initializing a class using upcasted object

Hi

Here is the code:

class Base {
public:
    Base() {...}
    virtual ~Base() {}
    virtual void foo() const =0;
};

class Der1 : public Base {
public:
    Der():Base() {...}
    ~Der() {...}
    void foo() const {...}
};

There are several Der.. classes, and the problem is that I want to make
some X class, that will have a handle to the Base, in order to use
polymorphism.
Initialization of X objects will determine which actual object in the
Base's hierarchy will be used (Der1 or Der2 or ..), but I have no idea
how to do that in "nice" way.

If I use references as the handle, I end with something like this:

class X {
    const Base& ref;
public:
    X(const Base& ref):ref(ref) {...}
    ~X() {...}
    complicated_function_that_uses_foo();
};

But it must be used like that:

Der temp();
X x(temp);
x.complicated_function_that_uses_foo();

And it can be easily misused:

X x(Der());
x.complicated_function_that_uses_foo(); //calling pure virtual method

I thought about pointers, but pointers are evil.. erm, I mean that I
haven't came up with anything "nice" using pointers.

Any ideas on how it _should_ be done?

Thanks in advance.

--
mati