I'd like to restart the discussion on how conversions will work in
Boost.Python v3. Here's the starting point:
I'd like to see support for static, template-based conversions. These
would be defined by [partial-]specializing a traits class, and I tend to
think they should only be invoked after attempting all registry-based
conversions. Users would have to include the same headers in groups of
interdependent modules to avoid specializing the same traits class
multiple times in different ways; I can't think of a way to protect them
from this, but template-based specializations are a sufficiently
advanced featured that I'm comfortable leaving it up to users to avoid
this problem.
We've had some discussion of allowing different modules to have
different registries (in addition to a global registry shared across
modules). Leaving aside implementation questions, I have a little
survey for interested parties:
1) Under what circumstances would you want a conversion that is
completely limited to a specific module (or a group of modules that
explicitly declare it)?
2) Under what circumstances would you want a conversion to look in a
module-specific registry, and then fall back to the global registry?
3) Considering that we will have a "best-match" overloading system, what
should take precedence, an inexact match in a module-specific registry,
or an exact match in a global registry? (Clearly this is a moot point
for to-Python conversion).
Finally, can anyone give me a reason why having a global registry can
lead to a violation of the "One Definition Rule"? This was alluded to
many times in the earlier discussion, and there's no doubt that a global
registry may lead to unexpected (from a given module's perspective)
behavior - but I do not understand the implication that the global
registry can result in formally undefined behavior by violating the ODR.
Thanks!
Jim
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