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

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2007-06-24 05:01:48

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