4.1. Assertions and Contract Programming

With assertions a programmer can check assumptions at runtime. The syntax is assert(condition). If an assertion fails the program will terminate with an appropriate error message. There are a few more assertion methods within the GLib standard namespace, e.g.:


return_if_fail(bool expr)


warn_if_fail(bool expr)


You might be tempted to use assertions in order to check method arguments for null. However, this is not necessary, since Vala does that implicitly for all parameters that are not marked with ? as being nullable.

void method_name(Foo foo, Bar bar) {
    /* Not necessary, Vala does that for you:
    return_if_fail(foo != null);
    return_if_fail(bar != null);

Vala supports basic contract programming features. A method may have preconditions (requires) and postconditions (ensures) that must be fulfilled at the beginning or the end of a method respectively:

double method_name(int x, double d)
        requires (x > 0 && x < 10)
        requires (d >= 0.0 && d <= 1.0)
        ensures (result >= 0.0 && result <= 10.0)
    return d * x;

result is a special variable representing the return value.