Hi Michael,

Michael Powell wrote:

Hello,

I've got some structs in a AST of sorts which the main player in the
mix is a node. In at least one of the use cases, one of the variant
types must be forward declared at the time the node can be declared.
Possibly one or two other similar use cases.

Error 2 error C2139: 'xml::xelement' : an undefined class is not
allowed as an argument to compiler intrinsic type trait
'__is_nothrow_constructible'
i:\installs\development\boost.org\1.55.0\boost_1_55_0\stage\include\boost\type_traits\has_nothrow_constructor.hpp
24 1 xml

However, I am bumping into C2139, above.

Is there any way for variant to work with forward declared structs (or
classes, etc)? Or am I just asking too much from C++?

Like undiscriminated unions, variant cannot hold incomplete types. Without knowing the sizes of its potential members, it wouldn't know the maximum size it should allocate.

The solution to problems like this is usually to hold a pointer to the incomplete type (not sure if variant works with move-only types like std::unique_ptr). Variant has some syntactic sugar to allow recursive variants (where one child could be another variant of the same type) for tree-like structures.

HTH,
Nate