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
VALA_X_Y	Set if Vala API version is equal or higher to version X.Y

Examples

How to conditionally compile code based on a valac option -D.

Sample code:

vala-test:examples/advanced.vala

// Vala preprocessor example
public class Preprocessor : Object {

    /* public instance method */
    public void run () {
#if PREPROCESSOR_DEBUG
        // Use "-D PREPROCESSOR_DEBUG" to run this code path
        stdout.printf ("debug version");
#else
        // Normally, we run this code path
        stdout.printf ("production version");
#endif
    }
}

/* application entry point */
void main () {
    var sample = new Preprocessor ();
    sample.run ();
}

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