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upvar(n)                                    Tcl Built-In Commands                                   upvar(n)


       upvar - Create link to variable in a different stack frame

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

       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 is not # or a digit
       (it defaults to 1).  For each otherVar argument, upvar makes the variable by that name in the  proce-dure procedure
       dure 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.  There must not exist a variable by the name myVar at the time upvar is invoked.  MyVar  is
       always  treated  as  the  name of a variable, not an array element.  An error is returned if the name
       looks like an array element, such as a(b).  OtherVar may refer to a scalar variable, an array, or  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 proce-dure: 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.

       Upvar interacts with traces in a straightforward but possibly unexpected manner.  If a variable trace
       is defined on otherVar, that trace will be triggered by actions involving myVar.  However, the  trace
       procedure  will  be  passed the name of myVar, rather than the name of otherVar.  Thus, the output of
       the following code will be "localVar" rather than "originalVar":
              proc traceproc { name index op } {
                 puts $name
              proc setByUpvar { name value } {
                 upvar $name localVar
                 set localVar $value
              set originalVar 1
              trace variable originalVar w traceproc
              setByUpvar originalVar 2

       If otherVar refers to an element of an array, then variable traces set for the entire array will  not
       be  invoked  when myVar is accessed (but traces on the particular element will still be invoked).  In
       particular, if the array is env, then changes made to myVar will not be passed to  subprocesses  cor-rectly. correctly.

       A decr command that works like incr except it subtracts the value from the variable instead of adding
              proc decr {varName {decrement 1}} {
                  upvar 1 $varName var
                  incr var [expr {-$decrement}]

       global(n), namespace(n), uplevel(n), variable(n)

       context, frame, global, level, namespace, procedure, variable

Tcl                                                                                                 upvar(n)

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