Set attribute of superclass dynamically

Ruby
1

I have the following code

class foo
price_per_liter = 1
price
module bar
def set_price val
self.price_per_liter = val
end
end
end

In this way I can succesfully set the price per liter of the class. How
can I make it dynamically, so I can also set the attribute ‘price’?

I tried instance_variable_set(@variable, val), but that doesn’t work.
Probably because the variables of the class ‘foo’ and the module ‘bar’
lives independent of each other.

2

Jeroen v. wrote in post #994745:

I have the following code

class foo

That’s wrong - class names must start with a capital letter

price_per_liter = 1

That sets a local variable. It’s only visible between the ‘class’ and
‘end’ excluding any nested ‘def’, ‘class’ or ‘module’ (since each of
those start a new scope for local variables). This is unlikely to be
useful.

price

That will raise an error - it’s not a local variable so it must be a
method call, but your class Foo object doesn’t have a method ‘price’
either (i.e. def self.price … )

module bar

Error again: should be ‘Bar’ not ‘bar’. However I’m not sure why you’re
making a module Foo::Bar - it’s not a class, and it’s not a subclass of
Foo.

def set_price val
  self.price_per_liter = val

That will call method ‘price_per_liter=’, but you have not defined one.
Error again.

end

end
end

In this way I can succesfully set the price per liter of the class. How
can I make it dynamically, so I can also set the attribute ‘price’?

I’m not sure what it is you’re trying to achieve. Here’s a guess:

class Foo
def self.price_per_liter=(val)
@price_per_liter = val
end
def self.price_per_liter
@price_per_liter
end
end

class Bar < Foo
end

Foo.price_per_liter = 133.9
Bar.price_per_liter = 144.9

puts Foo.price_per_liter
puts Bar.price_per_liter

I tried instance_variable_set(@variable, val), but that doesn’t work.
Probably because the variables of the class ‘foo’ and the module ‘bar’
lives independent of each other.

It’s true that the two classes have separate instance variables (since
each class is a separate object, of class Class). But you should be able
to do

Foo.instance_variable_set(:@price_per_liter, 133.9)

3

You’re right. I wrote the example code too quick.

In the meantime, I also found another solution that is good for me
(however your solution would also worked):

In the module you can do:
self.send("#{method_name}=", value)
This creates a setter on the fly for the superclass :wink:

4

Jeroen v. wrote in post #994769:

In the module you can do:
self.send("#{method_name}=", value)
This creates a setter on the fly for the superclass :wink:

That calls the setter method, but only if it already exists. If it
doesn’t, it will raise an exception.

You need to provide some code to demonstrate what you’re saying.

5

On Sun, Apr 24, 2011 at 9:29 AM, Jeroen v. [email protected]
wrote:

end

In this way I can succesfully set the price per liter of the class. How
can I make it dunamically, so I can also set the attribute ‘price’?

I tried instance_variable_set(@variable, val), but that doesn’t work.
Probably because the variables of the class ‘foo’ and the module ‘bar’
lives independent of each other.

What was the error it gave you? From here, I have a hard time believing
you
tried setting the ivar, because your code has two syntax errors (classes
and
modules must be constants) a name error (price is neither a local
variable
nor a method), and you can not set the price per liter of the class as
you
say you did, since price_per_liter is a local variable and there is no
price_per_liter= method, anyway.

Anyway, everything in Ruby is an object, including classes. Every object
in
Ruby has a singleton class, which is a class just for that one object
(they
are created lazily). Why is this relevant? Because methods are defined
in
classes, so defining methods on the object’s singleton class will define
methods for that one object. This is what class methods are.

So what about modules? When you extend an object (and classes are
objects)
with a module, it gets put in the ancestry chain behind the singleton
class.
So when you ask the object for its methods, it looks in the singleton
class,
and when it doesn’t find it, it goes up the chain into the module, where
it
discovers the method.

So, we just define the #price= method in the module, then we extend the
class with the module, and now the class has access to that method.

define the price method on the

singleton class of Foo

class Foo
def self.price
@price
end
end

define the price= method as an

instance method of Bar

module Bar
def price=(new_price)
@price = new_price
end
end

put Bar as an ancestor of

Foo’s singleton class

Foo.extend Bar

tada, everything works

Foo.price # => nil
Foo.price = 5
Foo.price # => 5

You also look like you don’t understand the difference between local
variables, instance variables, and methods. In Ruby, all instance
variables
are private, and they always begin with the @ sigil. So @price is an
instance variable, and price is either a local variable or a method.
When
you say price_per_liter = 1 in your code above, you are setting a
local
variable. If you want to invoke a method on self, you need
self.price_per_liter = 1 to disambiguate. If you want to set the
instance
variable, you need @price_per_liter = 1

Note that Ruby has syntactic sugar to turn self.price = 1 into
self.price=(1), that assignment is just a method call.