ok, i’ve got some module and it has some instance variables i want to
be set by the classes extending the module, and aparently i’m not doing
this correctly:
module A
def test
raise ‘ARG’ if !@blah
@blah
end
end
class B
include A
extend A
@blah = ‘YES!’
end
irb(main):025:0> b = B.new
=> #<B:0x136376c>
irb(main):026:0> b.test
RuntimeError: ARG
from (irb):9:in `test’
from (irb):26
can anyone tell me what the syntax should be? how do i set @blah in the
B class so that its available to the test method
In class B, you are attepmting to use an instance variable as if it
were a class variable. To set @blah to a value when its instantiated,
you’ll need to stick that line in the constructor:
class B
include A
extend A
def initialize
@blah = ‘YES!’
end
end
Also, you dont need to both include and extend A inside B. Use extend
when you want to use the modules methods as class methods:
module X
def classy
puts “Yup, I’m classy”
end
end
class Y
extend X
end
Y.classy
=> Yup, I’m classy
You can also extend an instance of an object:
s = “Holy shniekes”
s.extend(X)
s.classy
=> Yup, I’m classy
Use include when you want to “mix-in” the modules methods with the
instance methods defined in the class:
module X
def classy
puts “Yup, I’m classy”
end
end
class Y
include X
end
Y.new.classy
=> Yup, I’m classy
Hope that helps 
Hi –
On Mon, 20 Mar 2006, [email protected] wrote:
end
=> #<B:0x136376c>
irb(main):026:0> b.test
RuntimeError: ARG
from (irb):9:in `test’
from (irb):26
can anyone tell me what the syntax should be? how do i set @blah in the
B class so that its available to the test method
When using instance variables, you have to match each one precisely to
the object whose instance variable it is. The @blah in @blah = ‘YES!’
belongs to the class object B. The @blah inside test belongs (or will
belong, upon execution) to whatever object is calling “test”. Unless
that object is the class object B, its @blah will be different (even
if it’s an instance of B).
So if you want them to match up, you have to call test on the class
object B:
B.test
That will run “test” with B as the receiver, so you’ll see B’s
instance variable @blah. (By the way, it’s a good idea not to name
your test methods “test”, because there’s already a method with that
name in Kernel, so sometimes you’ll get behavior you don’t expect.
That’s not the issue here, though.)
David
–
David A. Black ([email protected])
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)
“Ruby for Rails” chapters now available
from Manning Early Access Program! Ruby for Rails
Marcin MielżyÅ?ski wrote:
It can be easily seen who is the default receiver (class object or
instance object) by doing this:
class A
p self
def initialize
p self
end
end
A.new
lopex
[email protected] wrote:
end
=> #<B:0x136376c>
irb(main):026:0> b.test
RuntimeError: ARG
from (irb):9:in `test’
from (irb):26
can anyone tell me what the syntax should be? how do i set @blah in the
B class so that its available to the test method
Try this one:
module A
def test
raise ‘ARG’ if !@blah
@blah
end
end
class B
include A
def initialize
@blah = ‘YES!’
end
end
b = B.new
p b.test
Module#include will make module methods to become instance ones.
Object#extend adds methods to an instance (to the class B in this case,
you could say then B.test)
consider also:
class A
@a=4
end
p A.class_eval{@a}
=> 4
which means that @a is an instance variable of class object, not an
instance variable of class instance.
lopex
Marcin MielżyÅ?ski wrote:
def initialize
@blah = ‘YES!’
end
end
b = B.new
p b.test
You can even automate this with proper use of initializers:
module A
def initialize(*a,&b)
super
@blah = “set”
end
def test
raise ‘ARG’ if !@blah
@blah
end
end
class B
include A
end
class C
include A
def initialize(x)
super()
@another_member = x
end
end
B.new.test
=> “set”
C.new(1).test
=> “set”
Module#include will make module methods to become instance ones.
Object#extend adds methods to an instance (to the class B in this case,
you could say then B.test)
Adding to that, usually the OP should decide whether to do one or
another.
Kind regards
robert