class Base
{
public:
    Base(int val, int pval): val{val}, pval{pval} {}
    virtual ~Base() = default;
    virtual void fun() const { }
    void foo() const
    {
        do_foo();
    }
    void bar() const {}
protected:
    int val;
private:
    virtual void do_foo() const = 0;
    int pval;
};

class Derived1: public Base
{
public:
    void fun() const override {}
    Derived1(int val, int pval, double d)
        : Base{val, pval}, d{d} {}

private:
    double d;
};

class Derived11 final: public Derived1
{
    using Derived1::Derived1;
    void do_foo() const override {}
};
class Derived2: public Base
{
    using Base::Base;
    void do_foo() const final {}
public:
    void bar() const {}
};
struct Derived21: Derived2
{
    using Derived2::Derived2;
    void fun() const override {}
};