C++ Constructors

Default Constructor

A constructor is a special method that runs automatically when an object is created. It has the same name as the class and no return type.

#include <iostream>
using namespace std;

class Student {
  public:
    string name;
    int    grade;

    // Default constructor
    Student() {
        name  = "Unknown";
        grade = 0;
        cout << "Student created!" << endl;
    }
};

int main() {
    Student s;   // Constructor called automatically
    cout << s.name << ", Grade: " << s.grade << endl;
    return 0;
}

Output:

Student created!
Unknown, Grade: 0

Parameterized Constructor

class Student {
  public:
    string name;
    int    grade;

    Student(string n, int g) {
        name  = n;
        grade = g;
    }

    void display() {
        cout << name << " — Grade: " << grade << endl;
    }
};

int main() {
    Student s1("Alice", 95);
    Student s2("Bob",   82);
    s1.display();
    s2.display();
    return 0;
}

Output:

Alice — Grade: 95
Bob — Grade: 82

Member Initializer List

A more efficient way to initialize members — required for const and reference members:

class Circle {
  public:
    const double PI;
    double radius;

    Circle(double r) : PI(3.14159), radius(r) { }

    double area() { return PI * radius * radius; }
};

Circle c(5.0);
cout << c.area();   // 78.54

#include <iostream>
using namespace std;

class Product {
  public:
    string name;
    double price;
    int    quantity;

    Product(string n, double p, int q) : name(n), price(p), quantity(q) { }

    double totalValue() { return price * quantity; }
};

int main() {
    Product p("Widget", 9.99, 50);
    cout << p.name << " total: $" << p.totalValue() << endl;
    return 0;
}

Summary

• Constructors initialize objects when they are created.
• The default constructor takes no arguments.
• Parameterized constructors accept arguments to set initial values.
• Member initializer lists are more efficient and required for const members.