Re: [boost] [MPL][vector] Is there interest in mpl::vector using variadic templates?

On 03/01/2017 05:07 PM, Larry Evans via Boost wrote:

On 03/01/2017 12:56 PM, Antony Polukhin via Boost wrote:

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2017-02-21 13:52 GMT+03:00 Михаил Максимов via Boost :

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Dear community,

I've recently started contributing to boost::variant. To speed up variant's compilation I'm implementing mpl::vector on variadic templates. For now status of new vector implementation is: <...> made me wonder, is there interest in variadic templates implementation? Are there limitations for it's usefulness, which I did not see?

I'm very interested in patching MPL to be able to use variadic templates.

There was a variadic template mpl try from 7 years ago:

https://github.com/cppljevans/variadic_templates/tree/master/boost/mpl

Maybe it could give you some ideas. OTOH, maybe it's *too* old :(

One problem with the above cppljevans mpl is there's no at.hpp. Instead, the non-variadic boost/mpl/at.hpp was used. The reason no variadic at was created was because, AFAICT, there was no non-recursive method for picking the I-th element from T... , and, IIUC, recursive templates cause compile-time slow downs. Since then, I've stumbled across other methods as shown in the attached (where, type_at is used instead of at). In addition there have been language proposals to do essentially the same: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0535r0.html#paramet... http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4235.htm http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0327r1.pdf I don't know of any compiler's that implement those "tuple slicing" operations; however, if someone does know of some, I'd like to see their use as an alternative to the existing mpl::at. Mike, if you find such an tuple-slicing compiler, maybe in your variadic mpl implementation you could provide a flag which uses that or one of the methods in the type_at.hpp attachment. Then, you could profile them to demonstrate which is best :) -regards, Larry