Re: [boost] Library Proposal: Reflection for C++

From: Edward Diener (eddielee_at_[hidden])
Date: 2006-02-03 21:12:36

David Abrahams wrote:
> Edward Diener <eddielee_at_[hidden]> writes:
> 
>> I think it is a commendable goal in Reflex to be able to reflect the 
>> parameter types for any given template instantiation, since this 
>> information may be valuable and usable at run-time. After all, at 
>> run-time, information about a template type should be just as accesible 
>> regarding finding type information than any other type. One of the 
>> pieces of information about a template type is the parameter types which 
>> are used to instantiate the template, so run-time reflection should 
>> include such information. But I feel that information about the template 
>> declaration/definition itself belongs in the realm of a compile-time 
>> reflection system, not a run-time reflection system, and can serve no 
>> purpose in a run-time reflection system.
> 
> Even if the run-time reflection system is generating code?
> The boundaries are a lot more fluid than you probably imagine.

I do not doubt that a compile-time reflection mechanism would be 
invaluable to template programming at compile time, but I do not see how 
such a reflection mechanism can work there unless it were integrated 
into the language/compiler itself since the benefits of a compile-time 
reflection mechanism would be to manipulate template code at compile 
time, similar to the way MPL does but with more built-in langauge 
functionality. I do not say that such an integration is not a worthy 
project, and to be considered as an addition to the C++ language, but I 
would rather separate compile-time reflection and run-time reflection 
since the latter can work without changing the language/compiler, as 
Reflex shows, while the former appears to me to need such a change. So I 
think it would be worthy to approach the easier task first, under the 
aegis of run-time functionality rather than the harder, and not mix the 
two in an initial attempt to provide C++ reflection.

At the same time, regarding run-time reflection, and given the fact that 
Reflex can reflect directly from a type or an object, I would still love 
to see, if C++ adapts such an approach to run-time reflection, an 
equivalent to the built-in "typeid" operator, eg. maybe 
"reflection_typeid" or maybe "extended_typeid" or maybe just "typeid" 
generating a full reflection type ala Reflex, as part of the C++ 
language. But barring that, using the specific classes of Reflex at 
run-time works also. I just think the built-in operator way of doing it 
would be more elegant and more in keeping with the "typeid" idea, and 
the feeling that run-time reflection is part of the language.