Preprocessor#
The Vala preprocessor is a particular part of Vala that acts at syntax level only, allowing you to conditionally write pieces of your software depending upon certain compile-time conditions. Preprocessor directives will never be generated in the resulting code.
Directives syntax#
All preprocessor directives start with a hash (#), except for the first line of a file starting with #! (used for Vala scripts).
vala-code:
[ any vala code ] [ pp-condition ] [ any vala code ]
pp-condition:
#if pp-expression vala-code [ pp-elif ] [ pp-else ] #endif
pp-elif:
#elif pp-expression vala-code [ pp-elif ]
pp-else:
#else vala-code
pp-expression:
pp-or-expression
pp-or-expression:
pp-and-expression [ || pp-and-expression ]
pp-and-expression:
pp-binary-expression [ && pp-binary-expression ]
pp-binary-expression:
pp-equality-expression
pp-inequality-expression
pp-equality-expression:
pp-unary-expression [ == pp-unary-expression ]
pp-inequality-expression:
pp-unary-expression [ != pp-unary-expression ]
pp-unary-expression:
pp-negation-expression
pp-primary-expression
pp-negation-expression:
! pp-unary-expression
pp-primary-expression:
pp-symbol
( pp-expression )
true
false
pp-symbol:
identifier
The semantics of the preprocessor are very simple: if the condition is true then the Vala code surrounded by the preprocessor will be parsed, otherwise it will be ignored. A symbol evaluates to true if it is defined at compile-time. If a symbol in a preprocessor directive is not defined, it evaluates to false.
Defining symbols#
It’s not possible to define a preprocessor symbol inside the Vala code
(like with C). The only way to define a symbol is to feed it through the
valac
option -D
.
Built-in defines#
Name |
Description |
POSIX |
Set if the profile is posix |
GOBJECT |
Set if the profile is gobject |
DOVA |
Set if the profile is dova |
VALA_X_Y |
Set if Vala API version is equal or higher to version X.Y |
DBUS_GLIB |
Set if using dbus-glib-1 package |
Examples#
How to conditionally compile code based on a valac
option -D
.
Sample code:
vala-test:examples/advanced.vala
1// Vala preprocessor example
2public class Preprocessor : Object {
3
4 public Preprocessor () {
5 }
6
7 /* public instance method */
8 public void run () {
9#if PREPROCESSOR_DEBUG
10 // Use "-D PREPROCESSOR_DEBUG" to run this code path
11 stdout.printf ("debug version");
12#else
13 // Normally, we run this code path
14 stdout.printf ("production version");
15#endif
16 }
17
18 /* application entry point */
19 public static int main (string[] args) {
20 var sample = new Preprocessor ();
21 sample.run ();
22 return 0;
23 }
24}
Compile and Run#
Normal build/run:
$ valac -o preprocessor Preprocessor.vala
$ ./preprocessor
Debug build/run:
$ valac -D PREPROCESSOR_DEBUG -o preprocessor-debug Preprocessor.vala
$ ./preprocessor-debug