Subject: Re: [boost] [variant] Common base type of variant sub-types
From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2010-01-09 07:25:40
Adam Badura wrote:
> Obviously it would not be wise to provide a function wrapper for each
> and every visitor. However the extended "get" I proposed is unique in
> its ways.
But it wouldn't be 'get'.
There is a reason why it works like this. It's designed to test and
extract for a specific type in one of the variant possibilities, without
doing any implicit conversion.
Adding the enhancement you suggest would change the meaning of 'get' and
potentially break other code.
So I think it would be best as a separate function.
Here is a possible implementation:
namespace boost
{
namespace detail
{
template<typename T>
struct extract_visitor : static_visitor<T&>
{
T& operator()(T& t) const
{
return t;
}
T operator()(...) const
{
throw bad_visit();
}
};
template<typename T>
struct extract_pointer_visitor : static_visitor<T*>
{
T* operator()(T& t) const
{
return &t;
}
T* operator()(...) const
{
return 0;
}
};
} // namespace detail
template<typename T, typename Variant>
T& extract(Variant& v)
{
return apply_visitor(detail::extract_visitor<T>(), v);
}
template<typename T, typename Variant>
const T& extract(const Variant& v)
{
return apply_visitor(detail::extract_visitor<const T>(), v);
}
template<typename T, typename Variant>
T* extract(Variant* v)
{
return apply_visitor(detail::extract_pointer_visitor<T>(), *v);
}
template<typename T, typename Variant>
const T* extract(const Variant* v)
{
return apply_visitor(detail::extract_pointer_visitor<const T>(), *v);
}
} // namespace boost
> Secondly
> it provides for variant a feature available in union.
That could only work with unions if the types are binary compatible,
which isn't necessarily the case.