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.

On Thu, 03 Feb 2005 00:33:37 +0200, Yuval Ronen wrote:

Eric Friedman wrote:

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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

Great! Thanks a lot.

BTW, another two small remarks: The "variant::types" typedef that was used here is not documented, and it will be helpful if I could read about it and about its possible usages. And if you wish not to burden the users with mpl, then it's possible to wrap mpl::contains in a constant boolean value "can_be", as I suggested.

variant::types is documented here: http://boost.org/doc/html/boost/variant.html Bruno

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BTW, another two small remarks: The "variant::types" typedef that was used here is not documented, and it will be helpful if I could read about it and about its possible usages.

variant::types is documented here: http://boost.org/doc/html/boost/variant.html

Bruno

Well, first of all, you're right. My mistake, I missed it. However, when thinking about why I missed it, I realized that its because it's not a colored link, like all other classes/functions. Why should a typedef be underprivileged compared to classes/functions? It's part of the interface and should be documented in the same way (and not just as a side note). I hope I'm not too much of a nagger... :-)