Constructors: Initializing Objects

A constructor is a special method that runs automatically when an object is created, used to set up the object's initial state. It typically has the same name as the class (or a reserved name like __construct in PHP or __init__ in Python) and no return type.

class Rectangle {
    width
    height

    function __construct(width, height) {
        this.width = width
        this.height = height
    }

    function area() {
        return this.width * this.height
    }
}

rect = new Rectangle(4, 5) // constructor runs immediately, setting width=4, height=5
print(rect.area()) // 20

Constructors ensure an object can never exist in an incomplete or invalid state — by requiring certain parameters up front, you make it impossible to accidentally create a Rectangle with no dimensions at all.

Constructor chaining in inheritance: a subclass constructor typically needs to call its parent's constructor to ensure inherited properties are also properly initialized:

class Shape {
    function __construct(color) { this.color = color }
}

class Rectangle extends Shape {
    function __construct(color, width, height) {
        super(color) // calls Shape's constructor first
        this.width = width
        this.height = height
    }
}

Default constructors: most languages provide an implicit, empty constructor if you don't define one yourself. Multiple constructors (constructor overloading) are supported in some languages (Java, C++, C#) to allow creating objects in different ways; others (PHP, Python) achieve similar flexibility with default parameter values or static factory methods instead.