Constructors

Constructors are a language feature.

In class-based object-oriented programming, a constructor (abbreviation: ctor) is a special type of subroutine called to create an object. It prepares the new object for use, often accepting arguments that the constructor uses to set required member variables.

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Languages with Constructors include Java, JavaScript, Python, C++, PHP, C#, MATLAB, Objective-C, Visual Basic .NET, Object Pascal, CFML, Action!

Example from JavaScript:

class Person {
 constructor(name) {
   this._name = name
 }
}
new Person("Jane")

Example from C++:

class Foobar {
 public:
  Foobar(double r = 1.0,
         double alpha = 0.0)  // Constructor, parameters with default values.
      : x_(r * cos(alpha))    // <- Initializer list
  {
    y_ = r * sin(alpha);  // <- Normal assignment
  }

 private:
  double x_;
  double y_;
};
Foobar a,
       b(3),
       c(5, M_PI/4);

Example from C#:

public class MyClass
{
    private int a;
    private string b;

    // Constructor
    public MyClass() : this(42, "string")
    {
    }

    // Overloading a constructor
    public MyClass(int a, string b)
    {
        this.a = a;
        this.b = b;
    }
}
// Code somewhere
// Instantiating an object with the constructor above
MyClass c = new MyClass(42, "string");

Example from CFML:

component {
   // properties
   property name="cheeseName";
 
   // constructor
   function Cheese init( required string cheeseName ) {
      variables.cheeseName = arguments.cheeseName;
      return this;
   }
}

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