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From: Andreas Huber (ahd6974-spamgroupstrap_at_[hidden])
Date: 2005-08-02 12:26:40

David Abrahams wrote:
> Why are you requiring that the user pass mpl::list<> as opposed to
> some other mpl sequence?

I need to modify that sequence internally. If any sequence was allowed
(including immutable ones), I would need to copy that sequence into an
mpl::list<> first. I suspect that the copying slows down compilation
noticeably (I never had the chance to make measurements, so this may
well be FUD), so I figured that requiring a list is the best way to give
the user the opportunity to keep compilation as fast as possible. Sure,
I could make a copy only if the user doesn't pass an mpl::list<> and
document that in the interface but nobody has ever made a request in
this direction. I guess this is due to the fact that pretty much
everybody specifies the InnerInitial states inline:

struct Y;
struct Z;
struct X : sc::simple_state< X, Machine, mpl::list< Y, Z > > {};

That is, I can't really think of a real-world example where you would
want to use an algorithm to fill that list...

Would you allow any mpl sequence anyway?


Andreas Huber
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