What is the difference between the VarIsEmpty and VarIsEmptyParam functions

delphi, delphi-7, variant

Solution

In COM it is possible to have optional parameters in a method call at any position, while in Delphi this is only possible at the end. So if you want to omit the parameter you can write `EmptyParam` instead. `EmptyParam` is a global variable initialized with the correct values.

Now when you are implementing a COM interface you have to deal with these optional parameters, too. The way to find out these omitted parameters is `VarIsEmptyParam`.

Note that even an empty variant given as a parameter yields VarIsEmptyParam = false, because the param is not omitted. It is just empty, but it is there.

So normally there is:

VarIsEmpty(v) ==> not VarIsEmptyParam(v)

and

VarIsEmptyParam(v) ==> not VarIsEmpty(v)

Problem

Working in Delphi7 just now, I noticed that not only a `VarIsEmpty` function exists, but also a `VarIsEmptyParam`. Since the help of Delphi does not give much explanation: VarIsEmptyParam returns true if the given variant represents an unassigned optional parameter. If the variant contains any other value, the function result is false. I was just wondering if anyone has used this function, and if so, how this function is meant to be used.

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