Passing a Pointer as an Argument in C++

 Passing a Pointer as an Argument

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Introduction

pass arguments to a function in three ways:

1. Call by Value
2. Call by Reference
3. Call by Pointer

Mainly focus on Call by Pointer à Passing the address of a variable by & operator to a function.

Inside the function, a pointer parameter receives this address and can directly modify the original variable.

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Syntax

void functionName(int *ptr) {

    // function body

}

 

int main() {

    int x = 10;

    functionName(&x);   // passing address of x

}

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Concept Diagram

+-----------+         +--------------------+

|  main()   |         |   function()       |

+-----------+         +--------------------+

| int x=10  |  --->   | int *ptr = &x      |

| &x (address) -> | *ptr accesses x    |

+-----------+         +--------------------+

→ Both x and *ptr refer to the same memory location.

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Example 1 — Modify a variable using a pointer

#include <iostream>

using namespace std;

 

void changeValue(int *p) {

    *p = 100;  // modifies the original variable

}

 

int main() {

    int a = 10;

    cout << "Before function: " << a << endl;

    changeValue(&a);   // passing address of a

    cout << "After function: " << a << endl;

    return 0;

}

Output:

Before function: 10

After function: 100

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Example 2 — Swapping two numbers using pointers

#include <iostream>

using namespace std;

 

void swapNumbers(int *x, int *y) {

    int temp = *x;

    *x = *y;

    *y = temp;

}

 

int main() {

    int a = 5, b = 10;

    cout << "Before Swap: a = " << a << ", b = " << b << endl;

    swapNumbers(&a, &b);

    cout << "After Swap: a = " << a << ", b = " << b << endl;

    return 0;

}

Output:

Before Swap: a = 5, b = 10

After Swap: a = 10, b = 5

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Example 3 — Passing a pointer to an array

#include <iostream>

using namespace std;

 

void displayArray(int *arr, int n) {

    for (int i = 0; i < n; i++)

        cout << arr[i] << " ";

}

 

int main() {

    int num[] = {10, 20, 30, 40};

    displayArray(num, 4);  // array name = base address

    return 0;

}

Output:

10 20 30 40

 

The array name acts as a pointer to its first element.

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Advantages

Efficient — avoids copying data
Allows modification of the caller’s variables
Useful for arrays and dynamic memory

Summary Table

Drawback	Explanation
1. Null pointer risk	Can cause segmentation fault
2. Complex syntax	Use of * and & reduces readability
3. Unintended changes	Directly modifies original variable
4. Pointer arithmetic	May corrupt data
5. Security issues	Exposes raw memory addresses
6. Type unsafety	Can cast to wrong type
7. Debugging difficulty	Hard to trace pointer-related bugs

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Difference Table

Type of Call	Function Prototype	Passes	Can Modify?
Call by Value	void fun(int x)	Copy of variable	No
Call by Pointer	void fun(int *x)	Address of variable	Yes
Call by Reference	void fun(int &x)	Alias of variable	Yes

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