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       itclvars - variables used by [incr Tcl]


       The  following  global  variables  are created and managed automatically by the [incr Tcl]
       library.  Except where noted below, these variables should normally be  treated  as  read-
       only by application-specific code and by users.

              When  an  interpreter  is created, [incr Tcl] initializes this variable to hold the
              name of a directory containing the  system  library  of  [incr Tcl]  scripts.   The
              initial value of itcl::library is set from the ITCL_LIBRARY environment variable if
              it exists, or from a compiled-in value otherwise.

              When an interpreter is created, [incr Tcl] initializes this variable  to  hold  the
              current  patch  level  for  [incr Tcl].   For  example, the value "2.0p1" indicates
              [incr Tcl] version 2.0 with the first set of patches applied.

              When an interpreter is created  containing  Tcl/Tk  and  the  [incr Tcl]  namespace
              facility, this variable controls a "backward-compatibility" mode for widget access.

              In  vanilla Tcl/Tk, there is a single pool of commands, so the access command for a
              widget is the same as  the  window  name.   When  a  widget  is  created  within  a
              namespace,  however,  its access command is installed in that namespace, and should
              be accessed outside of the namespace using a qualified name.  For example,
              namespace foo {
                  namespace bar {
                      button .b -text "Testing"
              foo::bar::.b configure -background red
              pack .b
       Note that the window name ".b" is still used in conjunction with commands  like  pack  and
       destroy.  However, the access command for the widget (i.e., name that appears as the first
       argument on a command line) must be more specific.

       The "winfo command" command can be used to query the fully-qualified  access  command  for
       any widget, so one can write:
              [winfo command .b] configure -background red
       and  this is good practice when writing library procedures.  Also, in conjunction with the
       bind command, the "%q" field can be used in place of "%W" as the access command:
              bind Button <Key-Return> {%q flash; %q invoke}
       While this behavior makes sense from the standpoint of encapsulation, it  causes  problems
       with  existing  Tcl/Tk applications.  Many existing applications are written with bindings
       that use "%W".  Many library procedures assume that the window name is the access command.

       The itcl::purist variable  controls  a  backward-compatibility  mode.   By  default,  this
       variable  is  "0",  and  the  window name can be used as an access command in any context.
       Whenever the unknown procedure stumbles across  a  widget  name,  it  simply  uses  "winfo
       command"  to determine the appropriate command name.  If this variable is set to "1", this
       backward-compatibility mode is disabled.  This gives better encapsulation, but  using  the
       window name as the access command may lead to "invalid command" errors.

              When  an  interpreter  is created, [incr Tcl] initializes this variable to hold the
              version number of the form x.y.  Changes to x represent major changes with probable
              incompatibilities  and changes to y represent small enhancements and bug fixes that
              retain backward compatibility.


       itcl, variables