81 lines
3.1 KiB
Plaintext
81 lines
3.1 KiB
Plaintext
'\"
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'\" Copyright (c) 1993 The Regents of the University of California.
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'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
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'\"
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'\" See the file "license.terms" for information on usage and redistribution
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'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
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'\"
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'\" SCCS: @(#) upvar.n 1.14 96/03/25 20:27:03
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'\"
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.so man.macros
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.TH upvar n "" Tcl "Tcl Built-In Commands"
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.BS
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'\" Note: do not modify the .SH NAME line immediately below!
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.SH NAME
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upvar \- Create link to variable in a different stack frame
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.SH SYNOPSIS
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\fBupvar \fR?\fIlevel\fR? \fIotherVar myVar \fR?\fIotherVar myVar \fR...?
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.BE
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.SH DESCRIPTION
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.PP
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This command arranges for one or more local variables in the current
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procedure to refer to variables in an enclosing procedure call or
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to global variables.
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\fILevel\fR may have any of the forms permitted for the \fBuplevel\fR
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command, and may be omitted if the first letter of the first \fIotherVar\fR
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isn't \fB#\fR or a digit (it defaults to \fB1\fR).
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For each \fIotherVar\fR argument, \fBupvar\fR makes the variable
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by that name in the procedure frame given by \fIlevel\fR (or at
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global level, if \fIlevel\fR is \fB#0\fR) accessible
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in the current procedure by the name given in the corresponding
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\fImyVar\fR argument.
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The variable named by \fIotherVar\fR need not exist at the time of the
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call; it will be created the first time \fImyVar\fR is referenced, just like
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an ordinary variable. There must not exist a variable by the
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name \fImyVar\fR at the time \fBupvar\fR is invoked.
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.VS
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\fIMyVar\fR is always treated as the name of a variable, not an
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array element. Even if the name looks like an array element,
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such as \fBa(b)\fR, a regular variable is created.
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\fIOtherVar\fR may refer to a scalar variable, an array,
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or an array element.
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.VE
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\fBUpvar\fR returns an empty string.
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.PP
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The \fBupvar\fR command simplifies the implementation of call-by-name
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procedure calling and also makes it easier to build new control constructs
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as Tcl procedures.
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For example, consider the following procedure:
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.CS
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\fBproc add2 name {
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upvar $name x
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set x [expr $x+2]
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}\fR
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.CE
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\fBAdd2\fR is invoked with an argument giving the name of a variable,
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and it adds two to the value of that variable.
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Although \fBadd2\fR could have been implemented using \fBuplevel\fR
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instead of \fBupvar\fR, \fBupvar\fR makes it simpler for \fBadd2\fR
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to access the variable in the caller's procedure frame.
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.PP
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.VS
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If an upvar variable is unset (e.g. \fBx\fR in \fBadd2\fR above), the
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\fBunset\fR operation affects the variable it is linked to, not the
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upvar variable. There is no way to unset an upvar variable except
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by exiting the procedure in which it is defined. However, it is
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possible to retarget an upvar variable by executing another \fBupvar\fR
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command.
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.SH BUGS
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.PP
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If \fIotherVar\fR refers to an element of an array, then variable
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traces set for the entire array will not be invoked when \fImyVar\fR
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is accessed (but traces on the particular element will still be
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invoked). In particular, if the array is \fBenv\fR, then changes
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made to \fImyVar\fR will not be passed to subprocesses correctly.
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.VE
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.SH KEYWORDS
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context, frame, global, level, procedure, variable
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