Question

Given an object initialized like so:

Base* a = new Derived();
Container<Base> c(a);

where

class Base {
  ...
  protected:
    ~Base();
}

class Derived : public Base {...};

template <typename T>
class Container {
  private:
    T* object;

  public:
    Container(T* o) : object(o) {}
    void deleteObject() {
      delete object;  // Object must be casted to (unknown) derived type to call destructor.
    }
};

Obviously this is very simplified from the actual code, but the question is how do I cast object from its templated type to its actual, derived type (if they are different), which is not known?

I cannot modify Base or Derived, or even any of the code calling Container, only the Container class itself.

Answer 1

If you’re able to change the creation code, you might get away with this:

template<class T>
void deleter(void* p){
  delete static_cast<T*>(p);
}

template<class T>
class Container{
private:
  T* obj;
  typedef void (*deleter_func)(void*);
  deleter_func obj_deleter;

public:
  Container(T* o, deleter_func df)
    : obj(o), obj_deleter(df) {}
  void deleteObject(){ obj_deleter(obj); }
};

And in the calling code:

Base* a = new Derived();
Container<Base> c(a, &deleter<Derived>);