raw/mann/upvar.n

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   8 '\" RCS: @(#) $Id: upvar.n,v 1.1 2003/12/20 03:31:54 bbbush Exp $
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  10 '\" The definitions below are for supplemental macros used in Tcl/Tk
  11 '\" manual entries.
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 208 .SH "STANDARD OPTIONS"
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 211 .ta 5.5c 11c
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 216 .fi
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 218 .LP
 219 See the \\fBoptions\\fR manual entry for details on the standard options.
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 225 .ta 4c
 226 Command-Line Name:      \\fB\\$1\\fR
 227 Database Name:  \\fB\\$2\\fR
 228 Database Class: \\fB\\$3\\fR
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 246 .TH upvar n "" Tcl "Tcl Built-In Commands"
 247 .BS
 248 '\" Note:  do not modify the .SH NAME line immediately below!
 249 .SH NAME
 250 upvar \- Create link to variable in a different stack frame
 251 .SH SYNOPSIS
 252 \fBupvar \fR?\fIlevel\fR? \fIotherVar myVar \fR?\fIotherVar myVar \fR...?
 253 .BE
 254
 255 .SH DESCRIPTION
 256 .PP
 257 This command arranges for one or more local variables in the current
 258 procedure to refer to variables in an enclosing procedure call or
 259 to global variables.
 260 \fILevel\fR may have any of the forms permitted for the \fBuplevel\fR
 261 command, and may be omitted if the first letter of the first \fIotherVar\fR
 262 isn't \fB#\fR or a digit (it defaults to \fB1\fR).
 263 For each \fIotherVar\fR argument, \fBupvar\fR makes the variable
 264 by that name in the procedure frame given by \fIlevel\fR (or at
 265 global level, if \fIlevel\fR is \fB#0\fR) accessible
 266 in the current procedure by the name given in the corresponding
 267 \fImyVar\fR argument.
 268 The variable named by \fIotherVar\fR need not exist at the time of the
 269 call;  it will be created the first time \fImyVar\fR is referenced, just like
 270 an ordinary variable.  There must not exist a variable by the
 271 name \fImyVar\fR at the time \fBupvar\fR is invoked.
 272 \fIMyVar\fR is always treated as the name of a variable, not an
 273 array element.  Even if the name looks like an array element,
 274 such as \fBa(b)\fR, a regular variable is created.
 275 \fIOtherVar\fR may refer to a scalar variable, an array,
 276 or an array element.
 277 \fBUpvar\fR returns an empty string.
 278 .PP
 279 The \fBupvar\fR command simplifies the implementation of call-by-name
 280 procedure calling and also makes it easier to build new control constructs
 281 as Tcl procedures.
 282 For example, consider the following procedure:
 283 .CS
 284 \fBproc add2 name {
 285         upvar $name x
 286         set x [expr $x+2]
 287 }\fR
 288 .CE
 289 \fBAdd2\fR is invoked with an argument giving the name of a variable,
 290 and it adds two to the value of that variable.
 291 Although \fBadd2\fR could have been implemented using \fBuplevel\fR
 292 instead of \fBupvar\fR, \fBupvar\fR makes it simpler for \fBadd2\fR
 293 to access the variable in the caller's procedure frame.
 294 .PP
 295 \fBnamespace eval\fR is another way (besides procedure calls)
 296 that the Tcl naming context can change.
 297 It adds a call frame to the stack to represent the namespace context.
 298 This means each \fBnamespace eval\fR command
 299 counts as another call level for \fBuplevel\fR and \fBupvar\fR commands.
 300 For example, \fBinfo level 1\fR will return a list
 301 describing a command that is either
 302 the outermost procedure call or the outermost \fBnamespace eval\fR command.
 303 Also, \fBuplevel #0\fR evaluates a script
 304 at top-level in the outermost namespace (the global namespace).
 305 .PP
 306 .VS
 307 If an upvar variable is unset (e.g. \fBx\fR in \fBadd2\fR above), the
 308 \fBunset\fR operation affects the variable it is linked to, not the
 309 upvar variable.  There is no way to unset an upvar variable except
 310 by exiting the procedure in which it is defined.  However, it is
 311 possible to retarget an upvar variable by executing another \fBupvar\fR
 312 command.
 313
 314 .SH Traces and upvar
 315 .PP
 316 Upvar interacts with traces in a straightforward but possibly
 317 unexpected manner.  If a variable trace is defined on \fIotherVar\fR, that
 318 trace will be triggered by actions involving \fImyVar\fR.  However,
 319 the trace procedure will be passed the name of \fImyVar\fR, rather
 320 than the name of \fIotherVar\fR.  Thus, the output of the following code
 321 will be \fBlocalVar\fR rather than \fBoriginalVar\fR:
 322 .CS
 323 \fBproc traceproc { name index op } {
 324         puts $name
 325 }
 326 proc setByUpvar { name value } {
 327         upvar $name localVar
 328         set localVar $value
 329 }
 330 set originalVar 1
 331 trace variable originalVar w traceproc
 332 setByUpvar originalVar 2
 333 }\fR
 334 .CE
 335
 336 If \fIotherVar\fR refers to an element of an array, then variable
 337 traces set for the entire array will not be invoked when \fImyVar\fR
 338 is accessed (but traces on the particular element will still be
 339 invoked).  In particular, if the array is \fBenv\fR, then changes
 340 made to \fImyVar\fR will not be passed to subprocesses correctly.
 341 .VE
 342
 343 .SH "SEE ALSO"
 344 global(n), namespace(n), uplevel(n), variable(n)
 345
 346 .SH KEYWORDS
 347 context, frame, global, level, namespace, procedure, variable