Is Variable Declared?
 

Is there any way to determine in a script if a variable has been declared or not? I want to be able to declare a variable using a macro, but only if it hasn't already been declared.

Variables aren't so much declared as that they either contain data, or they don't. So you could just use :
StrCmp $Var "" +2 0
MessageBox MB_OK "Var is 'declared'"

Yes but if a variable hasn't been declared from within a macro, then that will throw a compiler warning.
You should define a constant of the same name of that variable and check if that constant exists with !ifdef.

E.g.

code:
!macro Var VarName

 Var "${Var}"

 !define "${Var}"

!macroend



!insertmacro Var MyVar



Function MyFunc

!ifdef MyVar

 StrCpy $MyVar "blah"

!endif

FunctionEnd

-Stu

ah, good point!

code:
!ifndef VAR_DECLARED

  !define VAR_DECLARED

  Var VAR_NAME

!endif

-dandaman32

Thanks for the suggestions. That's certainly a clever and viable workaround!

But not exactly what I was looking for as it pollutes the macro namespace and is more apt to cause collisions when using headers from multiple sources. :(

OTOH, I rearchitected what I was doing so I didn't end up needing it anyway. At least not this time. :)