Re: [Boost-Users] Re: RE: PP Library...

From: Tanton Gibbs (thgibbs_at_[hidden])
Date: 2003-05-06 19:35:47

This makes sense.  So, replacement is done after substitution on the RHS,
not before macro argument substitution, correct?

In other words, with
#define EMPTY()
#define MACRO( x ) x

MACRO( EMPTY() )
would have EMPTY() substituted for the formal macro parameter x and the RHS
value x.  Then, a macros are executed on the RHS changing EMPTY() into
nothing (literally).

----- Original Message ----- 
From: "Paul Mensonides" <yg-boost-users_at_[hidden]>
To: <boost-users_at_[hidden]>
Sent: Tuesday, May 06, 2003 6:17 PM
Subject: [Boost-Users] Re: RE: PP Library...

> Tanton Gibbs wrote:
> > Also, I was asked by a coworker if the following would be std C++:
> >
> > BOOST_PP_LIST_FOR_EACH( DO_IT, BOOST_PP_EMPTY(), ARG_LIST )
> >
> > I said that it would not because BOOST_PP_EMPTY would be expanded
> > first to nothing, which would be the same as
> > BOOST_PP_LIST_FOR_EACH( DO_IT, , ARG_LIST )
> > which Paul explicitly said below was non-conformant.
> >
> > Is this correct?
>
> Well, not exactly, but close.  This, for example, is okay:
>
> #define EMPTY()
> #define MACRO(x) x
>
> MACRO(         ) // undefined
> MACRO( EMPTY() ) // okay
>
> However, this is not:
>
> #define MACRO_2(x) MACRO(x)
>
> MACRO_2( EMPTY() ) // okay for MACRO_2, undefined for MACRO
>
> The reason is relatively simple.  Before the argument is inserted into the
> replacement list of MACRO_2, it is macro expanded.  In this case, it
expands to
> nothing, which yields a replacement list like this:
>
> MACRO( )
>
> Which is rescanned for macros to expand and consequently introduces
undefined
> behavior.  BOOST_PP_LIST_FOR_EACH, as well as virtually every other pp-lib
> primitive uses other macros, so it is never safe to do this:
>
> BOOST_PP_SOME_MACRO( BOOST_PP_EMPTY() )
>
> C99 explicitly allows you to pass nothing as macro argument, but it is
still
> undefined in C++.
>
> Regards,
> Paul Mensonides
>
>
>
>
>
>
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