Why OOP Exists

Procedural languages (C) tie data and the functions that act on it loosely together — global data can be changed by any function, anywhere, making large programs hard to maintain and reason about. OOP fixes this by binding data and behavior into a single unit (the class), controlling exactly who can touch what, and modeling programs as a set of interacting objects rather than a sequence of instructions.

Classification / Structure of OOP Concepts (Big Picture)

Use this as the mental map before drilling into definitions — CSS questions often ask you to "classify" or "categorize" OOP concepts, not just define them.

• A. Basic Building Blocks
  • Object, Class, Attribute (data member), Method (member function), Message Passing
• B. The Four Pillars (Core Principles)
  • Abstraction, Encapsulation, Inheritance, Polymorphism
• C. Object Relationships (how classes relate to each other)
  • Association (general "uses-a" link) → Aggregation ("has-a," weak, parts can exist independently) → Composition ("has-a," strong, parts die with the whole) → Inheritance ("is-a")
• D. Binding Types (when a call is resolved)
  • Static/Early Binding — resolved at compile time (e.g., function/operator overloading)
  • Dynamic/Late Binding — resolved at run time (e.g., virtual functions, overriding)
• E. Design-Quality Metrics (how "good" an OOP design is)
  • Coupling — degree of interdependency between classes (aim: low coupling)
  • Cohesion — how closely related/focused a single class's responsibilities are (aim: high cohesion)

Glossary of Key OOP Terms (Definitions)

• Object — a runtime instance of a class; a self-contained unit combining state (data) and behavior (methods). Example: myCar is an object of class Car.
• Class — a blueprint/template that defines the attributes and methods an object of that type will have; no memory is allocated until an object is created from it.
• Attribute / Data Member — a variable that holds an object's state (e.g., balance in a BankAccount class).
• Method / Member Function — a function defined inside a class that operates on its data members (e.g., deposit()).
• Instance — another word for "object"; "instantiation" is the act of creating an object from a class.
• Message Passing — how objects communicate: one object invokes another object's method, optionally passing data.
• Constructor — a special member function automatically invoked when an object is created; typically initializes data members. Has the same name as the class, no return type.
• Destructor — a special member function automatically invoked when an object is destroyed; used to release resources. Prefixed with ~ in C++.
• Abstraction — exposing only essential features of an object while hiding implementation detail ("what it does," not "how it does it").
• Encapsulation — bundling data and methods into a single unit (class), typically restricting direct access to internal state via access modifiers.