Re: PATCH (libstdc++-v3): Fix libstdc++/7680 (and a reference fixfor the dynamic visibility of C99 features in system headers)

[Zack Weinberg <zack at codesourcery dot com>]

Loren James Rittle <rittle at latour dot rsch dot comm dot mot dot com> writes:

>>I'm wondering how much typing you would save with CPP...
>
> I wondered the same thing.  How do I put a #if region inside a macro
> defined with #define?
>
> I need to define something like this to tighten it:
>
> #define _GLIBCXX_C99_BIND(a,b) \
> #if (a) \
>   using ::b; \
> #endif \

This, I'm afraid, is completely impossible.

If you want to talk about extensions to allow it, I would suggest that
you instead allow

  if (a) using ::b;

where (a) is an integer-constant-expression.  This is a limited form
of the 'if-goes-everywhere' functionality I've been thinking about off
and on - the idea is to supersede #if completely (except for excluding
other preprocessing directives), by allowing if (x) { ... } anywhere,
as long as x is an integer constant expression, and then adding
builtin predicates that evaluate all the sorts of things you want to
test at compile time.

zw