class Foo {

public:

    Foo(double nw): x(nw) { };

    virtual ~Foo() = default;

    inline operator double() noexcept { return x; }

private:
    double x = double();

};

class Bar : public Foo {

    using Foo::Foo;

public:
    ~Bar() = default;

    inline operator double() noexcept { return Foo::operator double(); }
};


int _tmain(int argc, _TCHAR* argv[])
{
    Bar d(4.2);
    std::cout << d;
    _getch();

}