http://www.lwithers.me.uk/articles/covariant.html
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    Derived* d = new Derived;
    Base* b = d->clone();
    Derived* d2 = dynamic_cast<Derived*>(b);
    if(!d2) {
        delete b;
        throw SomeCrazyException();
    }

As you can see, this is pretty long-winded given that we know d2 should be a Derived instance.

If, instead, the clone() method could tell the compiler "actually, I always return a Derived*" (i.e. it became virtual Derived* Derived::clone() we could do away with the upcast and the check, moving all this away from run-time and into compile-time:

    Derived* d = new Derived;
    Derived* d2 = d->clone();

This is the ideal solution; there is no run-time cost associated with it, and it's completely safe: the compiler will check all our types for us. If somebody changes Derived::clone() to return a Base*, the compiler will complain;

続きがある

Fortunately, a language feature does exist to do this: covariant return types.