Re: [boost] Proposal: Polymorphic Value Objects

From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2007-09-26 14:16:56

Igor.Smirnov_at_[hidden] write:

> But should the destructor of D and T be declared as "virtual" in order to
> make this work? If yes, this going to be "intrusive" detail anyway!

Indeed, but this is part of the language.

> Regarding the second one, is this correct to write:
> 
> class T{...}
> class D: public class T {...}
> 
> 
> D my_derived_object;
> T& my_reference = my_derived_object; 
> template<typename T> Container( my_reference); 

[snip]

That's not valid C++.
Yet I still see what you mean, and I explained that problem several 
times months ago in my original message and in this thread.

 > I
> do not understand, why you propose value object but not smart pointer?

Since we deep-copy, we've got value semantics. Better make this a value 
than a pointer, then.
Plus, it is well known that pointers are sources of unsafety. By 
completely hiding pointers from the interfaces, safety is gained.

> How
> do you plan to access these objects and the functions of them? Through
> "."?

Unfortunately, operator. is not overloadable. So you'd have to use 
operator->.
You could have seen that in the implementation and examples I provided.

> How do you provide that the correct instance of virtual function is
> called, that is from type D?

The member functions must be virtual for them to be dynamically 
dispatched to the appropriate ones.
It simply forward operator-> to the pointer to the base.

> Why do you need this, at all?

There is a motivation part at the beginning of my first message.

> Indeed, the
> notation "->" is used to indicate possible call of virtual function,
> whereas "." is always understood as fixed call (if I understand correctly,
> correct me if not).

operator. has nothing to do with "fixed calls".
It is not the case, for example, when using references.

> So when you deal with polimorphic objects, it is
> better to do through (smart) pointers. Why do you want to mix this?

Polymorphic value objects are a type of smart pointers, except that they 
have a value interface, not a pointer one.

> I also can not undesrand the code from your first mail. 

It's perfectly standard C++, and very simplistic. It's a very thin layer 
and easy to maintain.
Have you ever looked at the code of a boost library? It's usually much 
more complicated.