VBScript Constants Are Not Hoisted

re: VBScript Constants Are Not Hoisted

Jiho Han — Wed, 08 Dec 2004 14:13:00 GMT

So in these statements,

print TypeName(c)
print Eval("TypeName(c)")
Const c = 10

So on which line does the slot 'c' get created? on the first or the second? or is it both lines since they are in a different context?

re: VBScript Constants Are Not Hoisted

Mat Hall — Wed, 08 Dec 2004 14:54:00 GMT

I think we should go back to line numbers. At least then you KNOW what order things are going to happen in!

(Unless you use FORTRAN, in which case line numbers make no difference to execution order.)

re: VBScript Constants Are Not Hoisted

Eric Lippert — Wed, 08 Dec 2004 16:41:00 GMT

The second. As you can see from the code gen, the first line is exactly equivalent to 'print typename(10)'.

re: VBScript Constants Are Not Hoisted

Nicholas Allen — Wed, 08 Dec 2004 22:03:00 GMT

From a design standpoint, was there an advantage to not hoisting the allocation of a slot for a constant to the top of scope? Or did noone notice that it was a problem until it was too late to go and change it?

By the way, thanks for the conversion help the other day. By bad design indeed, unfortunately.

re: VBScript Constants Are Not Hoisted

Eric Lippert — Wed, 08 Dec 2004 23:07:00 GMT

I'm pretty sure that this was a "no one noticed it" issue. But that particular design decision predates my time here, so I can't be sure.

re: VBScript Constants Are Not Hoisted

DWalker — Wed, 08 Dec 2004 23:46:00 GMT

Shouldn't "Call Print 1" be "Call Print 10" instead? Otherwise, something is REALLY wrong.

This, on the other hand

print c
Const c = 10

generates this code:

Bos 0
IntConst 10
Call 'print' 1
Bos 1
IntConst 10
ConstSt 'c'

re: VBScript Constants Are Not Hoisted

Eric Lippert — Thu, 09 Dec 2004 00:02:00 GMT

> Shouldn't "Call Print 1" be "Call Print 10"

No. Read my comments above again:

LocalAdr 1 put reference to slot 1 on stack
Call 'print' 1 call print with one argument

So "IntConst 10" puts 10 on the argument stack, "Call print 1" calls print and passes one argument from the argument stack.

If it had been "print 10, 20" then it would have been

IntConst 10
IntConst 20
Call 'print' 2

Make sense?

re: VBScript Constants Are Not Hoisted

Anthonyt — Fri, 12 Aug 2005 19:37:52 GMT

I want to do this:

dim strValue
strValue = "Hello World"
const MY_CONST = "Hi" & strValue

But it doesn't work. Is there a way around it? I need to take the string value of the variable and put it into the constant.

re: VBScript Constants Are Not Hoisted

Eric Lippert — Wed, 17 Aug 2005 09:34:02 GMT

Read the link "how constants work in VBScript" above for the answer to your question.

re: VBScript Constants Are Not Hoisted

Seamus — Mon, 26 Feb 2007 21:22:11 GMT

Thanks so much for posting this info on how constants are evaluated, it's been very very useful. Given that constants are not hoisted and they are handled using very limited constant folding, is there any possibility that same named constants could unintentionally get their values crossed? For example:

Given the following .asp files

common/common.asp:

--------------

Sub printConstValue

response.write "Const Value: " & MY_CONST

End Sub

app_A/page_A.asp:

-----------------

const MY_CONST = "A"

app_A/page_A1.asp:

-----------------

call printConstValue

app_B/page_B.asp:

-----------------

const MY_CONST = "B"

app_B/page_B1.asp:

-----------------

call printConstValue

Sometimes calling printconstValue from page B will print out const value declared in file A? I have situation similar to this and I've been scratching my head trying to figure out why every now and then the values of the constants are getting "crossed".