Re: [Boost-users] [preprocessor] BOOST_PP_IF with BOOST_PP_SEQ_ENUM as then-argument

From: Peter Bartlett (pete_at_[hidden])
Date: 2008-07-16 07:16:04

Quoting Maximilian Matthe <Maxi.Matthe_at_[hidden]>:

> Hello!
>
> #define seq (int)(int)
>
> BOOST_PP_IF(1, BOOST_PP_SEQ_ENUM(seq), BOOST_PP_EMTPY)
>
> does not expand to what i want: it should be int, int . G++ gives the
> following error:
>
> test.cpp:20:1: macro "BOOST_PP_IIF" passed 4 arguments, but takes just 3
> BOOST_PP_IIF
>
> which is logical, because SEQ_ENUM expands to int, int and so if gets 4
> arguments. I tried to surround SEQ_ENUM with IDENTITY but it's the same
> problem. Only surrounding SEQ_ENUM with () works, but then I have the
> () in the code, which I do not want. How can I solve this problem?
>
> Max

You need to delay the evaluation of the SEQ_ENUM til after the IF:

#define DO_NOTHING(seq)

BOOST_PP_IF(1, BOOST_PP_SEQ_ENUM , DO_NOTHING)()