Re: [Boost-users] BOOST_PP_SEQ_ENUM troubles

Subject: Re: [Boost-users] BOOST_PP_SEQ_ENUM troubles
From: Edward Diener (eldiener_at_[hidden])
Date: 2016-07-07 11:59:46

On 7/7/2016 3:04 AM, Cherotek Music wrote:
> Thanks I see. I'm at work now and can't test it but would replacing the
> comma in ARG_PAIR with *BOOST_PP_COMMA help?*

I do not think so.

>
> On Thu, Jul 7, 2016 at 1:49 AM, Edward Diener <eldiener_at_[hidden]
> <mailto:eldiener_at_[hidden]>> wrote:
>
>     On 7/6/2016 5:27 PM, Cherotek Music wrote:
>
>         Hi,
>         I'm trying to make a macro chain that expands something like this
>
>         *A, B, C*
>
>         into something like this
>
>         *{"A", create<AFile>}, {"B", create<BFile>}, {"C", create<CFile>}*
>
>         This is what I have come up with
>
>         *#define FORMATS A, B, C // Add new formats here*
>         *#define SEQ_FORMATS BOOST_PP_VARIADIC_TO_SEQ(FORMATS)*
>         *#define ARG_PAIR(s, data, x) {#x, create<x##File>}*
>         *#define SEQ_ARG_PAIRS BOOST_PP_SEQ_TRANSFORM(ARG_PAIR, ~,
>         SEQ_FORMATS)*
>         *#define ARG_PAIRS BOOST_PP_SEQ_ENUM(SEQ_ARG_PAIRS)*
>
>         The SEQ_ARG_PAIRS macro does expand into
>
>         *({"A", create<AFile>}) ({"B", create<BFile>}) ({"C",
>         create<CFile>})*
>
>         but ARG_PAIRS fails with the following error:
>
>         test.cpp:20:1: error: macro "BOOST_PP_SEQ_SIZE_0" passed 2
>         arguments,
>         but takes just 1
>              ARG_PAIRS
>          ^
>             BOOST_PP_SEQ_ENUM_0 ({"B", create<BFile>}) ({"C",
>         create<CFile>})
>         ({"A", create<AFile>}) ({"B", create<BFile>}) ({"C", create<CFile>})
>
>         Any ideas?
>
>
>     The reason for this is because after your transformation you do not
>     have a valid seq anymore for BOOST_PP_SEQ_ENUM. Each seq element
>     must be a one element tuple. The comma ( ',' ) in each seq element
>     no longer makes that seq element valid, producing a two element
>     tuple. Something like '({"A", create<AFile>})' really containes two
>     elements of data, the first one being '{"A"' and the second one
>     being 'create<AFile>}'.
>
>         thanks a lot