Re: Lost in Ashok's teachings

* Luc <luc@sep.invalid>
| ----------- oop.tcl -----------------
| oo::class create Account
| oo::define Account {
|     variable AccountNumber Balance
| }

>

| oo::define Account {
|     method UpdateBalance {change} {
|         set Balance [+ $Balance $change]
|         return $Balance
|     }
|     method balance {} { return $Balance }
|     method withdraw {amount} {
|         return [my UpdateBalance -$amount]
|     }
|     method deposit {amount} {
|         return [my UpdateBalance $amount]
|     }
| }

>

| oo::define Account {export UpdateBalance}
| set acct [Account new 3-14159265]
| Account create smith_account 2-71828182
| $acct deposit 132
| ----------------------------------

>

| There, I broke the toy.

>

| can't read "Balance": no such variable
|     while executing
| "+ $Balance $change"
|     (class "::Account" method "UpdateBalance" line 2)
|     invoked from within
| "my UpdateBalance $amount"
|     (class "::Account" method "deposit" line 2)
|     invoked from within
| "$acct deposit 132"
|     (file "oop.tcl" line 41)
| Compilation failed.

>

| I tried a lot of ideas and all of them run into the same problem:
| the compiler has no knowledge of this Balance variable I speak of.

You have ommitted the constructor from Ashoks example which initializes
the 'Balance' variable.  In your class, this variable is simply not
initialized, which leads to the error.

This is the same with every variable in TCL:

  global foo
  set foo
  => can't read "foo": no such variable

  namespace eval foo {
    variable bar
    set bar
  }
  => can't read "bar": no such variable

You need to initialize (=set) a variable before you can use it.
How the initialization is done depends on the context. 

  namespace eval foo {
    variable bar init-value
    set bar
  }
  => init-value
 
Thus if you have variables in a class, you most probably need a
constructor to initialize them.

This is also true for many OO languages...

HTH
R'