'\"
'\" 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.
'\" 
'\" RCS: @(#) $Id: upvar.n,v 1.1.1.1 2007/07/10 15:04:23 duncan Exp $
'\" 
'\" The definitions below are for supplemental macros used in Tcl/Tk
'\" manual entries.
'\"
'\" .AP type name in/out ?indent?
'\"	Start paragraph describing an argument to a library procedure.
'\"	type is type of argument (int, etc.), in/out is either "in", "out",
'\"	or "in/out" to describe whether procedure reads or modifies arg,
'\"	and indent is equivalent to second arg of .IP (shouldn't ever be
'\"	needed;  use .AS below instead)
'\"
'\" .AS ?type? ?name?
'\"	Give maximum sizes of arguments for setting tab stops.  Type and
'\"	name are examples of largest possible arguments that will be passed
'\"	to .AP later.  If args are omitted, default tab stops are used.
'\"
'\" .BS
'\"	Start box enclosure.  From here until next .BE, everything will be
'\"	enclosed in one large box.
'\"
'\" .BE
'\"	End of box enclosure.
'\"
'\" .CS
'\"	Begin code excerpt.
'\"
'\" .CE
'\"	End code excerpt.
'\"
'\" .VS ?version? ?br?
'\"	Begin vertical sidebar, for use in marking newly-changed parts
'\"	of man pages.  The first argument is ignored and used for recording
'\"	the version when the .VS was added, so that the sidebars can be
'\"	found and removed when they reach a certain age.  If another argument
'\"	is present, then a line break is forced before starting the sidebar.
'\"
'\" .VE
'\"	End of vertical sidebar.
'\"
'\" .DS
'\"	Begin an indented unfilled display.
'\"
'\" .DE
'\"	End of indented unfilled display.
'\"
'\" .SO
'\"	Start of list of standard options for a Tk widget.  The
'\"	options follow on successive lines, in four columns separated
'\"	by tabs.
'\"
'\" .SE
'\"	End of list of standard options for a Tk widget.
'\"
'\" .OP cmdName dbName dbClass
'\"	Start of description of a specific option.  cmdName gives the
'\"	option's name as specified in the class command, dbName gives
'\"	the option's name in the option database, and dbClass gives
'\"	the option's class in the option database.
'\"
'\" .UL arg1 arg2
'\"	Print arg1 underlined, then print arg2 normally.
'\"
'\" RCS: @(#) $Id: man.macros,v 1.1.1.1 2007/07/10 15:04:23 duncan Exp $
'\"
'\"	# Set up traps and other miscellaneous stuff for Tcl/Tk man pages.
.if t .wh -1.3i ^B
.nr ^l \n(.l
.ad b
'\"	# Start an argument description
.de AP
.ie !"\\$4"" .TP \\$4
.el \{\
.   ie !"\\$2"" .TP \\n()Cu
.   el          .TP 15
.\}
.ta \\n()Au \\n()Bu
.ie !"\\$3"" \{\
\&\\$1	\\fI\\$2\\fP	(\\$3)
.\".b
.\}
.el \{\
.br
.ie !"\\$2"" \{\
\&\\$1	\\fI\\$2\\fP
.\}
.el \{\
\&\\fI\\$1\\fP
.\}
.\}
..
'\"	# define tabbing values for .AP
.de AS
.nr )A 10n
.if !"\\$1"" .nr )A \\w'\\$1'u+3n
.nr )B \\n()Au+15n
.\"
.if !"\\$2"" .nr )B \\w'\\$2'u+\\n()Au+3n
.nr )C \\n()Bu+\\w'(in/out)'u+2n
..
.AS Tcl_Interp Tcl_CreateInterp in/out
'\"	# BS - start boxed text
'\"	# ^y = starting y location
'\"	# ^b = 1
.de BS
.br
.mk ^y
.nr ^b 1u
.if n .nf
.if n .ti 0
.if n \l'\\n(.lu\(ul'
.if n .fi
..
'\"	# BE - end boxed text (draw box now)
.de BE
.nf
.ti 0
.mk ^t
.ie n \l'\\n(^lu\(ul'
.el \{\
.\"	Draw four-sided box normally, but don't draw top of
.\"	box if the box started on an earlier page.
.ie !\\n(^b-1 \{\
\h'-1.5n'\L'|\\n(^yu-1v'\l'\\n(^lu+3n\(ul'\L'\\n(^tu+1v-\\n(^yu'\l'|0u-1.5n\(ul'
.\}
.el \}\
\h'-1.5n'\L'|\\n(^yu-1v'\h'\\n(^lu+3n'\L'\\n(^tu+1v-\\n(^yu'\l'|0u-1.5n\(ul'
.\}
.\}
.fi
.br
.nr ^b 0
..
'\"	# VS - start vertical sidebar
'\"	# ^Y = starting y location
'\"	# ^v = 1 (for troff;  for nroff this doesn't matter)
.de VS
.if !"\\$2"" .br
.mk ^Y
.ie n 'mc \s12\(br\s0
.el .nr ^v 1u
..
'\"	# VE - end of vertical sidebar
.de VE
.ie n 'mc
.el \{\
.ev 2
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.ti 0
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\h'|\\n(^lu+3n'\L'|\\n(^Yu-1v\(bv'\v'\\n(^tu+1v-\\n(^Yu'\h'-|\\n(^lu+3n'
.sp -1
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.ev
.\}
.nr ^v 0
..
'\"	# Special macro to handle page bottom:  finish off current
'\"	# box/sidebar if in box/sidebar mode, then invoked standard
'\"	# page bottom macro.
.de ^B
.ev 2
'ti 0
'nf
.mk ^t
.if \\n(^b \{\
.\"	Draw three-sided box if this is the box's first page,
.\"	draw two sides but no top otherwise.
.ie !\\n(^b-1 \h'-1.5n'\L'|\\n(^yu-1v'\l'\\n(^lu+3n\(ul'\L'\\n(^tu+1v-\\n(^yu'\h'|0u'\c
.el \h'-1.5n'\L'|\\n(^yu-1v'\h'\\n(^lu+3n'\L'\\n(^tu+1v-\\n(^yu'\h'|0u'\c
.\}
.if \\n(^v \{\
.nr ^x \\n(^tu+1v-\\n(^Yu
\kx\h'-\\nxu'\h'|\\n(^lu+3n'\ky\L'-\\n(^xu'\v'\\n(^xu'\h'|0u'\c
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..
'\"	# DS - begin display
.de DS
.RS
.nf
.sp
..
'\"	# DE - end display
.de DE
.fi
.RE
.sp
..
'\"	# SO - start of list of standard options
.de SO
.SH "STANDARD OPTIONS"
.LP
.nf
.ta 5.5c 11c
.ft B
..
'\"	# SE - end of list of standard options
.de SE
.fi
.ft R
.LP
See the \\fBoptions\\fR manual entry for details on the standard options.
..
'\"	# OP - start of full description for a single option
.de OP
.LP
.nf
.ta 4c
Command-Line Name:	\\fB\\$1\\fR
Database Name:	\\fB\\$2\\fR
Database Class:	\\fB\\$3\\fR
.fi
.IP
..
'\"	# CS - begin code excerpt
.de CS
.RS
.nf
.ta .25i .5i .75i 1i
..
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.de CE
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..
.de UL
\\$1\l'|0\(ul'\\$2
..
.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
isn't \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.  Even if the name looks like an array element,
such as \fBa(b)\fR, a regular variable is created.
\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 add2 name {
   \fBupvar\fR $name x
   set x [expr $x+2]
}
.CE
\fBadd2\fR is invoked with an argument giving the name of a variable,
and it adds two to the value of that variable.
Although \fBadd2\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
.VS
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 \fBlocalVar\fR rather than \fBoriginalVar\fR:
.CS
proc \fBtraceproc\fR { name index op } {
   puts $name
}
proc \fBsetByUpvar\fR { name value } {
   \fBupvar\fR $name localVar
   set localVar $value
}
set originalVar 1
trace variable originalVar w \fBtraceproc\fR
\fBsetByUpvar\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.
.VE
.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