source: downloads/tcl8.5.2/doc/upvar.n @ 25

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'\" Copyright (c) 1993 The Regents of the University of California.
'\" Copyright (c) 1994-1997 Sun Microsystems, Inc.
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'\" RCS: @(#) $Id: upvar.n,v 1.16 2007/12/13 15:22:33 dgp Exp $
'\" 
.so man.macros
.TH upvar n "" Tcl "Tcl Built-In Commands"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
upvar \- Create link to variable in a different stack frame
.SH SYNOPSIS
\fBupvar \fR?\fIlevel\fR? \fIotherVar myVar \fR?\fIotherVar myVar \fR...?
.BE

.SH DESCRIPTION
.PP
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.
\fILevel\fR may have any of the forms permitted for the \fBuplevel\fR
command, and may be omitted if the first letter of the first \fIotherVar\fR
is not \fB#\fR or a digit (it defaults to \fB1\fR).
For each \fIotherVar\fR argument, \fBupvar\fR makes the variable
by that name in the procedure frame given by \fIlevel\fR (or at
global level, if \fIlevel\fR is \fB#0\fR) accessible
in the current procedure by the name given in the corresponding
\fImyVar\fR argument.
The variable named by \fIotherVar\fR need not exist at the time of the
call;  it will be created the first time \fImyVar\fR is referenced, just like
an ordinary variable.  There must not exist a variable by the
name \fImyVar\fR at the time \fBupvar\fR is invoked.
\fIMyVar\fR 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 \fBa(b)\fR.
\fIOtherVar\fR may refer to a scalar variable, an array,
or an array element.
\fBUpvar\fR returns an empty string.
.PP
The \fBupvar\fR 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:
.CS
proc \fIadd2\fR name {
   \fBupvar\fR $name x
   set x [expr {$x + 2}]
}
.CE
If \fIadd2\fR is invoked with an argument giving the name of a variable,
it adds two to the value of that variable.
Although \fIadd2\fR could have been implemented using \fBuplevel\fR
instead of \fBupvar\fR, \fBupvar\fR makes it simpler for \fBadd2\fR
to access the variable in the caller's procedure frame.
.PP
\fBnamespace eval\fR 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 \fBnamespace eval\fR command
counts as another call level for \fBuplevel\fR and \fBupvar\fR commands.
For example, \fBinfo level 1\fR will return a list
describing a command that is either
the outermost procedure call or the outermost \fBnamespace eval\fR command.
Also, \fBuplevel #0\fR evaluates a script
at top-level in the outermost namespace (the global namespace).
.PP
If an upvar variable is unset (e.g. \fBx\fR in \fBadd2\fR above), the
\fBunset\fR 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 \fBupvar\fR
command.
.SH "TRACES AND UPVAR"
.PP
Upvar interacts with traces in a straightforward but possibly
unexpected manner.  If a variable trace is defined on \fIotherVar\fR, that
trace will be triggered by actions involving \fImyVar\fR.  However,
the trace procedure will be passed the name of \fImyVar\fR, rather
than the name of \fIotherVar\fR.  Thus, the output of the following code
will be
.QW "\fIlocalVar\fR"
rather than
.QW "\fIoriginalVar\fR" :
.CS
proc \fItraceproc\fR { name index op } {
   puts $name
}
proc \fIsetByUpvar\fR { name value } {
   \fBupvar\fR $name localVar
   set localVar $value
}
set originalVar 1
trace variable originalVar w \fItraceproc\fR
\fIsetByUpvar\fR originalVar 2
.CE
.PP
If \fIotherVar\fR refers to an element of an array, then variable
traces set for the entire array will not be invoked when \fImyVar\fR
is accessed (but traces on the particular element will still be
invoked).  In particular, if the array is \fBenv\fR, then changes
made to \fImyVar\fR will not be passed to subprocesses correctly.
.SH EXAMPLE
A \fBdecr\fR command that works like \fBincr\fR except it subtracts
the value from the variable instead of adding it:
.CS
proc decr {varName {decrement 1}} {
    \fBupvar\fR 1 $varName var
    incr var [expr {-$decrement}]
}
.CE

.SH "SEE ALSO"
global(n), namespace(n), uplevel(n), variable(n)

.SH KEYWORDS
context, frame, global, level, namespace, procedure, variable