[Boost-users] [variant] is_a/can_be

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

Hi.
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);
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 <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:

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