From: Brian Candler <b.candler@...>
Date: 2010-09-22T04:38:21+09:00
Subject: Re: temporarily override instance variable in a block

Jean-denis Vauguet wrote:
> Two approach are illustrated: variable swapping and instance_eval. The
> former does the trick but it's cumbersome and not thread-safe. The
> latter, I don't understand why, fails, for the block seems to be called
> in the
> 
> If anyone can help with this, thanks in advance!

There is no general solution to this. It will depend on the actual 
problem you're trying to solve.

You're right that replacing the instance variable is not thread-safe. 
(Note that you should use begin .. ensure .. end to make sure the 
variable is put back even in the case of an exception)

The self.class.new (or dup/clone) approach won't work for all objects. 
Some objects have compulsory arguments to their constructors, or have 
all sorts of side effects in their 'initialize' methods. The nearest 
you're likely to get is:

o = self.class.allocate
instance_variables.each do |v|
  o.instance_variable_set(v, instance_variable_get(v))
end

But this almost certainly means you're thinking about the problem the 
wrong way. Try a more functional approach: pass the proxy as an 
argument, instead of looking at the @proxy instance variable. Or use the 
instance variable as the default.

  def whatever(proxy = @proxy)
    .. do something with proxy ..
  end
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