Re: Issue with hide-ifdef and cc-mode

[Ergus]

On Sun, May 30, 2021 at 01:01:36PM +0200, Daniel Mart�n wrote:
> Ergus <spacibba@aol.com> writes:
>
> > I am trying to use hide-ifdef to shadow code like:
> >
> > #if 0
> > <code>
> > #endif
> >
> > But I found that either
> >
> > #if 0
> >
> > or
> > #if 1
> >
> > produces the same result disabling the code inside.
>
> I tested the following from emacs -Q in Emacs 28:
>
> New C buffer with
>
> #if 0
> int i = 0;
> #endif
>
> M-x hide-ifdef-mode
> M-x hide-ifdef-toggle-shadowing
> M-x hide-ifdefs
>
> And the variable definition is shown with a comment font-lock face, as
> expected.
>
> If I change it to "#if 1" and M-x hide-ifdefs again, the comment
> font-lock face is removed, as expected.  Perhaps some Emacs
> customization is getting in the way?
>
> For something more complex than a #if 0, you probably need to ask the C
> preprocessor or build system for the defined variables and configure
> hide-ifdef-mode accordingly.
Hi Daniel:

Thanks for the reply. It looks like I didn't understand the
documentation properly. (It is only a few lines)

I didn't know I need to manually call hide-ifdefs every time I change
the preprocessor argument. Because none of this is documented in the
manual. But also calling that makes the functionality pretty
sub-optimal.

It is bad that this functionality works fine out of the box either in
vim and in VS Code and other editors but not in emacs without user
explicit intervention.

So I am not sure how difficult may be to do it automatic without killing
the performance... Like font-lock does with everything else... Soes it
really kills performance to add his to font-lock?

I hope probably Alan may have an idea about how to solve this in a more
cc-mode way... Without forcing the user to interactively call
hide-ifdefs.

We do something similar with /* */ multi-line comments... I can't
understand why this may be so different for if 0 macros... But I may be
wrong.