Variable scope in VBScript functions

Question

I have a question about variable scope in VBScript. I know there's the following keywords (from autoitscript.com):

1. Dim = Local scope if the variable name doesn't already exist globally (in which case it reuses the global variable!)
2. Global = Forces creation of the variable in the Global scope
3. Local = Forces creation of the variable in the Local/Function scope

Imagine that I have the following .vbs file:

Dim strPath

strPath = "C:\folder"

DisplayPath strPath

Sub DisplayPath(strPath) 'Does this strPath get it's own local scope?
  MsgBox strPath
End Sub

In the function: DisplayPath(strPath), is strPath a local variable? Or do functions/subs have access to the strPath defined at the top of the main section of the script as a global variable?

Also, what's the point of explicitly using Dim versus just defining variables as I use them, which is possible in scripting languages?

Answer 1

The strPath in the DisplayPath procedure will be a new variable but not for the reasons you expect, there is subtle problem with your code that will cloud the issue.

When calling Sub procedure the VBScript syntax does not include parentheses. For example:-

Sub MyProc(Param1, Param2)
  '' # Do stuff
End Sub

MyProc("Hello", "World")

the above would result in a syntax error. It should be called:-

MyProc "Hello", "World"

Now when there is only one parameter a syntax error does not occur. This is because another use of parentheses is as part of an expression e.g. '(a + b) * c'. In the case of:-

DisplayPath(strPath)

VBScript resolves the "expression" (strPath) and pass the result to DisplayPath. Its this result that gives rise to new storage hold the result the expression.

Had you called with

DisplayPath strPath

no new created.

However what about this:-

Sub DisplayPath(something)
  MsgBox something
End Sub

There is still no new storage allocated. something will point at the same memory that strPath does.

Edit

The code below works:-

Dim strPath

strPath = "c:\folder"

Display


Sub Display()
  MsgBox strPath
End Sub

The declaration of strPath outside of a procedure causes it to have global scope.

As to the point of using explicit Dim what would happen if the assignment line above looked like this?

 strPath = "c:\folder"

A new variable called strPath would come into existence and strPath would remain empty. You should always begin your VBScript files with the line:-

Option Explicit

This will force you to explicitly Dim all variables to be used and will save you hours of debugging time.