Default Arguments & Constant Arguments in C++

1. What are Default Arguments? (In Simple Words)

In C language, if a function takes 3 inputs, you must pass all 3 inputs every single time you call it. If you pass only 2, you get an error.

C++ introduces Default Arguments.

If you do not pass a value for a parameter during the function call, the compiler automatically assigns a pre-set default value to it!

Real-Life Analogy: Think of opening a Fixed Deposit in a bank. By default, the bank sets the time period to 1 year and the interest rate to 7.0%. But if you want, you can change them to 3 years and 8.5%. If you don’t mention anything, the bank’s default values are used!

2. The Strict “Right-to-Left” Rule (Golden Exam Rule)

When adding default values to parameters, you must start from the rightmost parameter and move left.

You cannot give a default value to a left parameter while leaving a parameter to its right without a default!

3. Practical Code Example: Simple Interest Calculator

#include <iostream>
using namespace std;

// PROTOTYPE: Default years = 1, default rate = 7.0%
float computeInterest(float principal, int years = 1, float rate = 7.0f);

int main() {
    // 1. Pass only 1 argument: years defaults to 1, rate defaults to 7.0
    cout << "Case 1: Rs. " << computeInterest(10000.0f) << endl;

    // 2. Pass 2 arguments: years becomes 3, rate defaults to 7.0
    cout << "Case 2: Rs. " << computeInterest(10000.0f, 3) << endl;

    // 3. Pass all 3 arguments: overrides both defaults!
    cout << "Case 3: Rs. " << computeInterest(10000.0f, 3, 9.5f) << endl;

    return 0;
}

// DEFINITION: Note that we DO NOT repeat the '= 1' or '= 7.0f' here!
float computeInterest(float principal, int years, float rate) {
    return (principal * years * rate) / 100.0f;
}

Crucial Rule: Define default values in the Function Prototype (declaration) only. Do NOT repeat them in the function definition header.

4. Constant Arguments (const Parameters)

Sometimes you want to make sure that a function cannot change an argument by mistake. You can mark that parameter with the const keyword.

A Constant Argument makes the parameter read-only. If someone tries to modify it inside the function, the compiler throws an error immediately.

void printBill(const float total) {
    // total = total - 100; // ERROR! Compiler stops you from changing a const parameter!
    cout << "Final Bill: Rs. " << total << endl;
}

Why Use const & (Constant Reference) in C++?

When passing large objects (like a Student record with 50 fields):

1. Pass by Value (Student s): Safe, but creates a duplicate copy in RAM, wasting time and memory.
2. Pass by Reference (Student &s): Super fast (no copying), but the function can accidentally change your original data!
3. Pass by Constant Reference (const Student &s): The best of both worlds!
   • Zero copying (super fast because it passes a reference).
   • 100% safe (because const prevents any modification).

DAVV Exam Preparation Tips

Frequently Asked 5-Mark Questions:

1. Explain the rules for using default arguments in C++ with an example.
   Model Answer:
   • Define default arguments as fallback values used when caller omits them.
   • Explain the Right-to-Left rule: defaults must be placed starting at the end of the parameter list.
   • Show valid vs invalid declarations.
   • Provide a quick 5-line interest calculation snippet.
2. What are constant arguments in C++? Why are they useful?
   Model Answer:
   • Parameters declared with const cannot be modified inside the function (read-only).
   • Useful to prevent accidental changes to original data, especially when passing large objects by reference (const Student &s).

Frequently Asked 10-Mark Questions:

1. Explain Default Arguments and Function Overloading. Can default arguments replace function overloading? What is ambiguity?
   Model Answer:
   • Explain how a single function with default arguments (e.g., add(int a, int b, int c = 0)) can handle calls with both 2 and 3 arguments, replacing 2 overloaded functions.
   • Explain the Ambiguity problem: If you write void print(int a); and void print(int a, int b = 5);, calling print(10); causes a compiler error because the compiler cannot tell which version you intended to run!