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

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

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

Tcl                                                                   upvar(n)