Question

Let’s say I have the following class hierarchy in C++:

class Base;
class Derived1 : public Base;
class Derived2 : public Base;


class ParamType;
class DerivedParamType1 : public ParamType;
class DerivedParamType2 : public ParamType;

And I want a polymorphic function, func(ParamType), defined in Base to take a parameter of type DerivedParamType1 for Derived1 and a parameter of type DerivedParamType2 for Derived2.

How would this be done without pointers, if possible?

Answer 1

You cannot have Base::func take different parameters depending on what class inherits it. You will need to change something.

You could make them both take a ParamType and handle an unexpected parameter with whatever mechanism you like (e.g. throw an exception or return an error code instead of void):

struct ParamType;
struct Base {
  void func(ParamType&);
}
struct Derived1 : Base {};
//...

Or template on the type of parameter they should take:

struct ParamType;
struct DerivedParamType1 : ParamType {};
struct DerivedParamType2 : ParamType {};

template<class ParamT>
struct Base {
  void func(ParamT&);
};
struct Derived1 : Base<DerivedParamType1> {};
struct Derived2 : Base<DerivedParamType2> {};

With the second solution, Derived1 and Derived2 won’t share a common base and cannot be used polymorphically.