ByRef vs ByVal in ASP Classic
In ASP Classic, there are two ways that a variable can be passed to
a Sub Routine (or Function), ByRef or ByVal. The differences between
them are shown in the table below:
| ByRef |
ByVal |
| By Reference |
By Value |
| A reference to the actual data in memory |
A copy of the actual data |
| Faster because it references the actual data at a given memory
location |
Slower because it involves copying the actual data |
| Passing a variable to a Sub Routine ByRef means that the data stored
in the variable can be permanently changed. So, if you want the change
made to a variable in a Sub Routine to persist, use ByRef. |
A new instance of the variable is created and given to the Sub
Routine. Any changes made to the value of this variable in the Sub
Routine
have no effect on the value of the original variable passed in. |
| Can permanently alter the data stored in the variable that was passed in. |
Cannot permanently alter the data stored in the variable that was passed in. |
| ByRef is the actual data, and
can affect the actual data. |
ByVal is a copy of the actual data, and can't affect the actual
data. |
| The default in VB Script |
The default in ASP.net |
Demo Script:
<%
Sub AddToValues(ByRef iSnakes, ByVal iRabbits, ByRef iOtters)
iSnakes = iSnakes + 1
iRabbits = iRabbits + 1
iOtters = iRabbits
End Sub
Dim iSnakes, iRabbits
'assign initial values
iSnakes = 1
iRabbits = 1
Dim iOtters
iOtters = 0
'call the Sub
AddToValues iSnakes, iRabbits, iOtters
%>
<
!--display variable values after the call to the Sub-->
iSnakes value is now <%=iSnakes%><br>
iRabbits value is now <%=iRabbits%><br>
iOtters value is now <%=iOtters%><br>
<br>
Explanation:<br>
iSnakes starts off with a value of 1, gets passed into the Sub Routine
ByRef, meaning the actual data is being used, gets incremented in
the Sub Routine by 1, giving 2. As the actual data is being used, the value
of iSnakes is permanently changed to 2, so this is the value we see
when we display iSnakes later.<br><br>
iRabbits starts off with a value of 1, gets passed into the Sub Routine
ByVal, meaning a copy of the data is created and used by the Sub Routine,
is incremented by 1 in the Sub Routine giving 2 in the Sub Routine
(as proven later by iOtters), but this only applies in the Sub Routine
and the actual data is not affected, so when iRabbits is displayed
later, its value is still 1.<br><br>
iOtters just proves that the value of iRabbits did indeed reach 2
whilst it was in the Sub Routine. iOtters has an initial value of 0,
it's passed into the Sub Routine ByRef so we are dealing with the actual
data for iOtters and it can be permanently changed. In the Sub Routine
first we have incremented iRabbits giving 2, then we set iOtters to
iRabbits, so iOtters has a value of 2, and this changes the actual
iOtters data permanently, so we see a value of 2 for iOtters when it
is displayed later.
Run the Demo Script (in a new window)
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