On 04/15/12 04:57, Florian Goujeon wrote:
Hi Boosters,
I'd like to know if anybody were working on a C++11 version (using variadic
templates) of Boost.Variant.
I currently use the 'classic' version in my project, but since I need
more-than-20-type variants, the compilation take a lot of time (extending the
number of types that variant can support forces you to disable the use of
precompiled headers). Plus, I'll need to use 70 type variants (MPL lists are
physically limited to 50 types).
If nobody has started to implement such a version yet, I'd be pleased to
contribute.
I've written an implementation. It's incomplete and far from perfect, but it
could (hopefully) be a viable working basis.
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There's this: http://svn.boost.org/svn/boost/sandbox/variadic_templates/boost/composite_st... which uses variadic templates; hence, doesn't have the limitations you mentioned about the number of bounded types. *HOWEVER* I'v not tested whether it takes more or less compile time than variant. One difference of one_of_maybe w.r.t. boost::variant is that it's a true tagged variant. IOW, it allows duplicates in the bounded types: http://www.boost.org/doc/libs/1_49_0/doc/html/variant/tutorial.html#variant.... For example: variant<int,int> is not allowed because variant<int,int> when assigned from an int: variant<int,int> ii; ii=int(999); wouldn't, IIRC, know which bounded type, the 1st int or the 2nd, was intended as the target. OTOH: container<tags::one_of_maybe,unsigned,int,int> ii; ii.inject<0>(999); would work because the tag (the template arg to inject member function), indicates which bounded type is intended. You might object that: ii = int(999); is less "user-friendly" than: ii.inject<0>(999); however, I thought making the structure more like the actual mathematical disjoint union: http://en.wikipedia.org/wiki/Disjoint_union was worth the extra trouble. In addition, one_of_maybe certainly doesn't have all the nice features of variant outlined here: http://www.boost.org/doc/libs/1_49_0/doc/html/variant.html#variant.abstract and in particular, it does not have: http://www.boost.org/doc/libs/1_49_0/doc/html/variant/design.html#variant.de... Instead, if one of the bounded types has not yet been injected, the which function will return a tag value indicating this and the user is required to check this. I thought this would be useful for implementing something like boost's optional, which would then be like one_of_maybe with just 1 bounded type. Tests for one_of_maybe (and other composite_storage types) are here: http://svn.boost.org/svn/boost/sandbox/variadic_templates/libs/composite_sto... HTH. -regards, Larry