Call by Value vs Call by Reference in C++

In C++, function arguments can be passed in two main ways: Call by Value and Call by Reference. Understanding the difference between them is crucial for writing efficient and predictable programs.

🔹 Call by Value

In Call by Value, a copy of the actual argument is passed to the function. Changes made inside the function do not affect the original variable.

Key Points:

Passes a copy of the value.
Original data remains unchanged.
Requires more memory for copies.

Example:

#include <iostream>
using namespace std;

void modify(int x) {
    x = 20;
}

int main() {
    int num = 10;
    modify(num);
    cout << "Value of num: " << num << endl;
    return 0;
}

Output:

Value of num: 10

🔹 Call by Reference

In Call by Reference, the address of the actual argument is passed to the function. Changes made inside the function directly affect the original variable.

Key Points:

Passes a reference (memory address).
Original data is modified.
More memory-efficient than call by value.

Example:

#include <iostream>
using namespace std;

void modify(int &x) {
    x = 20;
}

int main() {
    int num = 10;
    modify(num);
    cout << "Value of num: " << num << endl;
    return 0;
}

Output:

Value of num: 20

🔹 Call by Pointer (Alternative Reference)

Another way is passing pointers, which also modifies the original variable.

void modify(int *x) {
    *x = 20;
}

📋 Comparison Table

Feature	Call by Value	Call by Reference
Data passed	Copy of actual data	Address of actual data
Effect on original data	No change	Can be modified
Memory usage	Higher (creates copies)	Lower (uses same variable)
Use case	When you don’t want changes	When you want to modify

Best Practices

Use call by value when you want to protect the original data.
Use call by reference for large data (like arrays/objects) to improve performance.
Prefer const references if you want to avoid copying but don’t want modification.