Question

i have two independent class hierarchies, starting at BaseClass1, BaseClass2

this is what I want to do:

struct BaseClass1 {
  virtual void obtain_map(std::map<int,BaseClass2> &map) = 0;
}

Subclasses of BaseClass1 override the obtain_map. But the problem is that those sublcasses should be able to use subclasses of BaseClass2 in the map parameter. (So in this sense, subclasses of the two “independent” hierarchies are actually related, or rather can be if the subclass designers want)

How can I accomplish this, or am I forced to create my own map class from scratch?

Answer 1

If you derive your classes like so and use pointer as the second map template parameter (a smart pointer like std::unique_ptr will save you some memory management trouble):

struct BaseClass1 {
    virtual void obtain_map(std::map<int, std::unique_ptr<BaseClass2>> &map) = 0;
};
struct DerivedClass1 : public BaseClass1 {
    virtual void obtain_map(std::map<int, std::unique_ptr<BaseClass2>> &map) override
    {
    }
};

You can still pass in maps with derived versions of BaseClass2 as the second type:

struct DerivedClass2 : public BaseClass2;
// ...

std::map<int, std::unique<DerivedClass2>> mdc2;
DerivedClass1 dc1;
dc1.obtain_map(mdc2);