Re: [Boost-users] [Preprocessor] Comma in sequence element

Subject: Re: [Boost-users] [Preprocessor] Comma in sequence element
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2013-02-12 14:26:40

On Tue, Feb 12, 2013 at 9:53 AM, Adam Badura <abadura_at_[hidden]> wrote:

> I have following code:
>
> #include <boost/preprocessor/seq/enum.hpp>
> #include <boost/preprocessor/seq/transform.hpp>
>
> template< int I, int J >
> struct Base
> {
> };
>
> #define SEQ ( 1 ) ( 2 ) ( 3 ) ( 4 )
>
> #define OP( s, data, elem ) Base< elem, elem >
>

Try something like
  #define OP(s, data, elem) (Base< elem)(elem >)
and...

> struct Derived
>   : BOOST_PP_SEQ_ENUM( BOOST_PP_SEQ_TRANSFORM( OP, nil, SEQ ) )
> {
> };
>

...use something like BOOST_PP_SEQ_FOREACH instead of
BOOST_PP_SEQ_TRANSFORM.

> The Derived has bases classes generated from a sequence (SEQ). The bases
> classes themselves are instantiations of Base template type. I expected
> the code to be preprocessed into something like (just the important part):
>
> struct Derived
>   : Base< 1, 1 >, Base< 2, 2 >, Base< 3, 3 >, Base< 4, 4 >
> {
> };
> but instead it is preprocessed into:
>
> struct Derived
>   : Base< 1, Base< 2, Base< 3, Base< 4
> {
> };
> This obviously is caused by Base being template with to parameters and
> thus having a comma.
>
> Adding parenthesis around Base in OP solves the issue by generating
> expected code fully. But it still does not compile because parenthesis are
> not allowed in that place (base classes list).
>
> Is there any other workaround? How to deal with it?
>

Hopefully that puts you on the right track. If you need more detail, holler
back.

- Jeff