Re: [Boost-users] [Preprocessor] Expand to nothin - BOOST_PP_SEQ_PUSH_BACK

From: Manuel Jung (gzahl_at_[hidden])
Date: 2007-06-24 03:31:27

Hi

Thanks for your explanations. I'll need a moment to understand and read
more. Maybe i'll be back here then. But thanks a lot for this!
Greetings
Manuel Jung

> Manuel Jung wrote:
>> Hi
>> 
>> The BOOST_PP_SEQ_PUSH_BACK Macro expands to the seq with the extra
>> element. But i don't want it to expand.
> 
> So if the macro wouldn't expand it stays BOOST_PP_SEQ_PUSH_BACK(foo,bar)
> (literally), which obviously is not a sequence...
> 
>> I want to use the sequence later instead.
> 
> Just do so (I'm not sure I understand what you're up to)!
> BOOST_PP_SEQ_PUSH_BACK is a trivial macro mostly for completeness:
> Instead of
> 
>      BOOST_PP_SEQ_PUSH_BACK(SEQ, c)
> 
> you might as well say:
> 
>      SEQ (c)
> 
> (subtle differences omitted, not to cause total confusion).
> 
>> How can i supress the expanding?
> 
> As it's the very nature of the preprocessor to expand all macro
> invocations it sees, you can't in general.
> 
> You can delay the expansion of function-like macros by supplying the
> argument list at a later point,
> 
>       #define MACRO(optional) foo optional() baz
> 
>       MACRO(BOOST_PP_EMPTY) // foo baz
>       MACRO(BOOST_PP_IDENTITY(bar)) // foo bar baz
> 
>       // BTW: this techniques is used to avoid empty macro arguments
>       // which provoke undefined behavior with C++98
> 
> or making sure it's connected at a later point:
> 
>       #define E()      // Empty
>       #define X(x) x   // eXpand
> 
>       MACRO E () (BOOST_PP_EMPTY) // MACRO (BOOST_PP_EMPTY)
> 
>       X( MACRO E () (BOOST_PP_EMPTY) ) // foo baz
>       X( MACRO E E () () (BOOST_PP_EMPTY) ) // MACRO (BOOST_PP_EMPTY)
>       X(X( MACRO E E () () (BOOST_PP_EMPTY) )) // foo baz
> 
> .
> 
>> I'm new too Preprocessorprogramming, so please excuse, if my question is
>> stupid. Not the easiest thing, starting Preprocessor and Metaprogramming
>> from nothing but the docs.
> 
> Search the list archives, there should be lots of good material, around.
> 
>      http://news.gmane.org/gmane.comp.lib.boost.user
>      http://news.gmane.org/gmane.comp.lib.boost.devel
> 
> Regards,
> Tobias