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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