Should i use dynamic_cast here? - c++

Question

Well, i have next code:
#include <type_traits>
#include <iostream>
#include <string>
#include <list>
#include <functional>
class base_main
{
public:
virtual ~base_main()
{
}
// some methods
};
class base_1 : virtual public base_main
{
// some methods
};
class base_2 : virtual public base_main
{
// some methods
};
class base_3 : virtual public base_main
{
// some methods
};
class object : public base_1, public base_2, public base_3
{
// some methods
};
// in other *hpp file
class object_controller_listener
{
public:
virtual void object_created( base_main* o )
{
// well, i want to work only with base_1 and base_2 interfaces, but not with base_3, and also i don't want to know something about object class in this *hpp
// is it good code design?
auto* xxx = dynamic_cast<base_1*>( o );
}
};
class objects_controller
{
void create()
{
std::unique_ptr<object> obj;
// ...
for( auto listener : m_listeners )
listener->object_created( obj.get() );
}
std::list<object_controller_listener*> m_listeners;
};
int main()
{
}
The question is - how can i work only with base_1 and base_2 interfaces? Should i create two separate listeners for them, and send two events in create() function, or should i use dynamic_cast for downcasting and send only one event in create() function? Is this good code design or is this feels like code smell?
UPD:
For example: base_1 - is render_base class, which contains render data, and have functions for set and get this data base_2 - collider base class which contains collider data, and have functions for set and get this data base_3 is physic base class and object is inheritance of all this classes. And when i want work only with render class i use create event which send only render_base class to the render_system, which works only with renders objects and truly use polymorphism. But if i want in some other place work with collider and physic objects, without knowledge about render - how can i use polymorphism here in base classes?

Answer 1

It is hard to tell what design you should choose as this heavily depends on the overall structure of the application.
Generally, I would avoid having a function with the signature virtual void object_created( base_main* o ) in which you dynamically cast to base_* and work on that directly in this function. Because the function signature is part of the documentation of the API.
So I would create distinct functions for base_1 and base_2 and call those.
How to do that depends again on the overall structure. You could create a helper function, that forwards the call to the other functions (this is just a fast implementation how that could look like:
template <typename DestT, typename SrcT, typename T>
void forward_if(SrcT obj, T *o, void (T::*f)(DestT)) noexcept {
if (auto tmp = dynamic_cast<DestT>(obj); tmp != nullptr) {
(o->*f)(tmp);
}
}
And then you could use it like this:
#include <iostream>
#include <vector>
class base_main {
public:
virtual ~base_main() {}
};
class base_1 : virtual public base_main {};
class base_2 : virtual public base_main {};
class base_3 : virtual public base_main {};
class object : public base_1, public base_2, public base_3 {};
template <typename DestT, typename SrcT, typename T>
void forward_if(SrcT obj, T *o, void (T::*f)(DestT)) noexcept {
if (auto tmp = dynamic_cast<DestT>(obj); tmp != nullptr) {
(o->*f)(tmp);
}
}
struct listener_base {
virtual void object_created(base_main *o) = 0;
};
struct specific_listener : public listener_base {
void object_created(base_main *o) override {
forward_if<base_1 *>(o, this, &specific_listener::object_created);
forward_if<base_2 *>(o, this, &specific_listener::object_created);
}
void object_created(base_1 *o) {
std::cout << "object created base_1" << std::endl;
}
void object_created(base_2 *o) {
std::cout << "object created base_2" << std::endl;
}
};
int main() {
std::vector<listener_base *> listeners;
listeners.push_back(new specific_listener());
object o;
for (auto listener : listeners) {
listener->object_created(&o);
}
return 0;
}