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>());
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
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 (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:
BOOST_STATIC_ASSERT(Variant::can_be<T>::value);
IMO, the is_a/can_be features are general and useful enough to be
included in the library. Wouldn't you agree?
Yuval
