do we need to "unset" variables in TCL ?

Question

Is it a requirement of good TCL code? What would happen if we don't use the "unset" keyword in a script? Any ill-effects I should know about?

I'm inheriting some legacy code and the errors that come about due to "unset"-ing non-existent variables are driving me up the wall!

Answer 1

It's possible to determine whether a variable exists before using it, using the info exists command. Be sure that if you're not using unset, that you don't upset the logic of the program somewhere else.

There's no Tcl-specific reason to unset a variable, that is, it's not going to cause a memory leak or run out of variable handles or anything crazy like that. Using unset may be a defensive programming practice, because it prevents future use of a variable after it's no longer relevant. Without knowing more about the exact code you're working with, it's hard to give more detailed info.

Answer 2

In addition to the other responses, if your Tcl version is new enough, you can also use:

unset -nocomplain foo

That'll unset foo if it exists, but won't complain if it doesn't.

Jeff

Answer 3

For note as I don't seem able to comment on the "info exists" above;

I use this form often..

if { [info exists pie] && [$pie == "ThisIsWhatIWantInPie"]} {
    puts "I found what I wanted in pie."
} else {
    puts "Pie did not exist; but I still did not error,TCL's evaluation \
          will see the conditional failed on the [info exists] and not \
          continue onto the comparison."
}