🧩 What are Default Parameters?

A default parameter in C++ means giving a default value to a function’s parameter.
If the caller doesn’t provide an argument for that parameter, the default value is used automatically.

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🧠 Syntax:

void functionName(int a, int b = 10) {
    cout << "a: " << a << ", b: " << b << endl;
}

👉 Here, b has a default value of 10.

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✅ Example:

#include <iostream>
using namespace std;

void greet(string name = "Guest") {
    cout << "Hello, " << name << "!" << endl;
}

int main() {
    greet("Amit");   // passes a custom argument
    greet();         // uses default parameter
    return 0;
}

🖨️ Output:

Hello, Amit!
Hello, Guest!

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⚙️ Example with Multiple Parameters:

#include <iostream>
using namespace std;

void display(int a, int b = 20, int c = 30) {
    cout << "a = " << a << ", b = " << b << ", c = " << c << endl;
}

int main() {
    display(10);        // uses default for b and c
    display(10, 40);    // uses default for c
    display(10, 40, 50);// uses all given values
    return 0;
}

🖨️ Output:

a = 10, b = 20, c = 30
a = 10, b = 40, c = 30
a = 10, b = 40, c = 50

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⚠️ Important Rules:

1. Default arguments must be on the right side.

    void test(int a = 10, int b = 20); ✅
    void test(int a = 10, int b); ❌  // Not allowed
   

2. Default values are assigned from right to left.

3. You can define defaults either in the function declaration or definition, not both.

   Example:

    // Declaration (header)
    void print(int a = 5, int b = 10);
   
    // Definition (implementation)
    void print(int a, int b) {
        cout << a << " " << b << endl;
    }
   

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🧑‍🏫 Use Case Example:

Default parameters are very helpful when you want to give flexibility but avoid overloading multiple functions.

void printBill(float price, float tax = 0.18) {
    cout << "Total amount: " << price + (price * tax) << endl;
}

int main() {
    printBill(1000);      // uses default 18% tax
    printBill(1000, 0.10);// uses custom 10% tax
}

Output:

Total amount: 1180
Total amount: 1100