create a class object as variable in Tcloo - oop

Question

I start to use tclOO.
I need to create a Aclass instance anotherClassIns in my main class MainClass as below:
oo:class create Aclass {
variable num;
constructor {argv} {
set num [lindex $argv 0]
}
}; #end of Aclass
oo:class create MainClass {
variable anotherClassIns ; # this variable is another class's instance!
constructor {argv} {
Aclass create anotherClassIns {$argv};
}
}
And in main function, I create a MainClass instance:
MainClass create Ins $argv
If I want to print $num in Aclass, how can I do?
Thanks.

Answer 1

All instance variables in TclOO have globally-addressable names so that they can be used with other parts of Tcl that expect such (especially vwait and trace, but also Tk's -textvariable options).
The easiest way for now is to make an accessor method:
oo:class create Aclass {
variable num
constructor {argv} {
set num [lindex $argv 0]
}
method num {} {
return $num
}
}
oo:class create MainClass {
variable anotherClassIns
constructor {argv} {
set anotherClassIns [Aclass new {*}$argv]
}
destructor {} {
# Assuming you're owning the inner instance
$anotherClassIns destroy
}
method printTheValue {} {
puts [$anotherClassIns num]
}
}
You can also access the variable directly by using the fact that variables exist in a namespace, and that namespace can be looked up with info object namespace:
method printTheValue {} {
# Probably easiest to do this
namespace upvar [info object namespace $anotherClassIns] num v
puts $v
}
But that won't work with 8.7's private variables, as those are name-mangled in a way that is decidedly non-accessible to outside code. In that case, you'd need to get Aclass to provide access into its internals; that's literally the point of private variables. The varname method of oo::object can help there; it's not exposed by default, but it's available internally via my, and you can expose it for a class or even just an instance via the export definition.
# Probably at object creation time
oo::objdefine $anotherClassIns export varname
# Then, to access
puts [set [$anotherClassIns varname num]]
# Or
upvar 0 [$anotherClassIns varname num] v
puts $v
There's a similar variable method, but that requires that you don't have a colliding local num variable:
# Object creation time
oo::objdefine $anotherClassIns export variable
# Access
$anotherClassIns variable num
puts $num