Passing Values by Reference (in out)

Java and Groovy are both "pass-by-value" languages. That is, if you pass a value to a method and the method modifies it, you don't see a change in the calling scope.

COM, on the other hand, supports both "pass-by-value" (or "byval") and "pass-by-reference" (or "byref"). "Byval" is sometime referred to as an "in" parameter, and "byref" is sometimes referred to as an "in/out" parameter, reflecting the direction the data is flowing. When you call a method that accepts a "pass-by-reference" parameter, the method can modify the parameter value, and this will be reflected in the calling scope.

Byref argument passing was always a favorite of C/C++ programmers, who often use the result of a method to pass exception information. It was the default way to pass values in versions of Visual Basic through 6. With the advent of modern try/catch exception handling, modifying values inside a method is generally considered bad programming practice. Visual Basic (and COM) still supports byref, but byval is now defined as the default setting. You should not have to deal with byref parameters often, especially when dealing with APIs that are designed for scripting. That doesn't mean it will never happen, though.

It shouldn't surprise you that Scriptom supports passing parameters by-reference, just in case you need it.

There are actually two ways that values are passed by-reference. For COM Events, which do not support returning values, the only way to pass back information is through a byref parameter. That is all explained in the article, and it is actually pretty transparent. You don't have to do anything special for it to work. Your event handler can just change one of the arguments passed to it, and the caller sees the changed value.

The other way that values are passed by reference is through method calls. This one is a little more complicated, but only a little. Scriptom takes care of most of the scary details for you.

How Does it Work?

The basic concept is that you place the value in a wrapper object, pass the wrapper to the method, and finally get the new value back from the wrapper when the method returns. Scriptom's wrapper object is VariantByref (org.codehaus.groovy.scriptom.VariantByref).

One cool feature is that Scriptom allows you to just pass a String or an Integer or whatever type the method requires, if you don't care about getting back the changed value. Most of the time, even when a parameter is defined as byref, you don't really care about seeing the changed value. So you can ignore all of this.

Okay, so let's assume that you do care. The changed value matters to you. Otherwise you would have stopped reading in the last paragraph, right? Consider the following VB6 object. It simply takes a byref String and modifies it.

The following example illustrates how to call the VB DLL so that the modified String is not lost.

That is really all there is to it. Again, you shouldn't have to use this feature very often. But when the API is designed around byref parameters, this can be a lifesaver!