From: Eric Friedman (ebf_at_[hidden])
Date: 2003-11-15 20:25:38
Peter Dimov wrote:
> David Abrahams wrote:
>>"Peter Dimov" <pdimov_at_[hidden]> writes:
>>>I take it back. The only correct interface is variant<mpl-seq>,
>>>without the variable-length enhancement.
>>I'm confused about what you mean. Isn't that the one we already have?
> We have
> variant<T1, T2, ..., Tn>
> There are two problems with it. First, there is a collision when n == 1.
Since the thread you're referring to, I've actually changed it to
variant<T1, T2, ..., Tn>
make_variant_over< mpl-seq >::type
where the latter produces an unspecified variant<...> of some sort.
> Second, if I write
> template<class Seq> void f( variant<Seq> const & v );
> my function will not match all variants. I think that the correct but
> painful thing to do is to just drop the convenience T1..Tn form.
The approach I currently recommend in the variant documentation is the
template < BOOST_VARIANT_ENUM_PARAMS(typename T) >
void f( variant<BOOST_VARIANT_ENUM_PARAMS(T)> const & v );
I don't know if this is to be considered better or worse than
template <typename V>
typename enable_if< is_variant<V> >::type
f( V const & v );
Of course, the latter only works on compilers with SFINAE. So perhaps we
could offer both. I'd have to provide is_variant, though.
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