Re: Collections of derived objects

Noah Roberts <>
Fri, 19 Mar 2010 10:13:23 -0700
In article <4ba31cbc$0$2858$>, says...


Let's say I have a base class Event and some sub classes EventType1,
EventType2, ...

I would like to have a collection (vector or list for example) of a mix
of EventType1, EventType2, ...

I think I have to create a vector<Event*> and starts playing with "new".
And of course, I will forget the "delete" !

Are there any other options, to keep the code clean ?

You can make many higherarchies that you have control over value types
with some work. It involves the use of the handle/body idiom:

// event.h
struct event
  // NVI
  typedef ? event_type_id;
  event_type_id event_type() const { return pimpl->event_type(); }

  event(event_type_id id);

  struct impl
    virtual event_type_id event_type() const = 0;
    // ...

  std::auto_ptr<impl> pimpl;

// event.cpp

struct x_event : event::impl
  event_type_id event_type() const { return ??; }

event::impl * impl_factory(event_type_id id)
  event::impl * = new ?? discovery;

event::event(event_type_id id) : pimpl(impl_factory(id)) {}


Issue with this design: subclasses MUST all have the exact same requests
to implement. For example, mouse_event could not have extra or
different functionality from keyboard_event. It could only implement
that functionality with differing behavior. You're probably better off
with the pointer solution but I figured more knowledge can't hurt.

I would like to transform (serialize in string or something else) these
events (contained into a collection). As I will have different
transformations, I do not want to put the serialization code inside the
events, to not pollute them. How can I do that, in a clean way ? Visitor
pattern or something like that?

Visitor is debatably clean. I have found it quite useful. You could
also get "Modern C++ Design" and read the chapter on multimethods.

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