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 07:34:48

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

> Peter Bartlett schrieb:
>> 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)()
>
> Ah thank you. This is quite tricky :-)

Sorry there was a typo in my post:

BOOST_PP_IF(1, BOOST_PP_SEQ_ENUM , DO_NOTHING)()

should've been

BOOST_PP_IF(1, BOOST_PP_SEQ_ENUM , DO_NOTHING)(seq)