Re: [Vala] Conditional compilation: Passing #ifdefs through to generated C code

[Jürg Billeter <j bitron ch>]

On Mon, 2009-03-23 at 20:35 -0400, Michael Terry wrote:
> I'd like to know more about how to use C preprocessor commands in
> Vala.  I know that Vala allows some sort of conditional compilation
> via [Conditional], but I'm interested in a particular use case that it
> doesn't seem to really help with.
>
> Specifically, I have a GTK+ app, and I want to use a bit of code only
> if the compile-time GTK+ version is high enough.  So something like:
>
> #if GTK_CHECK_VERSION(2, 4, 0)
>     GtkFileChooserAction action;
>     ...
> #endif
>
> I can have vala conditionally use a bit of code, but the conditional
> is resolved at Vala-compile-time, not C-compile-time.  I need
> 'passthrough' conditionals that show up in the generated C code, since
> my software distributes the C code in the release tarball.  The
> typical user just compiles the C.

I understand the issue. Unfortunately, it's not easy to solve. One issue
is that you might need multiple #if in the generated code for a single
#if in the Vala code - e.g. for #include directives. It would also
require the Vala compiler to support dealing with multiple declarations
of the same method for different conditions, which would make semantic
and flow analysis a lot more difficult.

I'd be happy to hear proposals how we could solve this in a
non-intrusive way.

Jürg