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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

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 Preprocessor () {
    }

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

    /* application entry point */
    public static int main (string[] args) {
        var sample = new Preprocessor ();
        sample.run ();
        return 0;
    }
}

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