# 🧩 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.

---

### 🧠 Syntax:

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

👉 Here, `b` has a **default value** of `10`.

---

### ✅ Example:

```plaintext
#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:

```plaintext
Hello, Amit!
Hello, Guest!
```

---

### ⚙️ Example with Multiple Parameters:

```plaintext
#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:

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

---

### ⚠️ Important Rules:

1. **Default arguments must be on the right side.**
    
    ```plaintext
    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:
    
    ```plaintext
    // Declaration (header)
    void print(int a = 5, int b = 10);
    
    // Definition (implementation)
    void print(int a, int b) {
        cout << a << " " << b << endl;
    }
    ```
    

---

### 🧑‍🏫 Use Case Example:

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

```plaintext
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:

```plaintext
Total amount: 1180
Total amount: 1100
```