Boost logo

Boost Users :

From: Yuval Ronen (ronen_yuval_at_[hidden])
Date: 2005-01-29 10:56:59

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

variant<int, string> v(3);

This can be done using the which() method, but its drawback is that I
rely on the order of the types in the variant declaration rather on the
type, which is really what I'm interested in.

I can implement the variant_is_a<>() feature using a visitor such as:

template <typename T>
struct is_a_visitor : public boost::static_visitor<bool>
    template <typename U>
    bool operator()(const U &) const { return false; }

    bool operator()(const T &) const { return true; }

template <typename T, class Variant>
bool variant_is_a(const Variant &v)
     return v.apply_visitor(is_a_visitor<T>());

but this way has a problem: calling variant_is_a<double>() on a variant
of type variant<int, string> (no double for this variant) will simply
return false, rather than just not compile, which is what I prefer.
Making this not compile will require a new feature from variant: a
boolean constant 'can_be'. With this, the variant_is_a() function could
be written with additional line:


IMO, the is_a/can_be features are general and useful enough to be
included in the library. Wouldn't you agree?


Boost-users list run by williamkempf at, kalb at, bjorn.karlsson at, gregod at, wekempf at