c++ double dispatch with mirrored hierarchies

the following class hierarchies represent abstract resource handler and resource hierarchies. Both have the interfaces as base classes. Now imagine you write a system where you can implement multiple specific resource systems under these interfaces. Here is just one example. The specific main class creates the resources derived from stuff. Now开发者_运维问答 when the created resource is handed to the base interface it is handed as a pointer to the base resource class but i want to handle the specific resource and have access to its specific attributes.

I know about double dispatch, but i don't think it works in this case. I would like to prevent RTTI and dynamic_casts. Do you have suggestions as to handle such cases?

class resource;

class main_resource_handler
{
public:
   virtual resource* create_resource() = 0;
   virtual void do_some(resource* st) = 0;
};

class resource
{
};

class specific_resource : public resource
{
public:
    int i;
};

class specific_resource_handler : public main_resource_handler
{
public:
    stuff* create_resource) {
        return new specific_resource);
    }
    void do_some(resource* st) {
        // in here i want to work with specific resource
    }
    void do_some(specific_resource* st) {
        // i want to get here
    }
}

main_resource_handler* handler = new specific_resource_handler();
resource* res = handler->create_resource();
handler->do_some(res); /// here

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I think you're not asking the right question.

To do what you're asking, all you need is to add this:

template<typename T>
class helper : public main_resource_handler
{
public:
   virtual resource* create_resource() { return new T; }
   virtual void do_some(resource* st) { do_some_specific(static_cast<T*>(st)); }
private:
   virtual void do_some_specific(T* st) = 0;
};

And change this:

class specific_resource_handler : public helper<specific_resource>
{
private:
   virtual void do_some_specific(T* st) { ... }
}

The static_cast is only safe if you can guarantee that you will always call do_some on the right type of handler. But if you already know it's the right kind of handler, then there's no need to make a base-class method call. So presumably what you want is to get some sort of resource, not knowing its exact type, and pass it to the appropriate handler. That's trickier...

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I wonder if the Curiously recurring template pattern is what you are looking for. There is already a thread on stackoverflow on this topic.

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I'm not sure why you need both the resource and the handler - it seems you're exposing an extra coupling to something with would be encapsulated. The problem wouldn't exist if create resource just returned a resource that the client could call methods on directly.

If you want safety, have the resource remember the address of the handler which created it, then check that in do_some(resource* st). If the resource was created by the current handler, and a handler can only create resources of a given kind, then it's safe to cast it and call the specific function. Though as above, if the function was just a virtual function on the resource it would be by definition type safe.

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I think you may be looking for this:

http://www.artima.com/cppsource/cooperative_visitor.html