From: Josh Cheek <josh.cheek@...>
Date: 2013-03-31T22:51:31+09:00
Subject: Private setters can be called by self, why not getters?

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class Counter
  def initialize
    self.count = 0
  end

  def increment1
    self.count = count + 1
  end

  def increment2
    self.count = self.count + 1
  end

  def increment3
    self.count += 1
  end

  private

  attr_accessor :count
end

counter = Counter.new
counter.increment1  rescue $!.message # => 1
counter.increment2  rescue $!.message # => "private method `count' called
for #<Counter:0x007f8c998c7c60 @count=1>"
counter.increment3  rescue $!.message # => "private method `count' called
for #<Counter:0x007f8c998c7c60 @count=1>"
(counter.count = 0) rescue $!.message # => "private method `count=' called
for #<Counter:0x007f8c998c7c60 @count=1>"


So in Counter#increment1, we can see that we can invoke Counter#count=,
even though it's private. But in increment2, we try `self.count`, which
raises a NoMethodError, because we're calling a private method with an
explicit receiver. But isn't that what we're doing when we say `self.count
= ...` ? Somehow it works for self, but we can see in the last example that
we can't do this from outside.

So why can we invoke private setters with self as the explicit receiver,
but not private getters? This comes up primarily because what we really
want to do is increment3, but this whole time I've thought it was the
setter that was causing the issue, realized last night it was the getter
(b/c I was going to recommend making these protected instead of private, in
order to avoid this issue).

-Josh

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<div>class Counter</div><div>=A0 def initialize</div><div>=A0 =A0 self.coun=
t =3D 0</div><div>=A0 end</div><div>=A0=A0</div><div>=A0 def increment1</di=
v><div>=A0 =A0 self.count =3D count + 1</div><div>=A0 end</div><div>=A0=A0<=
/div><div>=A0 def increment2</div>
<div>=A0 =A0 self.count =3D self.count + 1</div><div>=A0 end</div><div>=A0=
=A0</div><div>=A0 def increment3</div><div>=A0 =A0 self.count +=3D 1</div><=
div>=A0 end</div><div>=A0=A0</div><div>=A0 private</div><div>=A0=A0</div><d=
iv>=A0 attr_accessor :count</div>
<div>end</div><div><br></div><div><div>counter =3D Counter.new</div><div>co=
unter.increment1 =A0rescue $!.message # =3D&gt; 1</div><div>counter.increme=
nt2 =A0rescue $!.message # =3D&gt; &quot;private method `count&#39; called =
for #&lt;Counter:0x007f8c998c7c60 @count=3D1&gt;&quot;</div>
<div>counter.increment3 =A0rescue $!.message # =3D&gt; &quot;private method=
 `count&#39; called for #&lt;Counter:0x007f8c998c7c60 @count=3D1&gt;&quot;<=
/div><div>(counter.count =3D 0) rescue $!.message # =3D&gt; &quot;private m=
ethod `count=3D&#39; called for #&lt;Counter:0x007f8c998c7c60 @count=3D1&gt=
;&quot;</div>
</div><div><br></div><div><br></div><div>So in Counter#increment1, we can s=
ee that we can invoke Counter#count=3D, even though it&#39;s private. But i=
n increment2, we try `self.count`, which raises a NoMethodError, because we=
&#39;re calling a private method with an explicit receiver. But isn&#39;t t=
hat what we&#39;re doing when we say `self.count =3D ...` ? Somehow it work=
s for self, but we can see in the last example that we can&#39;t do this fr=
om outside.</div>
<div><br></div><div>So why can we invoke private setters with self as the e=
xplicit receiver, but not private getters? This comes up primarily because =
what we really want to do is increment3, but this whole time I&#39;ve thoug=
ht it was the setter that was causing the issue, realized last night it was=
 the getter (b/c I was going to recommend making these protected instead of=
 private, in order to avoid this issue).</div>
<div><br></div><div>-Josh</div>

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