oops · high

Abstract Classes, Pure Virtual & Interfaces

Pure virtual functions, abstract classes you can't instantiate, and the C++ idiom for an interface (all pure virtual methods plus a virtual destructor) — what makes a class abstract, what a derived class must do to become concrete, and how abstract classes differ from pure interfaces.

The code

struct Drawable {                  // an interface
    virtual void draw() const = 0; // pure virtual -> abstract
    virtual ~Drawable() = default; // always: virtual dtor
};

struct Button : Drawable {
    void draw() const override { /* render */ } // must override
};

// Drawable d;             // ERROR: cannot instantiate abstract
Drawable* p = new Button; // OK: use via base pointer / reference
p->draw();                // dynamic dispatch -> Button::draw

What this lesson walks through

1. 01A pure virtual makes the class ABSTRACT
2. 02A concrete class overrides every pure virtual
3. 03Interface = all pure virtual + virtual dtor; use via base

Writing = 0 after a virtual function makes it pure virtual and its class abstract. You cannot create a Drawable object — it only defines an interface for derived classes to fulfil.