Tcl: Setting a private variable of an owned instance within a class

Question

Suppose the following code declarations:

itcl::class ObjectA {
    private variable m_ownedObject
    private variable m_someVariable

   constructor {} \
   {
        set m_ownedObject [ObjectA #auto]
   }

   protected method SetSomeVariable {newVal} {
       set m_someVariable $newVal
   }

   public method SomeMethod{} {
       $m_ownedObject SetSomeVariable 5
   }
}

This is the only way I know how to modify m_someVariable from within SomeMethod on m_ownedObject. In other languages (say C/C++/C#/Java to name a few), I'm pretty sure I could just say something like:

m_ownedObject.m_someVariable = 5

Is there a way to do something like this in tcl, or do I always need to create protected getters and setters? Hopefully this is reasonably clear.

Thanks,

Jordan

Answer 1

If you're declaring a variable as private, means that can be only accessed from within the class. And that's also valid for C/C++/Java ... so I'm not sure what are you expecting.

Anyway Tcl is a dynamic language, so you can do something like that.

  itcl::class tclass {
      foreach v {time distance} {
        method get$v {} [subst -nocommands { return [subst $$v] }]
        method set$v nuval [subst -nocommands { set $v \$nuval } ]
        protected variable $v "Var $v"
      }
  }

And it will create all the getters and setters that you need ;)

You can find more info here: http://wiki.tcl.tk/17667

Answer 2

You cannot directly do what you're asking for in itcl. However, this being Tcl, you can work around that, and directly set the member variable from anywhere. I use a helper routine called memv which you pass an instance and a variable name, and it returns a "reference" to that variable.

This obviously bypasses the private/protected mechanisms that Itcl set up, so you're violating abstractions using them. It's your call whether you want to use it. I find it invaluable for debugging, but don't it in production code.

The example usage is:

set [memv m_ownedObject m_someVariable] 5

The code for memv is:

proc memv {obj varname} {
  # have to look up the variable, which might be in a base class
  # so do 'info variable' to get that, and the full name is the 3rd element

  # next two lines handle pulling apart an array
  set aindex ""
  regexp -- {^(.+)\((.+)\)$} $varname ignore varname aindex

  set var [lindex [$obj info variable $varname] 2]

  if {$aindex == ""} {
    return [list @itcl $obj $var]
  } else {
    return [list @itcl $obj $var\($aindex\)]
  }
}

Similarly, I have a helper routine named memv which allows you to call any method (including private and protected methods). It's usage is similar

[memf m_ownedObject SetSomeVariable] 5

And it's code is:

proc memf {obj fcnname} {
  set f [$obj info function $fcnname]
  if {[llength $f] != 5} {
    error "expected '$obj info function $fcnname' to return something like 'private proc ::namespace::name args {...}' but got: $f"
  }
  set fullname [lindex [$obj info function $fcnname] 2]
  set namespace [namespace qualifiers $fullname]
  set function [namespace tail $fullname]
  return [itcl::code -namespace $namespace $obj $function]
}