NAME

       upvar - Create link to variable in a different stack frame

SYNOPSIS

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

TRACES AND UPVAR

       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 correctly.

EXAMPLE

       A decr command that works like incr except  it  subtracts  the  value  from  the  variable
       instead of adding it:

              proc decr {varName {decrement 1}} {
                  upvar 1 $varName var
                  incr var [expr {-$decrement}]
              }

SEE ALSO

       global(3tcl), namespace(3tcl), uplevel(3tcl), variable(3tcl)

KEYWORDS

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