Re: [Vala] Define constant at compile time

[Anatol Pomozov <anatol pomozov gmail com>]

On Tue, Nov 16, 2010 at 11:44 AM, Jan Hudec <bulb ucw cz> wrote:
> On Tue, Nov 16, 2010 at 18:51:12 +0100, Günther Wutz wrote:
> > It is possible to use cpp as Preprocessor. I'm working on a
> > wrapper-program to combine cpp and give it valac to compile. The easiest
> > way i found actually.
>
> Sorry. That's an extremely bad idea for most cases.
>
> The easiest way is to tell vala to emit the symbols that will then be defined
> when compiling the generated C code. That way vala will guard you from name
> conflicts and the code will be more readable.

"preprocessor" is probably a bad word as it makes people think about
C-style preprocessor. If I understand the request correctly one wants
"compile-time type-safe constants substitution".

> In C, the preprocessor is generally used for two things:
>  - symbolic constants -- but real constants serve the purpose better and

But what if you want to change the real constant value at the
compile-time? A classical example is DEBUG vs OPTIMIZED mode. During
development I want to log of information to stdout, like this:

if (DEBUG) {
  stdout.printf("User ID is: $user");
}

and I don't want to have this code in the release version. To make it
happen one can use something like

bool DEBUG=false;

and compile the vala program with -DDEBUG (or its equivalent
-DDEBUG=true). This compiler flag changes the default value of the
const.

Type-safety means that vala compile knows that DEBUG is boolean and
complains if someone tries to use number/string here (-DDEBUG=foo) or
tries to set value for unexisting constant.

>  - generic constructs -- but vala does have generics, so you don't need
>   those.
> Even in C++ you usually avoid the preprocessor for most things, because
> preprocessor symbols cannot be scoped. Which is doubly so applicable to vala.