upvar

upvar(3tcl)                   Tcl Built-In Commands                  upvar(3tcl)



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

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



Tcl                                                                  upvar(3tcl)