Difference Between Call by Value and Call by Reference in C++

Functions in C++ receive arguments either by call by value or call by reference. These parameter passing methods determine whether a function works on a copy of the data or the original variable.

• Call by Value passes a copy of the argument to the function.
• Call by Reference passes a reference to the original variable, allowing direct modification.

Actual Parameters vs Formal Parameters

Before understanding these methods, it is important to know the two types of function parameters.

• Actual Parameters (Arguments): Values passed during the function call.
• Formal Parameters: Parameters declared in the function definition that receive the arguments.

For example:

#include <iostream>
using namespace std;

void display(int x) {    // x is a formal parameter
    cout << x;
}

int main() {
    int num = 10;
    display(num);        // num is an actual parameter
    return 0;
}

Call by Value

In call by value, the function receives a copy of the original variable. Changes made inside the function affect only the copied value, leaving the original variable unchanged.

• Keeps the original variable unchanged after the function call.
• Suitable for small data types such as int and char.
• Useful when the function only needs to read the value.

#include <iostream>
using namespace std;

// Function receives a copy
void increment(int num)
{
    num++;
    cout << "Inside function: " << num << endl;
}

int main()
{
    int number = 5;

    increment(number);

    cout << "Outside function: " << number << endl;

    return 0;
}

Inside function: 6
Outside function: 5

Explanation

• number is passed by value to increment(), a copy of number is stored in num.
• Incrementing num changes only the copied value and the original variable number remains unchanged.

Call by Reference

In call by reference, the function receives a reference to the original variable instead of a copy. As both parameters refer to the same memory location, changes made inside the function directly modify the original variable.

• Avoids copying large objects, improving performance.
• Allows the function to modify the original variable.
• Commonly used for large objects or output parameters.

#include <iostream>
using namespace std;

// Function receives a reference
void increment(int& num)
{
    num++;
    cout << "Inside function: " << num << endl;
}

int main()
{
    int number = 5;

    increment(number);

    cout << "Outside function: " << number << endl;

    return 0;
}

Inside function: 6
Outside function: 6

Explanation

• number is passed by reference using int&, num becomes another name for number.
• Incrementing num directly modifies the original variable and the updated value is visible after the function returns.

Call by Value Vs Call by Reference