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

From: Maximilian Matthe (Maxi.Matthe_at_[hidden])
Date: 2008-07-16 08:30:54

Peter Bartlett schrieb:
> 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)
  Yes, I've realized that before. Thank you