Default Parameters in C++ Functions

Default parameters allow you to provide default values for function parameters. If the caller doesn’t provide a value, the default is used.

Basic Syntax


returnType functionName(type param1 = defaultValue) {
    // function body
}

Simple Example


#include <iostream>
using namespace std;
 
void greet(string name = "Guest") {
    cout << "Hello, " << name << "!" << endl;
}
 
int main() {
    greet("Alice");  // Uses "Alice"
    greet();         // Uses default "Guest"
    return 0;
}

Output:


Hello, Alice!
Hello, Guest!

How It Works

When you call greet() without arguments:

The function uses the default value “Guest”

When you call greet("Alice"):

The provided value “Alice” overrides the default

Multiple Default Parameters

You can have multiple parameters with defaults:


#include <iostream>
using namespace std;
 
void displayInfo(string name = "Unknown", int age = 0, string city = "Not specified") {
    cout << "Name: " << name << endl;
    cout << "Age: " << age << endl;
    cout << "City: " << city << endl;
    cout << "---" << endl;
}
 
int main() {
    displayInfo("Alice", 25, "New York");
    displayInfo("Bob", 30);
    displayInfo("Charlie");
    displayInfo();
    return 0;
}

Output:


Name: Alice
Age: 25
City: New York
---
Name: Bob
Age: 30
City: Not specified
---
Name: Charlie
Age: 0
City: Not specified
---
Name: Unknown
Age: 0
City: Not specified
---

Important Rule: Right-to-Left

Default parameters must be specified from right to left. Once a parameter has a default, all following parameters must also have defaults.

✅ Correct


void function1(int a, int b = 10, int c = 20);           // ✓
void function2(int a = 5, int b = 10, int c = 20);       // ✓
void function3(string text, int count = 1);              // ✓

❌ Wrong


void function4(int a = 5, int b, int c = 20);  // ✗ Error: b has no default
void function5(int a = 5, int b);              // ✗ Error: a has default but b doesn't

Practical Example: Power Function


#include <iostream>
#include <cmath>
using namespace std;
 
// Calculate power with default exponent of 2 (square)
double power(double base, int exponent = 2) {
    return pow(base, exponent);
}
 
int main() {
    cout << "5^3 = " << power(5, 3) << endl;     // 125
    cout << "4^2 = " << power(4) << endl;        // 16 (default exponent)
    cout << "7^2 = " << power(7) << endl;        // 49 (default exponent)
    return 0;
}

Output:


5^3 = 125
4^2 = 16
7^2 = 49

Rectangle Area with Default Square


#include <iostream>
using namespace std;
 
// If height not provided, assumes it's a square
double calculateArea(double width, double height = -1) {
    if (height == -1) {
        height = width;  // Make it a square
    }
    return width * height;
}
 
int main() {
    cout << "Rectangle 5x3: " << calculateArea(5, 3) << endl;
    cout << "Square 4x4: " << calculateArea(4) << endl;
    return 0;
}

Output:


Rectangle 5x3: 15
Square 4x4: 16

Printing with Default Separator


#include <iostream>
#include <string>
using namespace std;
 
void printWithSeparator(string text, char separator = '-', int count = 20) {
    for (int i = 0; i < count; i++) {
        cout << separator;
    }
    cout << endl << text << endl;
    for (int i = 0; i < count; i++) {
        cout << separator;
    }
    cout << endl;
}
 
int main() {
    printWithSeparator("Title 1");
    printWithSeparator("Title 2", '*');
    printWithSeparator("Title 3", '=', 30);
    return 0;
}

Output:


--------------------
Title 1
--------------------
********************
Title 2
********************
==============================
Title 3
==============================

Interest Calculator


#include <iostream>
using namespace std;
 
// Calculate simple interest with default rate of 5%
double calculateInterest(double principal, double time, double rate = 5.0) {
    return (principal * time * rate) / 100;
}
 
int main() {
    cout << "Interest on $1000 for 2 years at 5%: $"
         << calculateInterest(1000, 2) << endl;
 
    cout << "Interest on $1000 for 2 years at 7%: $"
         << calculateInterest(1000, 2, 7.0) << endl;
 
    return 0;
}

Output:


Interest on $1000 for 2 years at 5%: $100
Interest on $1000 for 2 years at 7%: $140

Array Printer with Default Separator


#include <iostream>
using namespace std;
 
void printArray(int arr[], int size, string separator = ", ") {
    for (int i = 0; i < size; i++) {
        cout << arr[i];
        if (i < size - 1) {
            cout << separator;
        }
    }
    cout << endl;
}
 
int main() {
    int numbers[] = {1, 2, 3, 4, 5};
 
    cout << "With default separator: ";
    printArray(numbers, 5);
 
    cout << "With custom separator: ";
    printArray(numbers, 5, " | ");
 
    return 0;
}

Output:


With default separator: 1, 2, 3, 4, 5
With custom separator: 1 | 2 | 3 | 4 | 5

Logger Function


#include <iostream>
#include <string>
using namespace std;
 
void log(string message, string level = "INFO") {
    cout << "[" << level << "] " << message << endl;
}
 
int main() {
    log("Application started");
    log("User logged in", "DEBUG");
    log("Database error", "ERROR");
    log("Processing data");
    return 0;
}

Output:


[INFO] Application started
[DEBUG] User logged in
[ERROR] Database error
[INFO] Processing data

Function Overloading vs Default Parameters

You can achieve similar results with either approach:

Using Default Parameters


void display(int value, bool newline = true) {
    cout << value;
    if (newline) cout << endl;
}

Using Function Overloading


void display(int value) {
    cout << value << endl;
}
 
void display(int value, bool newline) {
    cout << value;
    if (newline) cout << endl;
}

Note: Default parameters are usually simpler and more maintainable.

Best Practices

Use sensible defaults: Choose defaults that cover the most common use case
Document default values: Make it clear in comments what the defaults are
Consider const references for complex default objects:
```
void process(const string& text = "default");
```
Don’t overuse: Too many defaults can make code hard to understand
Put required parameters first: This makes the function signature clearer

Common Mistakes

Default in Declaration AND Definition


// Wrong: default specified in both places
void greet(string name = "Guest");  // Declaration
 
void greet(string name = "Guest") {  // Definition - Error!
    cout << "Hello, " << name << endl;
}
 
// Correct: default only in declaration
void greet(string name = "Guest");  // Declaration
 
void greet(string name) {  // Definition
    cout << "Hello, " << name << endl;
}

Skipping Parameters


void display(int a, int b = 10, int c = 20);
 
// You cannot skip b and provide c
// display(5, , 30);  // ✗ Syntax error
 
// You must provide parameters in order
display(5);        // ✓ Uses defaults for b and c
display(5, 15);    // ✓ Uses default for c
display(5, 15, 30); // ✓ All provided

Practice Exercises

Greeting Function: Create a function that greets a user with optional time of day (default: “day”).
Temperature Display: Write a function that displays temperature with optional unit (default: “C”).
Draw Rectangle: Create a function that draws a rectangle with optional fill character (default: ’*’).
Calculate Discount: Write a function that calculates price after discount (default: 10%).
Format Phone Number: Create a function that formats phone numbers with optional country code (default: “+1”).

When to Use Default Parameters

✅ Good use cases:

Common default configurations
Optional formatting options
Backward compatibility
Sensible fallback values

❌ Avoid when:

The “default” isn’t obvious
Most calls need to override the default
The default value might change frequently

Next Steps

Learn about function overloading to create multiple versions with different parameters
Explore templates for generic function parameters
Study reference parameters for efficient data passing

Default parameters make your functions more flexible and easier to use!