UpvarX

NAME

UpvarX — Do upvar. If made new variable, then set to default.

SYNOPSIS

UpvarX VarName Var ?DefaultValue?

DESCRIPTION

Return Value

None.

Arguments

VarName	Name of variable to find/create in caller.
Var	Name of variable to use locally.
DefaultValue	Optional. Value to set a newly created variable to. Defaults to empty string.

More Info

This command was written to be able to write procs that have behavior similar to incr and append. When you use those commands, you can give it the name of a variable that does not yet exist and one will be created for you (and initialized to a default if you choose).

For information regarding exceptions / errors, see here.

EXAMPLES

The following example shows use of UpvarX in a proc that will create a variable and then immediately read it (a case where upvar will not work).

proc MySetZeroIfEmpty {VarName} {
     UpvarX $VarName Var
     if {[IsEmpty $Var]} {
          set Var 0
     }
     
     return $Var
}

% unset -nocomplain NewVariable
% MySetZeroIfEmpty NewVariable
0
% puts $NewVariable
0