Abstract classes

Learn abstract classes and pure virtual methods.

Abstract classes

An abstract class contains at least one pure virtual function, declared with = 0: virtual void draw() = 0;.

You cannot create an instance of an abstract class: Drawable d; gives a compile error. But a derived class that implements the pure virtual can be instantiated. class Circle : public Drawable { void draw() override { } }; is valid. Abstract classes define interfaces and force derived classes to implement specific methods.

Implementing pure virtual

A derived class that does not implement a pure virtual remains abstract. One that implements it becomes concrete and can be instantiated.

For example, if Drawable has virtual void draw() = 0;, both Circle and Square must implement it. Because Circle does, it is concrete; new Circle() is valid. This design guarantees at compile time which class does what.

Exercise Drawable abstract class and Circle implementation

Define a Drawable abstract class with virtual void draw() = 0;. A Circle class inherits from Drawable; its void draw() override body is std::cout << "Circle";. In main write Drawable* d = new Circle();, call d->draw(); and delete d;.

Show example solution

#include <iostream>

class Drawable {
public:
    virtual void draw() = 0;
};

class Circle : public Drawable {
public:
    void draw() override { std::cout << "Circle"; }
};

int main() {
    Drawable* d = new Circle();
    d->draw();
    delete d;
    return 0;
}

Test yourself

Complete the pure virtual declaration

virtual void draw() = ____;

Show answer

virtual void draw() = 0;

Match each piece to its meaning

= 0

Pure virtual
Drawable

Abstract class
override

Implementation
new Circle()

Concrete instance
How is a pure virtual function declared?
```
???
```
- static void draw();
- virtual void draw() { }
- void draw() virtual;
- virtual void draw() = 0;
Show answer

virtual void draw() = 0; — = 0 makes the function pure virtual and the class abstract.

Next lesson: dynamic_cast

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