Object-oriented programming concepts

Object-oriented programming organises a programme around objects rather than only procedures. A class is a blueprint that defines data members and member functions. An object is a runtime instance of a class, with its own state and access to the behaviour defined by the class. In an exam question, the key distinction is that a class occupies no full object storage until an object is created, while each object has separate instance data unless a member is declared static. For example, a `Student` class may define `rollNo`, `name` and `calculateGrade()`, while each student object stores a different roll number and name.

Objects have identity, state and behaviour. Identity distinguishes two objects even if their values are equal. State is represented by attributes or fields. Behaviour is represented by methods or member functions. This model appears in C++ through class instances, in Java through reference variables pointing to objects, and in Python through instances whose attributes are usually stored in a dictionary-like structure. Static members belong to the class, not to a particular object, and are common MCQ targets.

Exam takeaway: a class defines structure and behaviour; an object is the live instance with identity and state.