C++ Functions: Declarations, Parameter Passing Modes & Best Practices

A Function is a reusable block of code that performs a specific task. Functions improve code modularity, readability, and maintainability. In C++, choosing the correct parameter-passing mechanism is critical for performance and safety.

1. Pass by Value vs Pass by Reference

#include <iostream>

// Pass by Value: Operates on a COPY (Original unchanged)
void tryToModify(int x) {
    x = 999;
}

// Pass by Reference: Operates directly on the ORIGINAL variable
void actuallyModify(int& x) {
    x = 999;
}

// Pass by Const Reference: Fast (Zero copy), Read-Only (Guaranteed safe)
void printBigData(const std::string& text) {
    std::cout << "Text length: " << text.length() << "\n";
}

int main() {
    int score = 50;

    tryToModify(score);
    std::cout << "After tryToModify: " << score << "\n"; // Still 50

    actuallyModify(score);
    std::cout << "After actuallyModify: " << score << "\n"; // Now 999!

    return 0;
}

2. Default Arguments in C++

C++ allows functions to have default parameter values. Default parameters must be assigned from right to left in the parameter list:

#include <iostream>

// Default argument for 'taxRate' is 0.05 (5%)
double calculateTotal(double subtotal, double taxRate = 0.05) {
    return subtotal + (subtotal * taxRate);
}

int main() {
    std::cout << "Default tax: " << calculateTotal(100.0) << "\n";      // Uses 0.05 -> 105.0
    std::cout << "Custom tax:  " << calculateTotal(100.0, 0.18) << "\n"; // Uses 0.18 -> 118.0
    return 0;
}