Default Arguments Explained

Default arguments in C++ allow you to specify default values for function parameters. If a value for a parameter is not provided when the function is called, the default value is used. This feature enhances the flexibility and usability of functions by reducing the need for multiple overloaded functions.

Key Concepts

1. Default Argument Definition

Default arguments are specified in the function declaration by assigning a value to the parameter. This value is used if the corresponding argument is omitted from the function call.

Example:

#include <iostream>
using namespace std;

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

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

In this example, the function greet has a default argument "Guest" for the parameter name. If no argument is provided, the function uses the default value.

2. Positioning of Default Arguments

Default arguments must be placed at the end of the parameter list. This means that all parameters following a default argument must also have default values.

Example:

#include <iostream>
using namespace std;

void displayInfo(string name, int age = 0, string occupation = "Unknown") {
    cout << "Name: " << name << ", Age: " << age << ", Occupation: " << occupation << endl;
}

int main() {
    displayInfo("Alice");              // Output: Name: Alice, Age: 0, Occupation: Unknown
    displayInfo("Bob", 30);            // Output: Name: Bob, Age: 30, Occupation: Unknown
    displayInfo("Charlie", 25, "Engineer"); // Output: Name: Charlie, Age: 25, Occupation: Engineer
    return 0;
}
    

In this example, the default arguments age and occupation are placed at the end of the parameter list, ensuring that they can be omitted in function calls.

3. Overriding Default Arguments

When calling a function with default arguments, you can override the default values by providing explicit arguments. The provided arguments will take precedence over the default values.

Example:

#include <iostream>
using namespace std;

void calculate(int a = 10, int b = 20) {
    cout << "Sum: " << (a + b) << endl;
}

int main() {
    calculate();          // Output: Sum: 30
    calculate(5);         // Output: Sum: 25
    calculate(5, 15);     // Output: Sum: 20
    return 0;
}
    

In this example, the function calculate has default arguments a = 10 and b = 20. When called with different arguments, the provided values override the defaults.

4. Practical Use Cases

Default arguments are particularly useful in scenarios where a function has commonly used parameters with standard values. They reduce the need for multiple overloaded functions and make the function calls more concise.

Example:

#include <iostream>
using namespace std;

void drawShape(string shape = "circle", int size = 10) {
    cout << "Drawing a " << shape << " of size " << size << endl;
}

int main() {
    drawShape();          // Output: Drawing a circle of size 10
    drawShape("square");  // Output: Drawing a square of size 10
    drawShape("triangle", 15); // Output: Drawing a triangle of size 15
    return 0;
}
    

In this example, the function drawShape uses default arguments to draw a circle of size 10 by default. The user can override these defaults to draw different shapes and sizes.