(c1+1) which is 'r'
    c1[2] = 'z'; // like saying *(c1+2) = 'z'
    printf("%s\n",C); // Arzfat
    // C = c1; // Error
    c1+=2; // Now address = 202
    printf("%c\n",*c1); // z
}

• Remember, Arrays are always passed to function by reference, which means that we pass the base address of that array to that function

// Using pointer as a parameter
void MyPrintFun1(char *c)
{
    while (*c != '\0') {
        printf("%c", *c);
        c++;
    }
    printf("\n");
}
// Using an array as a parameter
void MyPrintFun2(char c[]) {
    int i = 0;
    while (c[i] != '\0') {
        printf("%c", c[i]);
        i++;
    }
    printf("\n");
}

int main() {
    char C[] = "Arafat";
    MyPrintFun1(C);
    MyPrintFun2(C);
}

• Note that in the pointer function, all the following are correct

while (*c != '\0')
while (c[i] != '\0')
while (*(c+i) != '\0')

9.2 Character Arrays & Pointers - Part 2

• You can write an array of characters like this

int main() {
    char *C = "Hello"; // this string is stored as a constant in text segment in memory during compilation
    C[0] = 'a'; // This will case a run-time error
    printf("%s",C);
}

In C, when you declare a character pointer and assign it a value using double quotes, such as char *C = "Hello";, the resulting string literal is stored in a read-only memory segment. This means that the memory location where the string is stored cannot be modified.
The compiler interprets char* C = "Hello" as char C[6] = {'H','e','l','l','o','\0'}

• If we have a character array & we passed it in a function, we can modify this array using this pointer like this

void MyPrintFun1(char *c)
{
    c[0] = 'Z'; // Allowed
    while (*c != '\0') {
        printf("%c", *c);
        c++;
    }
    printf("\n");
}

int main() {
    char C[] = "Hello";
    MyPrintFun1(C); // op: Zello
}

• Sometimes we want a function that read a string not be able to write or modify anything in it, to do so we will pass the parameter as const like this void MyPrintFun1(const char *c)

void MyPrintFun1(const char *c)
{
    c[0] = 'Z'; // Compilation Error
    while (*c != '\0') {
        printf("%c", *c);
        c++;
    }
    printf("\n");
}

int main() {
    char C[] = "Hello";
    MyPrintFun1(C); 
}

Now we can read C[] in MyPrintFun1() but not writing in it

10. Pointers & Multi-Dimensional Arrays

2-D Array : Means each index in the array refers to another array

• Carefully observe the following code

#include <bits/stdc++.h>
using namespace std;

int main() {
    int A[2];
    // A[0] -> refers to an int
    // A[1] -> refers to an int

    int B[2][3] = {{2, 3, 6},
                   {4, 5, 8}};
    // B[0] -> refer to 1-D array of 3 elements
    // B[1] -> refer to 1-D array of 3 elements

    //int *p = B; // wrong as B will return a pointer to 1-D array of 3 elements, while p points only on an integer
    int (*p)[3] = B;
}

• How to access the elements

    int B[2][3] = {{2, 3, 6},
                    {4, 5, 8}};
    // All have the same Output
    cout << B << " " << B[0] << " " << *B << " " << &B[0] << " " << &B[0][0] << endl; // 400 (element 2)
    cout<< B+1 << " " << &B[1] << endl; // 400 + size of 1-D array of 3 elements (12) = 400 + 12 = 412 (element 4)
    cout << *(B + 1) << " " << B[1] << " " << " " << &B[1] << " " << &B[1][0] << endl; // 412 (element 4)
    // Can't type in the following line B[1][2] as it will return the value
    cout << *(B + 1) + 2 << " " << B[1] + 2 << " " << &B[1][2] << endl; // 420 (element 8)
    cout << *(*B + 1) << endl; //404 (element 3)

So, For 2-D Array, Accessing an element equation is B[i][j] = *(B[i] + j) = *( *(B+i) + j)

3-D Array : Means each index in the array refers to another 2-D array

• Carefully observe the following code

#include <bits/stdc++.h>
using namespace std;

int main() {
    
    int C[3][2][2] = {{{2, 5}, {7,  9}},
                      {{3, 4}, {6,  1}},
                      {{0, 8}, {11, 13}}};
    
    int (*p)[2][2] = C;
   
    cout << C << " " << C[0] << " " << &C[0][0] << " "; // address 800 (element 2)
    cout << *(C[0][1] + 1) << " " << C[0][1][1] << " " << &C[0][0] << " "; // address of element 9
    cout << *(C[1] + 1) << " " << C[1][1] << " " << &C[1][1][0] << " "; // address 824 (element 6)
    
}

So, For 3-D Array, Accessing an element equation is B[i][j][k] = *(B[i][j] + k) = *( *(*(B+i) + j) + k)

Pass Multi-Dimensional Array to a Function

• Let's see the following code

// Created by Ahmed Arafat on 1/10/2023.
#include <bits/stdc++.h>
using namespace std;

// 1-D Array
void Fun1_1(int A[]) {}

void Fun1_2(int *A) {}

// 2-D Array
void Fun2_1(int B[][2]) {}

void Fun2_2(int (*p)[2]) {}

// 3-D Array
void Fun3_1(int C[][2][2]) {}

void Fun3_2(int (*p)[2][2]) {
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 2; j++) {
            for (int k = 0; k < 2; k++) {
                //cout<<p[i][j][k]<<" ";
                cout << *(*((*p + i) + j) + k) << " ";
            }
            cout << "\t";
        }
        puts("");
    }
}

// Wrong Syntax
// void Fun3_2(int ***C) {}

int main() {

    int C[3][2][2] = {{{2, 5}, {7,  9}},
                      {{3, 4}, {6,  1}},
                      {{0, 8}, {11, 13}}};

    int (*p)[2][2] = C;
    Fun3_2(C);
}

11. Pointers & Dynamic Memory Allocation

• Application's memory can be divided into 4 segments

  • Code (Text)
  • Static/Global Variables
  • Stack : Stores function calls & local variables
  • Heap

• Let's see the following code

#include <bits/stdc++.h>
using namespace std;
int total; // Global
int Square(int x) // x is local to Square()
{
    return x * x;
}
int SquareOfSum(int x, int y) // x,y are local to SquareOfSum()
{
    int z = Square(x + y); //  z is local to SquareOfSum()
    return z;
}
int main() {
    int a = 4, b = 8;// a,b are local to main()
    total = SquareOfSum(a, b);
    printf("%d", total);
}

• When our program starts, operating system allocates some amount of reserved space for stack like 1MB,but the actual allocation of the stack frame happens during run time, and if our call stack goes beyond reserved memory so then this will be called Stack Over Flow, then our program will crash (can be happened during recursion without adding base condition)

• So, there is some limitation inside the stack, the memory cannot be grown once allocated to the stack, application cannot request more memory for the stack so that the allocation & reallocation in the stack follows a rule, when a function is called it is pushed into the top of the stack or removed from the top of the stack when the function stops executing

• It is not possible to manipulate the scope of the variable if it is stored in the stack

• Another limitation is that we need to declare a large array as local variable, we need to know the size of the array during compile time if we have the scenario where we want to decide the size of an array based on a parameter during run time it is a problem for the stack

• Unlike stack, heap's size can vary during the lifetime of a program & there is no set rule for allocation and reallocation of memory, a programmer can totally control

• how much memory to use in a heap and to what time he wants to keep the data in the memory, and heap can grow as long as there is available space in the system, this is also a dangerous thing in heap that we want to be really careful while using heap, heap also can be called free pool of memory

• How heap is implemented is decided by operating system, compiler. It is something that can vary depending on the computer architecture

• Heap is also called Dynamic Memory & using the heap is referred to as Dynamic Memory Allocation

• To use dynamic memory in C Language we need to use 4 functions

  • malloc()
  • calloc()
  • realloc()
  • free()

• To use dynamic memory in C++ we need to know 2 operators

  • new
  • delete

• Note: in reality the size of the stack is decided by the operating system & the compiler

• Now lets allocated some memory in heap

int main() {
    int a;// stored in stack
    int *p;
    p = (int*) malloc(sizeof(int));
    *p = 10;
    // now p will point into another block in heap
    // the previous block still exists in heap
    // to deallocate the memory in heap that is referred by pointer p
    // type free(p);
    free(p);
    p = (int*) malloc(sizeof(int));
    *p = 20;
}

malloc(sizeof(int)) will return a void pointer to the starting address of block allocated in heap, so we need to typecast it like this (int*). Now it will return the address of 200

• It is the responsibility of the programmer to clear anything allocated by him in the heap, and it is not needed anymore

• To store an array in the heap in C Language type

int main() {
    int *p;
    // 4 * 20 = 120 Byte of memory will be allocated in heap
    // and now `p` contains the base address of that array
    p = (int*) malloc(sizeof(int) * 20);
    p[0] = 10;
    p[1] = 20;
    *p = 11; // override 10
    *(p+1) = 21; // override 20
}

• To write the above code in C++

int main() {
    int *p;
    p = new int;
    *p = 10;
    delete p;
    p = new int[20];
    p[0] = 10;
    p[1] = 20;
    *p = 11; // override 10
    *(p + 1) = 21; // override 20
    delete[] p;
}

• Let's now see some functions that dynamically allocate memory in C Language
• Allocate Block of memory :
  • malloc() - signature/definition of it is like : void * malloc(size_t size)
    • Its parameter asks you for size of memory blocks in bytes & return the address of the first Byte of block that is allocated in heap
    • It itself is a generic function that allocate some size of memory, it does not care whether you're allocating this memory to store char or int or any other datatype.
    • It simply returns a void pointer to the starting address
  • calloc() - signature/definition of it is like : void * calloc(size_t num, size_t size)
    • This means that if you want to allocate an array of size 3 you will type (int*)calloc(3,sizeof(int))
    • One main difference between malloc & calloc is that malloc don't initialize the bytes with any values, so there might be garbage values, while calloc initialize all allocated Bytes to 0
  • realloc() - signature/definition of it is like : void* realloc(void *ptr,size_t size)
    • The first argument is a pointer to starting address of existing block & the second argument is the size of new block
• Deallocate Block of memory :
  • free()

size_t is a datatype that stores positive integer only like unsigned, the size cannot be 0 or any negative value

Remember: you cannot de-referencing a void pointer as it is a generic pointer type, so that we have to typecast it

In realloc function, if the new size is greater than the old one, here the compiler see if it can extend the old block it will do that otherwise it will create another block of memory with new size and then copy all the bytes of the old block to it & then free the old block

• Now Let's write some code

int main() {
    int n;
    printf("Enter size of the array\n");
    scanf("%d", &n);
    int *arr = (int *) malloc(n * sizeof(int));
    for (int i = 0; i < n; i++) {
        arr[i] = i + 1;
    }
    for (int i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
}

• Remove initialize loop to see the value of the array

int main() {
    int n;
    printf("Enter size of the array\n");
    scanf("%d", &n);
    int *arr = (int *) malloc(n * sizeof(int));
    for (int i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
}

O/P: -957605056 324 -957611696 324 0, as you can see it is garbage value

• Now use the function calloc() to observe how it initializes the bytes to zero

int main() {
    int n;
    printf("Enter size of the array\n");
    scanf("%d", &n);
    int *arr = (int *) calloc(n,sizeof(int));
    for (int i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
}

O/P: 0 0 0 0 0

• Now let's see if we used free() function then we tried to access that array

int main() {
    int n;
    printf("Enter size of the array\n");
    scanf("%d", &n);
    int *arr = (int *) calloc(n, sizeof(int));
    free(arr);
    for (int i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
}

O/P: -199091392 707 -199098032 707 0

As you can see we can access that array even if the memory is freed, this is because pointer arr still refer to the same block ( it refers to the base address of the array) even if the memory is freed (as you stored in that pointer the address)
We can also access the array even after using free() function like this arr[2] = 10; but this line may cause crash in some machines
To prevent access that block of memory again type arr = NULL;, Now we cannot refer to that array anymore as the pointer arr now is referring to NULL
So, the purpose of using free() is that the compiler will be able to reallocate the released memory again in any other variable

• Now let's use realloc() function

int main() {
    int n;
    printf("Enter size of the array\n");
    scanf("%d", &n);
    int *arr = (int *) malloc(n*sizeof(int));
    for (int i = 0; i < n; i++) {
        arr[i] = i+1;
    }
    int *arr2 = (int*) realloc(arr,n*2*sizeof(int));
    printf("Address of arr %d , Address of arr2 %d\n",arr,arr2);
    for (int i = 0; i < n*2; i++) {
        printf("%d ", arr2[i]);
    }
}
/*
O/P:
Address of arr -9430176 , Address of arr2 -9430176
1 2 3 4 5 -1163005939 -1163005939 -1163005939 -1163005939 -1163005939
 */

As you can see in this case, the old block of code is extended to store more elements, that's because the address of arr2 is like address arr
to reduce the array's size we can type int *arr2 = (int*) realloc(arr,(n/2)*sizeof(int));
Note #1 : arr = (int*) realloc(arr,0); is like saying free(arr)
Note #2 : int *arr = (int*) realloc(NULL,n*size(int)); is like saying int *arr = (int*) malloc(n*size(int));

12. Pointers As Function Return

• Pointers are just another data type, so we can use them as a return type for a function

• Let's see again an example of passing a parameter by value

int Add(int a, int b) // Called function
{
    int c = a + b;
    return c;
}
int main() { // Calling function
    int a = 2 , b = 6;
    printf("Address of a in main : %d\n",&a);
    // value of a in main() is copied to value of a in Add()
    // value of b in main() is copied to value of b in Add()
    int c = Add(a,b);// Call by value
    printf("Sum = %d\n",c);
}

• What if we want to make function Add() returns the address of c rather than returning the value

int* Add(int *a, int *b) // Called function
{
    int c = *a + *b;
    return &c;
}
int main() { // Calling function
    int a = 2 , b = 6;
    int *c = Add(&a,&b);// Call by reference
    printf("Sum = %d\n",*c);
}

the following code may or may not work depending on your compiler, but the following code will not work definitely

void PrintHello()
{
    printf("Hello Arafat\n");
}
int* Add(int *a, int *b) // Called function
{
    int c = *a + *b;
    return &c;
}
int main() { // Calling function
    int a = 2 , b = 6;
    int *c = Add(&a,&b);// Call by reference
    PrintHello();
    printf("Sum = %d\n",*c);
}

Why?, Because c variable is a local variable to the function Add(), which means that is has been created inside its stack frame, so we are returning its address which located inside its frame, but after the function Add() is terminated & the other function PrintHello() is called, its stack frame will be created instead of the terminated Add() function, so that the address of c may during this call be overridden as it is now in stack frame of PrintHello() function, so that it won't work & if it works in some compilers then the value of address of c will be garbage value

• To solve this problem we will have to create the variable c in heap not stack like this :

void PrintHello()
{
    printf("Hello Arafat\n");
}
int* Add(int *a, int *b) // Called function
{
    int *c = new int; // allocate c in heap
    // In C Language
    //int *c = (*int) malloc(sizeof(int)); // allocate c in heap
    *c = *a + *b;
    return c;
}
int main() { // Calling function
    int a = 2 , b = 6;
    int *c = Add(&a,&b);// Call by reference
    PrintHello();
    printf("Sum = %d\n",*c); // O/P : 8
}

• Bottom line, it is okay to pass a pointer (address) from bottom to top (from main() to Add()), because it is a guarantee that main() is still occupying the stack, but passing a pointer (address) from top to bottom (from Add() to main()) is wrong as Add() sooner or later will be deallocated from the memory and its stack frame might be replaced by any other called function

13. Function Pointers

• Function pointers is used to store the address of functions
• So, pointers can point to data & can point to function
• We can use a pointer to function to dereference & execute that function
• But the question here is what really is the address of a function & what is the use-cases ?

• In memory the function will be one contiguous block of some instructions, the address of a function which is also called entry point of a function will be the address of first instruction in that function (address 206 in below picture)

• when we say function pointers, we mean pointers that store the starting address or entry point of block of memory contains all instructions for that function

• Let's see the syntax of writing the function pointers

int Add(int a, int b)
{
    return a + b;
}
int* Add1(int a, int b)
{
    int *c = new int;
    *c = a+b;
    return c;
}
int main() {
    //int is the return type of the function this pointer will refer to
    //(int,int) is the data type of the function this pointer will refer to
    int (*p)(int,int);
    // the return type of the function this pointer will refer to is pointer to int
    int* (*p2)(int,int);
    p = &Add;
    // p = Add; // function name will return the address of it
    int c = (*p)(5,10); // de-referencing and executing the function
    c = p(5,10); // correct
    printf("Sum = %d",c);
}

• Let's observe some Compilation Errors with function Pointers

int Add(int a, int b)
{
    return a + b;
}
int main() {
    void (*p)(int,int);
    p = &Add; // Compile Error as Add() returns int not void
}

• Another example :

int Add(int a, int b)
{
    return a + b;
}
int main() {
    int (*p)(int);
    p = &Add;
    int c = p(2,3); // Compilation Error as pointer p refers to a function that takes only one parameter not two
}

• Let's practice writing a function pointer to a void function

void PrintHello()
{
    printf("Hello Arafat\n");
}
int main() {
    void (*ptr)();
    ptr = PrintHello;
    ptr(); // (*ptr)();
}

• Let's practice writing a function pointer to a void function that takes char * as a parameter

void PrintHello(char *c)
{
    printf("Hello %s\n",c);
}
int main() {
    void (*ptr)(char *);
    ptr = PrintHello; // ptr = &PrintHello;
    ptr("Arafat"); // (*ptr)("Arafat");
}

In Function Pointers & Callbacks part, we will see the real use-cases of function pointer

14. Function Pointers & Callbacks

#include <bits/stdc++.h>
using namespace std;
void Swap(int *a, int *b) {
    int c = *a;
    *a = *b;
    *b = c;
}
void BubbleSort(int *arr, int n) {
    for (int i = 0; i < n; i++)
    {
        for (int j = 0; j < n - 1 - i; j++) {
            if (arr[j] > arr[j + 1]) Swap(&arr[j], &arr[j + 1]);
        }
    }
}
int main() {
    int arr[] = {5, 4, 3, 2, 1};
    int n = sizeof(arr) / sizeof(arr[0]);
    BubbleSort(arr, n);
    for (int i = 0; i < n; i++) printf("%d ", arr[i]);
}

#include <bits/stdc++.h>
using namespace std;
void Swap(int *a, int *b) {
    int c = *a;
    *a = *b;
    *b = c;
}
int Compare(int a, int b)
{
    return a > b ? 1 : 0 ;
}

void BubbleSort(int *arr, int n, int (*ptr)(int,int)) {
    for (int i = 0; i < n; i++) // 5 4 3 2 1
    {
        for (int j = 0; j < n - 1 - i; j++) {
            if (ptr(arr[j],arr[j + 1]) > 0) Swap(&arr[j], &arr[j + 1]);
        }
    }
}

int main() {
    int arr[] = {5, 4, 3, 2, 1};
    int n = sizeof(arr) / sizeof(arr[0]);
    BubbleSort(arr, n,Compare);
    for (int i = 0; i < n; i++) printf("%d ", arr[i]);
}

#include <bits/stdc++.h>
using namespace std;
void Swap(int *a, int *b) {
    int c = *a;
    *a = *b;
    *b = c;
}
int CompareASC(int a, int b)
{
    return a > b ? 1 : 0 ;
}

int CompareDESC(int a, int b)
{
    return a < b ? 1 : 0 ;
}


int CompareABS(int a, int b)
{
    return abs(a) > abs(b) ? 1 : 0 ;
}

void BubbleSort(int *arr, int n, int (*ptr)(int,int)) {
    for (int i = 0; i < n; i++) // 5 4 3 2 1
    {
        for (int j = 0; j < n - 1 - i; j++) {
            if (ptr(arr[j],arr[j + 1]) > 0) Swap(&arr[j], &arr[j + 1]);
        }
    }
}

int main() {
    int arr[] = {5, 4, 3, 2, 1};
    int n = sizeof(arr) / sizeof(arr[0]);
    BubbleSort(arr, n,CompareDESC);
    //BubbleSort(arr, n,CompareASC);
    //BubbleSort(arr, n,CompareABS);
    for (int i = 0; i < n; i++) printf("%d ", arr[i]);
}

int MyCompare(const void *a, const void *b)
{
    int A = *((int*)a); // Typecasting to int then getting the value
    int B = *((int*)b); // Typecasting to int then getting the value
    // it will return a positive number if A ranked higher than B (In this case it will swap as returned number is positive)
    // it will return a negative number if A ranked lower than B
    // it will return ZERO if A has the same rank as B
    return A-B;
    //return B-A;
    //return abs(A)-abs(B);
}
int main() {
    int arr[] = {5, 4, 3, 2, 1};
    int n = sizeof(arr) / sizeof(arr[0]);
    qsort(arr,n,sizeof(int),MyCompare);
    for (int i = 0; i < n; i++) printf("%d ", arr[i]);
}

VIP Note: The advantage of using function pointer in the above codes is that I'm not modifying any lines in the function BubbleSort(), the only thing I'm changing is while calling this function in main() like this BubbleSort(arr, n,CompareDESC);, I'm just changing the name of passed function so then in this case the function pointer ptr will refer to the passed function allowing me to just call that function using my pointer ptr (again without modifying any code in function BubbleSort()), this will help me making my code reusable & becoming a clean code`

15. What Is Memory Leak ?

• This phenomenon happened due to improper use of dynamic memory (memory in the heap)

• As we said before we can free memory in heap using function free() in C Language & operator delete in C++

• Let's create a simple game called Simple Betting Game

• The computer will shuffle Jack Queen King cards

• The player has to guess the position of the Queen

• If he wins, he takes 3 * bet

• If he looses, he will lose the bet amount

• Player has 100 L.E initially

• Let's write the code of this game :)

// Created by Ahmed Arafat on 1/10/2023.
#include <bits/stdc++.h>
using namespace std;
int cash = 100, bet;
void play() {
    printf("Shuffling The Cards .....\n");
    Here:
    printf("Please Enter The Position Of The Queen 1,2 or 3");
    int user_pos, pos;
    scanf("%d", &user_pos);
    if (!(user_pos >= 1 && user_pos <= 3)) {
        printf("Please Enter A Valid Number\n");
        goto Here;
    }
    user_pos--;
    srand(time(NULL));
    pos = rand() % 3;
    if (pos == user_pos) {
        printf("You Win ! Congratulations\n");
        cash += 3 * bet;
    } else {
        printf("Wrong Guessing The Position Of The Queen Was %d\n", pos + 1);
        cash -= bet;
    }
    printf("Your Cash Now : %d L.E\n", cash);
}

int main() {
    while (cash) {
        printf("What Is Your Bet?\n");
        scanf("%d", &bet);
        if (bet > cash) printf("You Only Have %d L.E\n", cash);
        else if (bet <= 0) printf("Enter A Positive Number Please\n");
        else
        {
            play();
            printf("\n*************************\n");
        }
    }
}

• You can modify the function play() like this

void play() {
    char Cards[] = {'Q', 'K', 'J'};
    printf("Shuffling The Cards .....\n");
    Here:
    printf("Please Enter The Position Of The Queen 1,2 or 3");
    int user_pos, pos;
    scanf("%d", &user_pos);
    if (!(user_pos >= 1 && user_pos <= 3)) {
        printf("Please Enter A Valid Number\n");
        goto Here;
    }
    srand(time(NULL));
    for (int i = 0; i < 10; i++) {
        int x = rand() % 3;
        int y = rand() % 3;
        int temp = Cards[x];
        Cards[x] = Cards[y];
        Cards[y] = Cards[temp];
    }
    if (Cards[--user_pos] == 'Q') {
        printf("You Win ! Congratulations\n");
        cash += 3 * bet;
    } else {
        printf("Wrong Guessing \n");
        cash -= bet;
    }
    printf("Your Cash Now : %d L.E\n", cash);
}

The memory consumption from the above code will be the same as time run, but if we modified it to be something like this

void play() {
    char *Cards = (char *) malloc(3 * sizeof(char));
    Cards[0] = 'Q';
    Cards[1] = 'K';
    Cards[2] = 'J';
        ......
        ......
}

Each time we call function play() we allocate a memory in heap, which will cause a memory lack because we are not deallocating this block of memory, to solve this we have to write free(Cards) at the end of the function `play()

Reference

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