Re: FSF GCC, #import and #pragma once

[Geoffrey Keating <geoffk at apple dot com>]

On Tuesday, March 4, 2003, at 02:01  PM, Neil Booth wrote:

> Geoffrey Keating wrote:-
>
> > On Tuesday, March 4, 2003, at 01:09  PM, Neil Booth wrote:
> >
> > > I've a suggestion to make that I'm hoping will break the impasse 
> > > here.
> > > ...
> >
> > I suggest waiting until my upcoming cppfiles rewrite is complete and 
> > we
> > discuss this then.
>
> I think that's a good reason to discuss it now.  We don't want to have
> big changes before agreeing what we want to do.

OK, then, I propose that:

1. We undeprecate #import in GCC for objective-C.
2. The semantics of #import become that:
   A. #import A followed by #import B will include B only if its content
      differs from the content of A.
   B. #import A followed by #include B will include B only if its 
content
      differs from the content of A.
   C. #include A followed by #import B will include B only if its 
content
      differs from the content of A.
3. Users are warned that #import without multiple-include guards can
   impose a compile-speed penalty.
4. #pragma once behaves as if the file was included with #import

The reason I propose this is that this is what I have to implement 
anyway for Apple, and it'll save me work if I can just use the same 
code in FSF cpplib.

I'm uncertain about:

5. We disable (not warn about, disable) #import and #pragma once in C 
and C++
   unless the user gives a flag to switch it on.

The reason for this is that the combination of 2(C) and PCH mean that 
every included file has to get checksummed when building a PCH file.  
I'm not sure how much overhead this might be, and am deferring an 
opinion on (5) until I get timing numbers.

--
Geoff Keating <geoffk at apple dot com>