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}] }
If add2 is invoked with an argument giving the name of a variable, 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.
namespace eval is another way (besides procedure calls) that the Tcl naming context can change. It adds a call frame to the stack to represent the namespace context. This means each namespace eval command counts as another call level for uplevel and upvar commands. For example, info level 1 will return a list describing a command that is either the outermost procedure call or the outermost namespace eval command. Also, uplevel #0 evaluates a script at top-level in the outermost namespace (the global namespace).
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.
proc traceproc { name index op } { puts $name } proc setByUpvar { name value } { upvar $name localVar set localVar $value } set originalVar 1 trace add variable originalVar write traceproc setByUpvar originalVar 2
If otherVar refers to an element of an array, then the element name is passed as the second argument to the trace procedure. This may be important information in case of traces set on an entire array.
proc decr {varName {decrement 1}} { upvar 1 $varName var incr var [expr {-$decrement}] }