Inheritance in C++

class Shape {
public:
    virtual double area() const = 0;   // pure virtual
    virtual ~Shape() = default;        // always virtual in a base class
};

class Circle : public Shape {
public:
    explicit Circle(double r) : r_(r) {}
    double area() const override { return 3.14159 * r_ * r_; }
private:
    double r_;
};

public inheritance means "is-a". Multiple inheritance is allowed but can create the diamond problem, solved with virtual inheritance.