Re: [boost] Boost.Guid - questions

From: Andy (atompkins_at_[hidden])
Date: 2007-05-07 12:58:51

Janek Kozicki <janek_listy_at_[hidden]> wrote in
news:20070506182937.0da237c8_at_absurd: 

> Dear Andy,
> 
> I remember that serialization library needs some sort of unique
> identifier. Currently it uses a class name. But Robert said several
> times that he will gladly allow using some other unique identifier.
> Is your library prepared for that?

I believe that my library can be used for this.  I would gladly look
into any issues.  I do not know a lot about the Boost Serialization
library but I do see the BOOST_CLASS_EXPORT_GUID macro. 

One could write:

class my_class {
public:
  static const my_class_guid("1234....");
};
BOOST_CLASS_EXPORT_GUID(my_class, my_class_guid.to_string());

> 
> When the identifier is assigned?
> - when the program starts?

I believe this is the case.

> - when the code is compiled?

My guid library can not generate guids at compile time.

> - when an attempt to read identifier for a given class is made?
> 
> Is the identifier the same across several program runs? (serialization
> library needs that to properly distinguish classes ;)

Yes, if one types in the guid as a string.

> 
> 
> Is it possible to get an identifier of a class:
> 
> - during compilation (so it could be used in template metaprogramming,
>   typedeffed enums or in #defines)

No. 

> 
> - during runtime when only a polymorphic(!) class instance is provided
>   (eg. I got a class Shape and classes Square and Circle deriving from
>   it. 
>    Then I want to get a unique identifier to distinguish whether a 
>    pointer Shape* is a Square* or a Circle*)

yes.

> 
> - during runtime when only a type is provided? Not an instance of a
> class. 

yes.

> 
> Is it possible to tell your library to assign numbers not random, but
> incrementing sequentially, starting from some given number? Maybe
> that's unfeasible, why?

yes.  Just use one of the constructors.

> 
> Is the unique identifier an 'int' number, or is it custom? Maybe a
> string? 

It is a custom type, a 16 byte number that has a string representation.

> 
> If the deadline can be extended till 11 or 12 May I should be able to
> write a review. Currently I completely have no time even to look at
> the documentation. However I'm curious to see your answers to my
> questions above.
> 
> I'm particularly interested in using your library in multimethods
> (see. Andrei Alexandrescu "Modern C++ Design" to check what's that ;)
> for constant time dispatch (classes will be indexed by their
> identifier). 

Multimethods do something based on the dynamic type of two or more
parameters.  I see no reason why this could not work for multimethods. 
One way would be to have a virtual base class method return the guid of
the class and each derived class would return the correct guid.  The
multimethod could determine what to do based on this information. 

> 
> best regards

Andy.