Re: [Boost-users] [fusion] nth runtime element of a sequence

Subject: Re: [Boost-users] [fusion] nth runtime element of a sequence
From: Larry Evans (cppljevans_at_[hidden])
Date: 2014-02-25 12:28:31

On 02/24/14 11:55, MM wrote:
>
>
> On 24 February 2014 14:28, Larry Evans <cppljevans_at_[hidden]
> <mailto:cppljevans_at_[hidden]>> wrote:
>
>     On 02/24/14 05:04, Krzysztof Czainski wrote:
>
>         2014-02-24 11:55 GMT+01:00 MM <finjulhich_at_[hidden]
>         <mailto:finjulhich_at_[hidden]>
>         <mailto:finjulhich_at_[hidden] <mailto:finjulhich_at_[hidden]>>>__:
>
>
>              Hello,
>
>              I just wanted to confirm there's no provided method to get
>         the n'th
>              element of say a fusion vector where n is known at runtime,
>         except
>              of handling it explicitkly with a switch case?
>
>              Thanks
>
>              MM
>
>
>         If such a function existed, what return type would it have?
>
>         Regards,
>         Kris
>
>     What about:
>        variant<T1,T2,T3> get(unsigned n, vector<T1,T2,T3>)
>     ?  For example:
>
>        vector<T1,T2,T3> vec(T1(),T2(),T3());
>        variant<T1,T2,T3> var;
>        var = get(2,vec);
>        assert(var.which(),2);
>
>     -regards,
>     Larry
>
> Possible, a variant over the unique types in the sequence.
It has to be over unique types because boost::variant cannot handle
duplicates.  However, if, instead of boost::variant, a truly tagged
union:

   http://en.wikipedia.org/wiki/Tagged_union

were used, then the types could be duplicated and there would be any
need to program some metafunction to find the unique types in the
fusion::vector<T1,T2,...,Tn>.

Tagged unions were also requested here:

http://stackoverflow.com/questions/17727563/tagged-unions-aka-variant-in-c-with-the-same-type-multiple-times/22021344#22021344

which also provided a link to an implementation of a tagged union.

> What would the impl look like though?
>
> MM
>
>
>
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