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More On ByRef vs ByVal

  • Article

In my previous entry I discussed VBScript's various syntaxes for passing values by reference. However it occurs to me that there may be some confusion about what exactly "byref" and "byval" mean in JScript and VBScript. This is frequently a source of confusion, as VBScript has byref behaviour not supported by JScript.

The confusion arises because VBScript uses "by reference" to mean two similar, but different things. VBScript supports reference types and variable references. JScript supports the former but not the latter.

The best way to illustrate the difference is with an example. Consider this class:

Class Foo
Public Bar
End Class

Now we can create an instance of this class:

Dim Blah, Baz
Set Blah = New Foo
Set Baz = Blah
Blah.Bar = 123

Both Blah and Baz are references to the same object. The fourth line changes both Blah.Bar and Baz.Bar because these are different names for the same thing.

That's the "reference type" feature. We say that VBScript treats objects as reference types.

Now consider this little program:

Sub Change(ByRef XYZ)
XYZ = 5
End Sub
Dim ABC
ABC = 123
Change ABC

This passes a reference to variable ABC. The local variable XYZ becomes an alias for ABC, so the assignment XYZ = 5 changes ABC as well.

JScript has reference types -- all object types are reference types. But JScript does not support variable references. There is no way for one scope to change the value of a variable in another scope.

UPDATE: articles on related topics can be found here:

https://blogs.msdn.com/ericlippert/archive/2003/09/15/52996.aspx

https://blogs.msdn.com/ericlippert/archive/2003/09/15/53006.aspx

https://blogs.msdn.com/ericlippert/archive/2003/09/29/53117.aspx

Comments

  • Anonymous
    February 18, 2004
    I was talking about reference types vs. by-reference variables a while back. Recall that both JScript and VBScript have reference types (ie, objects) but JScript does not have by-reference variables. COM supports passing variable references around, but unfortunately the intersection of early-bound COM and late-bound IDispatch is a little bit goofy.
  • Anonymous
    July 04, 2006
    The comment has been removed
  • Anonymous
    January 21, 2009
    PingBack from http://www.keyongtech.com/1945642-byval-was-it-ever-the