What is a constructor in object-oriented programming?

A constructor in object-oriented programming is a special type of function that is automatically called when an object of a class is created. Its primary role is to initialize the newly created object. This involves setting initial values for the object's attributes or performing any setup steps required before the object can be used. Constructors often have the same name as the class and may have parameters that allow passing initial values for the object's properties.

When an object is instantiated, the constructor runs, and whatever code is written within it executes. This establishes a consistent state for the object right from the moment it is created. By effectively managing initializations and ensuring that all necessary conditions are met before usage, constructors play a crucial role in object-oriented programming.

The other options describe aspects of programming but do not accurately capture the essence and function of a constructor. For example, a method that is called when a program starts could refer to the main method or starting point of execution in various programming languages, while an instance of a class refers to an object created from that class. Additionally, a variable that holds the state of an object refers to the object's attributes or state, which is separate from the initialization process that constructors handle.