What is the primary purpose of a constructor in a class?

The primary purpose of a constructor in a class is to initialize new objects of that class. When an instance of a class is created, the constructor is called automatically, which allows for the setup of initial values for the object's properties or to perform any necessary setup procedures needed immediately upon the creation of an object. This is a fundamental aspect of object-oriented programming, as it establishes the state of an object right from the start.

For example, if you have a class representing a 'Car', the constructor can set default values for attributes like color, model, and year. Without this initialization step, objects created from the class might hold uninitialized or undefined values, leading to unpredictable behaviors and potential errors in the program.

The other options describe different functionalities but do not pertain to the primary purpose of a constructor. Defining the structure of the class relates to the overall class design and its members. Cleaning up resources is the role of a destructor, which is called when an object is no longer needed. Executing code when a program starts is typically managed by the main function or similar entry points, rather than directly by constructors.