'\"
'\" Copyright (c) 1993 The Regents of the University of California.
'\" Copyright (c) 1994-1997 Sun Microsystems, Inc.
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\"
.TH upvar n "" Tcl "Tcl Built-In Commands"
.so man.macros
.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 (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:
.PP
.CS
proc \fIadd2\fR name {
    \fBupvar\fR $name x
    set x [expr {$x + 2}]
}
.CE
.PP
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 \fIadd2\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\fR \fB1\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" :
.PP
.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 add variable originalVar write \fItraceproc\fR
\fIsetByUpvar\fR originalVar 2
.CE
.PP
If \fIotherVar\fR refers to an element of an array, then the element
name is passed as the second argument to the trace procedure. This
may be important information in case of traces set on an entire array.
.SH EXAMPLE
A \fBdecr\fR command that works like \fBincr\fR except it subtracts
the value from the variable instead of adding it:
.PP
.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, upvar, variable
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