upvar(TCL)

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. upvar may only be invoked from within procedures. 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.