pointer

🧩 1️⃣ Basic Pointer

👉 A pointer stores the address of another variable.

#include <iostream>
using namespace std;

int main() {
    int a = 10;
    int *p = &a;

    cout << "Value of a: " << a << endl;
    cout << "Address of a: " << &a << endl;
    cout << "Pointer p: " << p << endl;
    cout << "Value pointed by p: " << *p << endl;
}

✅ Key Points:

• &a → gives address of a

• *p → gives value stored at that address

• Helps in dynamic memory and function parameter passing

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🧮 2️⃣ Pointer to Pointer

👉 Stores the address of another pointer.

#include <iostream>
using namespace std;

int main() {
    int x = 100;
    int *p = &x;
    int **q = &p;

    cout << "x = " << x << endl;
    cout << "*p = " << *p << endl;
    cout << "**q = " << **q << endl;
}

🧠 Memory Chain:

x = 100
p → address of x
q → address of p

✅ Use: Multi-level data like 2D arrays, dynamic structures (linked lists, trees).

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📦 3️⃣ Pointer with Arrays

👉 Array name acts like a pointer to its first element.

#include <iostream>
using namespace std;

int main() {
    int arr[3] = {10, 20, 30};
    int *p = arr; // same as &arr[0]

    cout << *p << endl;       // 10
    cout << *(p + 1) << endl; // 20
    cout << *(p + 2) << endl; // 30
}

✅ Key Points:

• arr[i] is same as *(arr + i)

• Useful for pointer arithmetic and array traversal

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🔄 4️⃣ Pointer with Functions (Call by Reference)

👉 Lets you modify variable values inside functions directly.

#include <iostream>
using namespace std;

void swap(int *x, int *y) {
    int temp = *x;
    *x = *y;
    *y = temp;
}

int main() {
    int a = 5, b = 10;
    swap(&a, &b);
    cout << "After swap: a = " << a << ", b = " << b << endl;
}

✅ Use:

• Pass variables by address

• Saves memory and improves performance

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⚙️ 5️⃣ Null Pointer

👉 A pointer that points to nothing (address = 0).

#include <iostream>
using namespace std;

int main() {
    int *p = NULL;  // or nullptr (C++11)

    if (p == NULL)
        cout << "Pointer is null!" << endl;
}

✅ Use:

• Initialize pointers safely

• Prevent crashes due to wild pointers

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☠️ 6️⃣ Dangling Pointer

👉 A dangling pointer points to memory that has been freed or deleted —
but the pointer still holds that old address.

Example 1: Returning local variable’s address

#include <iostream>
using namespace std;

int* fun() {
    int a = 10;
    return &a;  // ❌ 'a' will be destroyed after function ends
}

int main() {
    int *p = fun();
    cout << *p << endl;  // ❌ invalid, dangling pointer
}

Example 2: Deleting memory

#include <iostream>
using namespace std;

int main() {
    int *p = new int(5);
    delete p;    // memory released
    // p still holds the old address (dangling)
    cout << *p << endl;  // ❌ undefined behavior

    p = NULL;   // ✅ fix it
}

✅ Fix:

• Set pointer to NULL after deleting memory

• Avoid returning addresses of local variables