C++ Function Parameters

Types of Parameters

void func(int x) {
    x = 10; // Original unchanged
}

void func(int &x) {
    x = 10; // Original changed
}

void func(int *x) {
    *x = 10; // Original changed
}

void func(int x = 5) {
    cout << x;
}

🔹 Pass by Value

Pass by value copies the argument into the function, leaving the original variable unchanged. When you pass a variable by value, the function works with a local copy, so modifications inside the function don’t affect the original. This is safe and prevents accidental side effects but can be inefficient for large data structures. Use pass by value for primitive types or when you want to ensure the original data remains untouched, as in mathematical computations or immutable operations.

#include <iostream>
using namespace std;

void changeValue(int x) {
    x = 100;  // Only changes the copy
    cout << "Inside function: x = " << x << endl;
}

int main() {
    int number = 5;
    cout << "Before function: number = " << number << endl;
    
    changeValue(number);
    
    cout << "After function: number = " << number << endl;
    return 0;
}

🔹 Pass by Reference

Pass by reference allows functions to directly modify the original variable using the & symbol. This avoids copying, improves performance for large objects, and enables functions to alter arguments, as seen in swap operations. References maintain a direct link to the original data, making them ideal for mutating inputs, returning multiple values, or handling complex types efficiently. However, care is needed to avoid unintended changes, and const references can be used for read-only access to large data.

#include <iostream>
using namespace std;

void changeValue(int &x) {
    x = 100;  // Changes the original variable
    cout << "Inside function: x = " << x << endl;
}

void swapNumbers(int &a, int &b) {
    int temp = a;
    a = b;
    b = temp;
}

int main() {
    int number = 5;
    cout << "Before function: number = " << number << endl;
    
    changeValue(number);
    
    cout << "After function: number = " << number << endl;
    
    int x = 10, y = 20;
    cout << "Before swap: x = " << x << ", y = " << y << endl;
    swapNumbers(x, y);
    cout << "After swap: x = " << x << ", y = " << y << endl;
    
    return 0;
}

🔹 Default Parameters

Default parameters let you specify fallback values for function arguments when callers omit them. For example, void printInfo(string name, int age = 18, string city = "Unknown") allows flexible calls with partial data. Defaults enhance API usability, reduce overloaded functions, and provide sensible behavior for common cases. They’re especially useful in constructors, configuration functions, and libraries where parameters often have typical values, improving both code brevity and developer experience.

#include <iostream>
using namespace std;

// Function with default parameters
void printInfo(string name, int age = 18, string city = "Unknown") {
    cout << "Name: " << name << ", Age: " << age << ", City: " << city << endl;
}

int main() {
    printInfo("Alice");                    // Uses default age and city
    printInfo("Bob", 25);                  // Uses default city
    printInfo("Charlie", 30, "New York");  // All parameters provided
    
    return 0;
}

🔹 Multiple Parameters

Functions can accept multiple parameters of different types to perform complex operations. For instance, a recordStudent() function might take name (string), ID (int), GPA (double), and grade (char). This allows bundling related data into a single call, improving organization and reducing function proliferation. Proper use of multiple parameters enhances function clarity, supports structured data handling, and is common in data processing, reporting, and object initialization routines in C++ applications.

#include <iostream>
using namespace std;

double calculateGrade(int totalPoints, int maxPoints, bool extraCredit = false) {
    double percentage = (double)totalPoints / maxPoints * 100;
    
    if (extraCredit) {
        percentage += 5; // Add 5% for extra credit
    }
    
    return percentage;
}

void displayStudentInfo(string name, int id, double gpa, char grade) {
    cout << "Student: " << name << endl;
    cout << "ID: " << id << endl;
    cout << "GPA: " << gpa << endl;
    cout << "Grade: " << grade << endl;
}

int main() {
    double grade1 = calculateGrade(85, 100);
    double grade2 = calculateGrade(78, 100, true);
    
    cout << "Grade 1: " << grade1 << "%" << endl;
    cout << "Grade 2: " << grade2 << "%" << endl;
    
    displayStudentInfo("John Doe", 12345, 3.75, 'A');
    
    return 0;
}