Question about attr_accessor

ruby-talk
1

Hi,

I came across something unexpected today, and wondered if anyone could
explain it to me. If I have this class:

class Foo
  def initialize
    @bar = 0
  end

  attr_accessor :bar

  def add
    bar += 1
  end
end

then the "add" function doesn't work - it complains about a nil
object. You have to specifically put "self.bar += 1". Why is that - I
thought that the "self" was implicitly there?

Doing:

f = Foo.new
f.add

works as the class stands, so you only seem to need the self if you
are calling the accessor from within your class.

Confused...

TIA
Roland

2

Change your add method to:

def add
@bar+=1
end

If you use just 'bar' it will think of a local variable which is not defined.

martin

···

On Saturday 03 March 2007 13:00, Roland Swingler wrote:

Hi,

I came across something unexpected today, and wondered if anyone could
explain it to me. If I have this class:

class Foo
  def initialize
    @bar = 0
  end

  attr_accessor :bar

  def add
    bar += 1
  end
end

then the "add" function doesn't work - it complains about a nil
object. You have to specifically put "self.bar += 1". Why is that - I
thought that the "self" was implicitly there?

Doing:

f = Foo.new
f.add

works as the class stands, so you only seem to need the self if you
are calling the accessor from within your class.

Confused...

TIA
Roland

3

attr_accessor :bar refers to the instance variable @bar.
bar and @bar are not the same.

Harry

···

On 3/3/07, Roland Swingler <roland.swingler@gmail.com> wrote:

Hi,

I came across something unexpected today, and wondered if anyone could
explain it to me. If I have this class:

class Foo
  def initialize
    @bar = 0
  end

  attr_accessor :bar

  def add
    bar += 1
  end
end

then the "add" function doesn't work - it complains about a nil
object. You have to specifically put "self.bar += 1". Why is that - I
thought that the "self" was implicitly there?

Doing:

f = Foo.new
f.add

works as the class stands, so you only seem to need the self if you
are calling the accessor from within your class.

Confused...

TIA
Roland

--

Japanese Ruby List Subjects in English

4

Hi --

···

On Sat, 3 Mar 2007, Roland Swingler wrote:

Hi,

I came across something unexpected today, and wondered if anyone could
explain it to me. If I have this class:

class Foo
def initialize
   @bar = 0
end

attr_accessor :bar

def add
   bar += 1
end
end

then the "add" function doesn't work - it complains about a nil
object. You have to specifically put "self.bar += 1". Why is that - I
thought that the "self" was implicitly there?

For methods that end in = (like bar=), you always need an explicit
receiver, even if it's self, because otherwise Ruby will see it as an
assignment to a local variable.

David

--
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Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
A. Ruby Power and Light, LLC (http://www.rubypal.com)

5

bar += 1 is translated to "bar = bar + 1". When Ruby sees "bar =" it takes bar to be a local variable. Hence you have to prefix with "self.". See:
http://www.ruby-doc.org/docs/ProgrammingRuby/html/language.html#UO

Kind regards

  robert

···

On 03.03.2007 13:55, Roland Swingler wrote:

Hi,

I came across something unexpected today, and wondered if anyone could
explain it to me. If I have this class:

class Foo
  def initialize
    @bar = 0
  end

  attr_accessor :bar

  def add
    bar += 1
  end
end

then the "add" function doesn't work - it complains about a nil
object. You have to specifically put "self.bar += 1". Why is that - I
thought that the "self" was implicitly there?

6

Great - thanks for clearing that up.