oops · high

The Four Pillars of OOP

The four pillars of object-oriented programming in C++. Encapsulation bundles data with the methods that guard it and hides internal state (private fields + public methods preserve invariants). Abstraction exposes WHAT a type does, not HOW, through interfaces like a pure-virtual Shape::area(). Inheritance models an IS-A relationship and reuses a base's contract (Circle : Shape), specializing via overrides. Polymorphism lets one interface take many runtime forms — a single call site dispatches through the vptr/vtable to the real type, so new types slot in without changing existing code. Together they make code modular, extensible and safe.

The code

struct Account {              // 1) ENCAPSULATION: state behind methods
  long cents_ = 0;            //    private data — invariant kept inside
public: void deposit(long c){ cents_ += c; }
};
struct Shape {                // 2) ABSTRACTION: expose WHAT, not HOW
  virtual double area() const = 0;          // pure interface
};
struct Circle : Shape {       // 3) INHERITANCE: IS-A + reuse
  double r;  double area() const override { return 3.14159*r*r; }
};
void print(const Shape& s) {  // 4) POLYMORPHISM: one call, many forms
  std::cout << s.area();      //    virtual dispatch picks the real type
}

What this lesson walks through

1. 01OOP rests on four pillars
2. 02Encapsulation — hide state behind an interface
3. 03Abstraction — expose WHAT, not HOW
4. 04Inheritance — IS-A and code reuse
5. 05Polymorphism — one interface, many runtime forms

Object-oriented programming in C++ is built on four ideas. Encapsulation bundles data with the methods that guard it and hides the state. Abstraction exposes WHAT a type does, not HOW. Inheritance models an IS-A relationship and reuses code. Polymorphism lets one interface take many runtime forms. Everything else in OOP is built from these four.