To pass a variable by reference in C++, we use the “&” symbol in the function declaration. Let’s look at an example to understand how it works:

#include <iostream>

void modifyValue(int& num) {
    num = 10; // Modify the original variable directly
}

int main() {
    int num = 5;
    std::cout << "Before modification: " << num << "\n";
    
    modifyValue(num); // Pass `num` by reference
    
    std::cout << "After modification: " << num << "\n";
    
    return 0;
}

In this example, we have a function modifyValue, which takes an integer variable num by reference. Inside the function, we directly change the value of num to 10. When we call modifyValue from the main function and pass num as an argument, the original num variable gets modified.

The output of this code will be:

Before modification: 5
After modification: 10

By passing variables by reference, we eliminate the need to return a modified value from a function. This can be especially useful when dealing with large objects or when we want to modify multiple variables within a single function.

Remember to use this feature judiciously, as it can introduce side effects and make the code harder to reason about.