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Re: Intrinsics for N2965: Type traits and base classes


Don't worry, I'm not suggesting including boost::mpl at all, just leaving the return type of the bases trait unspecified. IMO, your example illustrates my point that without performance tuning, compiling metaprograms can be prohibitively expensive, so I want to avoid running the tuple metaprogram that creates the fields when we never need to instantiate the type. Benchmarks soon.

Mike

On 9/28/2011 2:53 AM, Jonathan Wakely wrote:
On 28 September 2011 04:22, Michael Spertus wrote:
Benjamin,
I think tuple is wrong both for performance reasons (I believe these are likely to be serious enough to depress use due to inordinately long compiles) and because it prematurely locks us into a rigid choice of how our typelists are implemented.

My inclination is to make it type-independent by returning an unspecified type that can have a sequence of types extracted from it (this is the approach taken by boost::mpl and has loads of experience that shows it is a good approach to metaprogramming). In other words, first<bases<A>>::type would be the first base of A, etc.
Citing Boost MPL as a good way to avoid inordinately long compiles ...
interesting!  Have you ever tried to reduce a GCC bug report from 20k
lines to 20, because most Boost libs include every MPL header?!

I hope we can get a simple typelist _without_ needing everything else
in MPL, such as the apply and lambda metafunctions (and maybe a lot of
that could be massively simplified using variadic templates anyway.)

.



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