upvar(3Tcl)

NAME
     upvar - Create link to variable in a different stack frame

SYNOPSIS
     upvar ?level? otherVar myVar ?otherVar myVar ...?


DESCRIPTION
     This command arranges for one or more local variables in the current
     procedure to refer to variables in an enclosing procedure call or to
     global variables.  Level may have any of the forms permitted for the
     uplevel command, and may be omitted if the first letter of the first
     otherVar isn't # or a digit (it defaults to 1).  For each otherVar
     argument, upvar makes the variable by that name in the procedure frame
     given by level (or at global level, if level is #0) accessible in the
     current procedure by the name given in the corresponding myVar argument.
     The variable named by otherVar need not exist at the time of the call;
     it will be created the first time myVar is referenced, just like an
     ordinary variable.  MyVar may not refer to an element of an array, but   |
     otherVar may refer to an array element.  Upvar returns an empty string.

     The upvar command simplifies the implementation of call-by-name procedure
     calling and also makes it easier to build new control constructs as Tcl
     procedures.  For example, consider the following procedure:

          proc add2 name {
              upvar $name x
              set x [expr $x+2]
          }

     Add2 is invoked with an argument giving the name of a variable, and it
     adds two to the value of that variable.  Although add2 could have been
     implemented using uplevel instead of upvar, upvar makes it simpler for
     add2 to access the variable in the caller's procedure frame.

     If an upvar variable is unset (e.g. x in add2 above), the unset operation|
     affects the variable it is linked to, not the upvar variable.  There is  |
     no way to unset an upvar variable except by exiting the procedure in     |
     which it is defined.  However, it is possible to retarget an upvar       |
     variable by executing another upvar command.


KEYWORDS
     context, frame, global, level, procedure, variable