C++ Default Arguments: How to Give Function Parameters Default Values

Sometimes a function parameter almost always gets the same value. Rather than forcing the caller to repeat it every time, C++ lets you declare a default value right in the function signature. If the caller provides an argument, that value is used. If not, the default kicks in.

Basic Syntax

Declare a default by writing = value after the parameter name:

#include <iostream>
#include <string>

void greet(std::string name = "World") {
    std::cout << "Hello, " << name << "!\n";
}

int main() {
    greet("Alice");  // Hello, Alice!
    greet();         // Hello, World!
    return 0;
}

The second call uses the default value "World" because no argument was passed.

Multiple Default Arguments

You can give defaults to multiple parameters. The rule is: defaults must be at the end of the list — you can’t have a non-default parameter after a default one.

#include <iostream>

void drawBox(int width, int height = 5, char fill = '*') {
    for (int row = 0; row < height; row++) {
        for (int col = 0; col < width; col++) {
            std::cout << fill;
        }
        std::cout << "\n";
    }
}

int main() {
    drawBox(4);           // width=4, height=5, fill='*'
    std::cout << "\n";
    drawBox(3, 2);        // width=3, height=2, fill='*'
    std::cout << "\n";
    drawBox(3, 2, '#');   // width=3, height=2, fill='#'
    return 0;
}

Output:

****
****
****
****
****

***
***

###
###

This works because you always provide the required parameter (width) and optionally override the defaults.

The “Trailing Defaults” Rule

This is valid:

void example(int a, int b = 10, int c = 20);  // fine

This is NOT valid:

void example(int a = 5, int b, int c = 20);  // error — b has no default but follows a

Think of it like a queue: once you start adding defaults, every parameter that follows must also have a default.

Default Arguments in Header Files

If you split your code into header (.h) and implementation (.cpp) files, declare the defaults only in the header — not in both places. Putting them in both causes a compiler error.

// myFunc.h
void connect(std::string host, int port = 8080, bool secure = false);

// myFunc.cpp
void connect(std::string host, int port, bool secure) {
    // no defaults here
}

Default Arguments vs Function Overloading

Both achieve similar results, but the right choice depends on your situation:

// Using defaults — single function, same logic
void log(std::string msg, bool newline = true) {
    std::cout << msg;
    if (newline) std::cout << "\n";
}

// Using overloading — separate logic per version
void log(std::string msg) { std::cout << msg << "\n"; }
void log(std::string msg, int level) {
    std::cout << "[" << level << "] " << msg << "\n";
}

Use defaults when the function body is the same and you just want optional parameters. Use function overloading when different argument types require genuinely different behaviour.

Practical Example: A Configurable Timer

#include <iostream>

void startTimer(int seconds, bool verbose = false, std::string label = "Timer") {
    if (verbose) {
        std::cout << label << " starting: " << seconds << "s\n";
    }
    // (actual timer logic would go here)
    if (verbose) {
        std::cout << label << " finished.\n";
    }
}

int main() {
    startTimer(10);                          // quiet mode
    startTimer(5, true);                     // verbose, default label
    startTimer(3, true, "Download timer");   // fully specified
    return 0;
}

Related Articles

• C++ Functions Tutorial: How to Write and Use Functions
• C++ Function Overloading: Same Name, Different Parameters Explained
• C++ Header Files Explained for Beginners
• C++ Pass by Value vs Pass by Reference

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