From: Brian Candler <B.Candler@...>
Date: 2007-03-08T23:57:03+09:00
Subject: Re: TkVariables and ".value" method

On Thu, Mar 08, 2007 at 11:40:09PM +0900, Alex DeCaria wrote:
> The code below, which puts "Hello" in a label on press of a button,
> works if the command for the button is { a.value = "Hello" }, but not if
> it is { a = "Hello" }.  I'm trying to understand "why", and can't find
> anything on the ".value" method in in the Pragmatic Programmers' book on
> Ruby.

http://rubycentral.com/book/tut_classes.html
Scroll down to "Writable Attributes"

> Can someone explain why {a = "Hello"} won't work, and why I need to do
> {a.value = "Hello"?}

a = TkVariable.new        # 'a' points to an instance of TkVariable

a.value = "Hello"         # calls method 'value=' on this object,
                          #   with "Hello" as the argument. Afterwards,
                          #   'a' still points to the same TkVariable object

BUT:

a = TkVariable.new        # 'a' points to an instance of TkVariable

a = "Hello"               # 'a' now points to a completely different object,
                          # which is a string. The TkVariable is not
                          # referenced from anywhere, and will be
                          # garbage-collected at some point

Try this in irb, see if it makes it any clearer:

  class Foo
    def bar=(x)
      @q = x
    end

    def bar
      @q
    end
  end

  a = Foo.new
  puts a.inspect
  a.bar = 99  
  puts a.inspect
  a = 123
  puts a.inspect

Brian.