behind the scenes in passing byref or byVal

Originally Posted by whburling

It is commonly explained that passing an object ByRef passes a pointer to an object and passing an object ByVal passes a copy of the object.

Reading your post, I don't think you mean "object." I think you mean "scalar variable." In computer science it is important to use precise language. Passing Object and array variables ByRef vs. ByVal is a whole discussion unto itself, which I'll be happy to get into if you are interested.

By Reference and By Value are passing mechanisms that are not unique to VBA, they are terms used for any language, although language support varies. For example, ALGOL also had By Result and By Value-Result. In C, the default is by value, and a symbol (not keyword) is needed to indicate a pass by reference.

The default mode in VBA is By Reference, so a pointer is passed. Any changes made in the called procedure to that variable will affect the caller. You may also explicitly specify ByRef.

Using ByVal creates a copy of the variable in the stack frame of the called procedure so that any changes to it are local to that procedure.

I am wondering if in passing ByVal, VBA actually copies the object before a subroutine is enterred and then VBA passes a pointer to that copied object

You are really getting down into the weeds here. At some level everything is a pointer. So don't think of it like this.

Another interesting thought is that it might be nice to be able to pass a reference but forbid changes.

Interesting indeed but I do not know of a language that does this. There really isn't any advantage to it because:

To me that might speed up execution compared to passing by value if one desires only to read

Probably not. Creating a copy of a pointer to a variable in the stack frame is not really any faster than creating a copy of the variable itself.

trying to identify who is changing the global variable (that was not supposed to be changed) might be tricky if the project is large?

People often use global variables because it's easier to code, but it's usually not a good practice. (Note that I did not say "always.") This is what was identified by Ed Yourdoun back in 1979 as "common environment coupling" and causes exactly the problem you have described. Bugs can be very difficult to diagnose because when a global variable is determined to have an unexpected value in one place, it can be extremely tedious to determine where and when this value was assigned. Two different code modules may make conflicting assumptions about how this variable is to be handled. There are loads of opportunities for things to go wrong.

Originally Posted by whburling

In fact, my question was really focused on the issue of objects being passed ByRef or ByVal.

Before I even read and think about and respond to the rest of this let me address this one sentence.

An object variable is itself a pointer that points to the state of the object. So if you pass an object ByVal, you are passing a copy of the pointer, but it is still pointing to the same object as the caller. This is completely different than how passing scalar variables works.

With regard to your original statement

It is commonly explained that passing an object ByRef passes a pointer to an object and passing an object ByVal passes a copy of the object.

This is 100% incorrect.

Here is an illustration of what happens if you pass an object by value and by reference.

Module code for class IntegerContainer:

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Test code:

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Output of test code:

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I wrote the above pseudo code in a manner in which the worksheet is passed ByVal as I did
Not want to alter the worksheet data and it seems the acceptable way to prevent changing of original data
in VBA is to pass an object ByVal (copy). It seems significantly wasteful to copy, four times, a worksheet that
has extensive data within it. Imagine a building having 244 rooms. That means the worksheet would have to be
copied 976 times.

That is not how it works. A worksheet is an object, so if you pass a Worksheet object variable ByVal, you are just passing a copy of the reference to the worksheet, not a copy of the entire worksheet.

Also, I would make Sub A a method of Room_Class, and pass in the actual data from the worksheet rather than passing the worksheet itself and allowing A to have knowledge of how the worksheet is organized. That information should be hidden from Sub A.

Originally Posted by whburling

It is commonly explained that passing an object ByRef passes a pointer to an object and passing an object ByVal passes a copy of the object.

Not a copy of the object itself, but a different pointer to it. That simply means that any changes made by the called procedure are still made to the object, but you cannot make the original pointer refer to a different object. So, for example, if you pass a ByRef variable that refers to a worksheet, the called routine can both alter the worksheet itself and change the passed variable to refer to a different worksheet and that will affect the calling procedure too. If you pass the same variable ByVal, the called code can still change the worksheet itself and can make the pointer refer to a different sheet but that will not affect the calling code.

For example:

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This is why event procedures pass objects byval - there can be multiple sinks for the event(s) of a given object and they should all work with the same object arguments.