David Abrahams wrote:
Edward Diener <eddielee@tropicsoft.com> 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.