Re: [Boost-users] [Fusion] Adapted struct with member optional<variant<A, B>> needs typedef

Subject: Re: [Boost-users] [Fusion] Adapted struct with member optional<variant<A, B>> needs typedef
From: Nathan Crookston (nathan.crookston_at_[hidden])
Date: 2010-09-27 12:19:04

Hi Pete,

On Mon, Sep 27, 2010 at 5:28 AM, PB <newbarker_at_[hidden]> wrote:
<snip code>
> BOOST_FUSION_ADAPT_STRUCT(
>    Transition,
>        (boost::optional<boost::variant<Word,Text>>, token)
> )
<snip code>
> Compiler output:
> error warning C4002: too many actual parameters for macro
> 'BOOST_FUSION_ADAPT_STRUCT_FILLER_0'.
> <etc>

> Why is this, and what are the constraints on the types fed to
> BOOST_FUSION_ADAPT_STRUCT?

The problem seems to be that the preprocessor is seeing the ',' in
boost::variant and is parsing it like a tuple of 3 objects:
"boost::optional<boost::variant<Word," "Text>>" and "token", while
Boost.Fusion expects 2 objects.  The normal solution is to add a set
of parentheses around the token with a comma, but that doesn't seem to
work here.  I think your typedef solution is probably the simplest and
best for this case -- but my skills with the preprocessor are
definitely incomplete.

HTH,
Nate