Proxying method calls

How do I safely proxy method calls from one class to another, as a
fallback for when the main class doesn’t respond to the call in
question? Is my implementation of respond_to? and method_missing below
correct (passing tests at bottom)?

class A
def initialize
@b = B.new
end

def respond_to? call
    unless super call
        @b.respond_to? call
    end
end

def method_missing call
    if @b.respond_to? call
        @b.send call
    else
        super
    end
end

end

class B
def b
‘b’
end
end

require ‘test/unit’

class MethodTest < Test::Unit::TestCase
def setup
@a = A.new
end

def test_method_missing
    assert_equal 'b', @a.b
    assert_raise NoMethodError do
        @a.c
    end
end

def test_respond_to
    assert @a.respond_to? :b
    assert_equal false, @a.respond_to?(:c)
end

end

Thomas

On 04/20/2010 11:48 PM, Thomas A. wrote:

def respond_to? call
    unless super call
        @b.respond_to? call
    end
end

I am wary about respond_to? - usually I would not bother to override
this. The reason is that there is no guarantee that an object really
responds to the method if respond_to? returns “true” - and the other way
round.

def method_missing call
    if @b.respond_to? call
        @b.send call
    else
        super
    end
end

There is at least one issue: you do not propagate method arguments
properly. Better do

 def method_missing(*args, &block)
   @b.send(*args, &block)
 end

Delegating to super is only necessary if the super class also implements
method_missing. Otherwise you’ll get an exception from @b anyway.

Please see also classes Delegator and SimpleDelegator.
http://ruby-doc.org/stdlib/libdoc/delegate/rdoc/index.html

Kind regards

robert

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