[Boost-users] Re: [variant] is_a/can_be

Hi, I can imagine cases where the current behavior might be preferable (e.g., uses of variant types in generic contexts). But I think for the vast majority of cases, and especially in non-generic contexts, checking the type as you propose is a good idea. I will determine a way to work this into the library, either by modifying the semantics of boost::get or by introducing a new accessor function. Thanks for the input. Eric Friedman Co-author of Variant library Yuval Ronen wrote:

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Hi. I encountered the need to query a boost::variant to which type it currently contain. I'm talking about something like:

variant v(3); assert(v.is_a<int>()); assert(!v.is_a<string>());

Take a look at get(), it's almost what you need. Unfortunately, it compiles even when the type cannot be inside the variant. This can be fixed with mpl::contains:

template bool variant_is_a(const Variant& v) { BOOST_MPL_ASSERT(( boost::mpl::contains

Variant::types,T> ));

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return boost::get<T>(&v) != 0; }

There should be a version of get that works as variant_is_a above. IMHO, the current behaviour is more dangerous than that of variant_is_a, so get() should be changed and maybe a new function be created that keeps the current behaviour. Some code may break with this change with a compile error that would go away if the other version of get is used.

Your code works beautifully. I really hope this feature will find its way to the variant library.