Error in Ruby quickstart tutorial

On the quickstart tutorial on the Ruby homepage,
http://www.ruby-lang.org/en/quickstart/3/ , it says,
“In Ruby, you can open a class up again and modify it. That doesn’t
change any objects that already exist, but it does affect any new
objects you create.” This is incorrect. For example:

irb(main):001:0> class FooClass; end
nil
irb(main):002:0> sally = FooClass.new
#FooClass:0xb7e0b214
irb(main):003:0> class FooClass; def foo; puts “bar”; end; end
nil
irb(main):004:0> sally.foo
bar
nil

Changing a class changes the objects of that class which already exist
(which is pretty cool).

Regards, Bret
Bret Jolly

oinkoink wrote:

On the quickstart tutorial on the Ruby homepage,
http://www.ruby-lang.org/en/quickstart/3/ , it says,
“In Ruby, you can open a class up again and modify it. That doesn’t
change any objects that already exist, but it does affect any new
objects you create.” This is incorrect. For example:

Nope, this is correct.

This in fact changes the object behaviour, but the objects themselves
are not changed. Methods belongs to classes, not objects.

lopex

Marcin Mielzynski wrote:

oinkoink wrote:

On the quickstart tutorial on the Ruby homepage,
http://www.ruby-lang.org/en/quickstart/3/ , it says,
“In Ruby, you can open a class up again and modify it. That doesn’t
change any objects that already exist, but it does affect any new
objects you create.” This is incorrect. For example:

Nope, this is correct.
This in fact changes the object behaviour, but the objects themselves
are not changed. Methods belongs to classes, not objects.
lopex

The problem is that the tutorial suggests that objects which have
already been created before a class is modified are affected
differently from objects which are created after the class is modified.

oinkoink wrote:

The problem is that the tutorial suggests that objects which have
already been created before a class is modified are affected
differently from objects which are created after the class is modified.

Ok, let’s put it another way. Modifying a class doesn’t change two thins

  • instance variables of existing objects
  • anything that is stored in singleton/eigen/meta/whatever class

You can define a method that belongs only to certain object and is not
stored in the object’s class:

class Foo

 def bar
     p "bar"
 end

end

f = Foo.new

defining a method on an object (it is stored in a special class that

belongs only to f)

def f.bar
p “my_bar”
end

redefining an instance method, class is changed

class Foo

 def bar
 	p "new_bar"
 end

end

not affected:

f.bar

-> “my_bar”

but new instances will have the new Foo#bar method

recommended reading:
http://www.whytheluckystiff.net/articles/seeingMetaclassesClearly.html

lopex

On 2006.10.05 07:50, Marcin Miel??y??ski wrote:

oinkoink wrote:

The problem is that the tutorial suggests that objects which have
already been created before a class is modified are affected
differently from objects which are created after the class is modified.

Ok, let’s put it another way. Modifying a class doesn’t change two thins

  • instance variables of existing objects
  • anything that is stored in singleton/eigen/meta/whatever class

What you are saying is correct, the problem is just that the tutorial
is definitely saying the same thing you are :slight_smile:

Any existing objects are affected by changes to the class.

This should be corrected or, if they actually did intend to say what
you have said here, ‘clarified’ with a heavy hand.

This forum is not affiliated to the Ruby language, Ruby on Rails framework, nor any Ruby applications discussed here.

| Privacy Policy | Terms of Service | Remote Ruby Jobs