From: Lyle Johnson <jlj@...>
Date: 2002-05-14T04:08:11+09:00
Subject: Re: Problems with the FXList

> I am writing my first Ruby program, which is a small GUI application,
> based on the Fox library. The GUI is quite simple, consisting of one
> list (FXList) which I want to add items, which again has two
> properties, name and password, and a few buttons. The items are
> implemented as a simple class Entry which extends FXListItem:
>
> class Entry < FXListItem
>
>   def initialize(name, password)
>     @name = name
>     @password = scramble(password)
>   end
>
>   def scramble(password)
>     # This method will be more sofisticated in next iteration :-)
>     password.reverse
>   end
>
>   def unscramble()
>     @password.reverse
>   end
>
>   def to_s
>     "Name: " + @name + ", Password: " + @password
>   end
> end
>
> When trying to add instances of this class to an FXList using the
> appendItem method, I get into trouble. Here is the offending code:
>
>     ...
>     @list = FXList.new(self, 10, nil, 0, LIST_SINGLESELECT |
> LAYOUT_FILL_X)
>     item = Entry.new("xx", "yy")
>     @list.appendItem(item)  # This line causes the error
>     ...
>
> and here is the error message I receive:
>
> c:/dev/ruby/lib/ruby/site_ruby/1.6/fox/overloads.rb:164:in
> `appendItem1': This FXListItem * already released (RuntimeError)
>         from c:/dev/ruby/lib/ruby/site_ruby/1.6/fox/overloads.rb:164:in
> `appendItem'
>         from PwdMgr.rb:71:in `initialize'
>         from PwdMgr.rb:100:in `new'
>         from PwdMgr.rb:100:in `run'
>         from PwdMgr.rb:109

You need to always be sure to call the base class initialize method when
deriving new classes from FOX classes. In your case, you failed to call
FXListItem#initialize in your Entry#initialize method and so it never
initialized the base class portion of this object. Try this for
Entry#initialize instead:

    class Entry < FXListItem

        def initialize(name, password)
            super("item text here") # calls FXListItem#initialize
            @name = name
            @password = scramble(password)
        end

        # other stuff as before
    end

> I also have tried to test the item variable using the
> item.instance_of?(FXListItem) method, and it returns false, so I guess
> I have got something wrong in my understanding of Ruby and
> inheritance.

Since it never got a chance to initialize the base class part you ended up
with a nil reference. After making the fix described above,
Entry#kind_of?(FXListItem) or Entry#is_a?(FXListItem) should return true.
Note that Entry#instance_of?(FXListItem) will not (and should not) return
true, however; Object#instance_of? only returns true if the receiver is an
instance of the specified class (not one of its subclasses).

> The C++ API doc says the following about appendItem: (Can't find a
> Ruby version of the API doc. Does it exist?)

I am slowly building up a set of RDoc-style source files from which I can
generate more Ruby-friendly documentation, but for the time being the C++
API is still your best source.

> FXint appendItem(FXListItem* item, FXbool notify=FALSE)
> Append a [possibly subclassed] item to the list
>
> Can anyone tell me what I am doing wrong here?

Your call to FXList#appendItem was perfectly fine. The error message was,
unfortunately, not very helpful (other than telling you that something was
wrong with the list item itself).

Hope this helps,

Lyle