The Wrapper extension creates a simple container type that can be used whenever indirection is needed in a BIR model. The polymorphic type declared here can be used to contain a single element of any (fixed) type. The wrapper itself (regardless of whether it contains a reference or primitive type) is heap-allocated so that the object can be passed between functions. C programmers may feel comfortable likening a wrapper to a pointer parameter passed into a function with the expectation of being filled. The C function signature void foo(int *fillme) would become the BIR function function foo(Wrapper.type<int> fillme) { ... }.

extension Wrapper for edu.ksu.cis.projects.bogor.module.wrapper.WrapperModule
{
    // The type itself
    typedef type<'a>;

    // Constructor. Always returns a non-null value
    expdef Wrapper.type<'a> create<'a>('a element);

    // Accessor. Return the contained element (possibly null if 'a is
    // a heap type) or throws a BIR NullPointerException 
    expdef 'a get<'a>(Wrapper.type<'a> wrapper);

    // Mutator. Installs a new elements into the the given wrapper.
    // Raises a BIR NullPointerException if the wrapper is null.
    actiondef set<'a>(Wrapper.type<'a> wrapper, 'a elem);
}

The following example uses a wrapper to get the return value from a function in an indirect way:

system Test
{
    extension Wrapper for edu.ksu.cis.projects.bogor.module.wrapper.WrapperModule
    {
        ...
    }

    const CONSTANTS
    {
        INITIAL_VAL = -1;
        RESULT_VAL = 101;
    }

    main thread MAIN()
    {
        // declare variable (initial value is null)
        Wrapper.type<int> container;

        // allocate new wrapper instance on heap
        container := Wrapper.create(CONSTANTS.INITIAL_VAL);

        // fill in value
        assert Wrapper.get(container) != CONSTANTS.RESULT_VAL;
        fillContainer(container);
        assert Wrapper.get(container) == CONSTANTS.RESULT_VAL;
    }

    function fillContainer(Wrapper.type<int> container)
    {
        Wrapper.set(container, CONSTANTS.RESULT_VAL);
    }
}