C++ covariance in parameters

问题

I wanted to know why C++ does not support co-variance in parameters like in example below or if there is a way to achieve it?

class base {
public:
virtual base* func(base * ptr) { return new base(); }
};

class derived : public base {
 public:
 virtual derived* func(derived * ptr) override { return new derived(); } //not allowed
};

回答1:

The return type is permissible since derived inherits from base, but the function parameter can't work - not all base instances will be a derived also. What's supposed to happen in the cases where func is called on a pointer to base with a parameter that's not a derived? The most derived implementation isn't callable.

来源：https://stackoverflow.com/questions/11821158/c-covariance-in-parameters