Re: [fusion] Proposal for deprecating fusion.tuple.

Kohei Takahashi <[email protected]> Sat, 30 Sep 2017 10:23:22 +0900
Newsgroups	gmane.comp.parsers.spirit.general
Message-ID	<[email protected]>
Hi Lee,

> How does `fusion::tuple` make `fusion::vector` implementation more
> complex? Are you referring to the C++03 or C++11 version?
C++11 is.
`fusion::vector` constructors have complex SFINAE expression and helper
functions in order to sieve its argument category: same type, other
convertible sequence, nested, ... That is reason why I did tag
dispatching in `fusion::detail::vector_data`.

I'm still debating about C++03 impl.

> For C++11 `fusion::tuple` I noticed that if the assignment from fusion
> sequences was removed (its constructor currently does not allow
> conversion from sequences and `std::tuple` has no such requirement),
> then the headers automatically included could be reduced. So
> `fusion::tuple` _could_ be a faster compiling sequence for when the
> conversion properties of `fusion::vector` was not needed.
Good point. Implicit conversion is one of cause why I proposed.  Since
`fusion::tuple` uses `fusion::vector`, if this goes on, it can't be
faster than `fusion::vector`. OK, re-implement `fusion::tuple` without
any other sequence? Perhaps it has no (or few) difference between
`fusion::vector` about performance.

However, no conversion performing guarantee is safer and meaningful
option for some user.

> Of course,
> since most have a good implementation of `std::tuple` perhaps this
> unnecessary? I would like to point out that recent tweaks to
> `fusion::vector` have [improved performance over libstdc++ `std::tuple`
> in construction][0][1].
Interesting. I'll try to benchmark simpler impl.

> Anyway, the bigger issue with deprecation will be on users of Boost.
> Hopefully the fix would be easy - just change to another sequence type.
> I'm not aware of an situations where this is _not true_ (i.e. where
> deprecation would cause major code re-write for Boost users).
Yes, I think so too. I'll post this proposal to devel/users ML and ask
who uses `fusion::tuple` in production code.

I my mind, just announce in next (or next to the next) release, hence no
code is changed. After 2 or more release passed, break the code
(removing, re-implementing, or aliasing).

Thanks,
Kohei

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