[OT] Clang seems to implement modules

[Walter Bright]

On 11/20/2012 3:51 PM, Leandro Lucarella wrote:
> Did you ever cared about reading those slides?!?!? You keep talking about
> problems with pre-compiled headers and what Doug Gregor is suggesting are NOT
> pre-compiled headers. Those are already in clang AFAIK.
>
> What he is proposing is a real module system, macros will not be re-evaluated
> inside modules. The symbols being global have nothing to do with this being
> pre-compiled headers.


Modules *are* a form of precompiled headers.


> Will this solve all the problems from C++ and make its compile time blazingly
> fast? Probably not, but will sure help, not only to avoid reading the same
> header over and over, but also by saving memory. But one thing is certain,
> THIS IS NOT PRE-COMPILED HEADERS (he even mention pre-compiler headers in the
> slides).
>
> For f*ck sake... Please, stop this misinformation madness.

Precompiled headers are:

1. compile a bunch of .h files into a symbol table
2. cache the symbol table (in memory or on disk)
3. read the symbol table instead of reparsing the .h files

Modules are:

1. compile a bunch of .h files into a symbol table
2. cache the symbol table (in memory or on disk)
3. read the symbol table instead of reparsing the .h files

Yes, I understand that there are semantic differences, and many differences in 
detail. C++11 does not support precompiled headers; all ph implementations are a 
kludge and are not standard compliant. The module proposal "legitimizes" them, 
i.e. changes the standard so that ph can be compliant. Yes, additional goodies 
are added like a separate scope for macros, an explicit syntax for them, etc.

The speed improvement should be comparable to what can be achieved with a good 
ph system.

Is this module proposal an improvement? I'd say yes. Is it going to solve C++'s 
compile speed problems? I doubt it. Is it a "true" module system? I don't know 
about that, but it doesn't address things like name collisions that are imported 
from different modules, at least not from what I saw in the slides.