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: PB (newbarker_at_[hidden])
Date: 2010-09-27 12:27:52

On Mon, Sep 27, 2010 at 5:19 PM, Nathan Crookston
<nathan.crookston_at_[hidden]> wrote:
> 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.

Nate,

Ahh. I didn't think about the comma. Perhaps the
BOOST_FUSION_ADAPT_STRUCT macro itself needs some parentheses
internally.

I'll stick to the typedef but hopefully if this can be fixed, the
right people are viewing this topic.

Thanks,

Pete