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In the example below, I want to have an instance variable in class B that is created/set whenever B is instantiated. Obviously I don't want to have to go redefine all the initialize methods of A.
class A
def initialize(a)
end
def initialize(a, b)
end
end
class B < A
# Here I want an instance variable created without
# redefining the initialize methods
#iv = "hey" #<-- Obviously does not work
# And I don't want to have to do #iv |= "hey" all over the place
end
I'm not sure what you have against defining initialize methods, but this is how it should be done.
class A
def initialize a
#a = a
end
attr_accessor :a
end
class B < A
def initialize a, b
#b = b
super(a)
end
attr_accessor :b
end
b = B.new 1, 2
b.a # => 1
b.b # => 2
Related
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What is the preferred way to call a method in a Ruby class, that lives in it's parent module?
like this..
#!/usr/bin/ruby
module Foo
def baz
123123
end
class Bar
def test
puts baz
end
end
class Bar
end
end
bar = Foo::Bar.new
bar.test
I can get this to work if I do this..
#!/usr/bin/ruby
module Foo
def baz
123123
end
class Bar
include Foo
def test
puts baz
end
end
class Bar
end
end
bar = Foo::Bar.new
bar.test
But is this the best way?
module Foo
def self.baz
123123
end
class Bar
def test
puts Foo.baz
end
end
end
bar = Foo::Bar.new
bar.test
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Suppose I have:
class Parent
def foo
"foo"
end
end
class Child < Parent
class << self
end
Then
c = Child.new.foo
does not give me "foo". Why not?
The question above came when I was messing around trying to figure out why some code with the class << self idiom was behaving in a way I didn't understand. Since I had a syntax error in this post it is obvious that the error in the code I was looking at had nothing to do with class << self breaking inheritance. I want to delete this Q. Stack overflow won't let me. Hope to post a better question later.
There is a syntax error here. You never close the singleton class. Should be:
class Child < Parent
class << self
end
end
Works perfectly for me:
irb(main):008:0> class Parent
irb(main):009:1> def foo
irb(main):010:2> "foo"
irb(main):011:2> end
irb(main):012:1> end
=> :foo
irb(main):013:0> class Child < Parent
irb(main):014:1> class << self
irb(main):015:2> def foo
irb(main):016:3> "class-foo"
irb(main):017:3> end
irb(main):018:2> end
irb(main):019:1> end
=> :foo
irb(main):020:0> c = Child.new
=> #<Child:0x000001021eae30>
irb(main):021:0> c.foo
=> "foo"
irb(main):022:0> Child.foo
=> "class-foo"
class Child < Parent
class << self
end
This definition is syntactically invalid, and doesn't make sense. The correct definition is
class Child < Parent
class << self
end
end
But even in this case, it's completely useless unless you need to define something inside the singleton class. The following code is more than enough.
class Parent
def foo
"foo"
end
end
class Child < Parent
end
Here's an example
2.1.1 :010 > Child.new.foo
=> "foo"
You're missing an "end". class << self is not a one-liner, and requires you to close the block. I just ran your code with an extra end and it worked.
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Heey I am new to Ruby. I need to create a factory method, which will return me an object of a class. Using that object I should be able to access the variables of the class. I have written the following code, but I surely have miss something.
class Super
##super_temp = 1
def Super.get_instance(world)
platform = world
if ##instance == nil
if platform==1
##instance = BaseA.new
else
##instance = BaseB.new
end
end
return ##instance
end
end
class BaseA < Super
##base_temp = 2
end
class BaseB < Super
##base_temp = 3
end
class Demo
def Demo.call_demo
obj = Super.get_instance(0)
puts "---------temp is #{obj.base_temp}"
end
end
Demo.call_demo
I need to retrieve the value of base_temp in class Demo.
Don't use ## (Why should we avoid using class variables ## in rails?) - # solves your problem just as easily.
Aside from that, all that is missing in your code is a getter:
class Super
#super_temp = 1
def Super.get_instance(world)
platform = world
if #instance == nil
if platform==1
#instance = BaseA.new
else
#instance = BaseB.new
end
end
return #instance
end
def base_temp
self.class.base_temp
end
def self.base_temp
#base_temp
end
end
class BaseA < Super
#base_temp = 2
end
class BaseB < Super
#base_temp = 3
end
class Demo
def Demo.call_demo
obj = Super.get_instance(0)
puts "---------temp is #{obj.base_temp}"
end
end
Demo.call_demo
# ---------temp is 3
The instance getter (implemented as self.class.base_temp) calls the class method base_temp of the instance's class. If we add prints of the internal products of the function, you can have some insights about its internals:
class Super
def base_temp
p self
p self.class
p self.class.base_temp
end
end
BaseA.new.base_temp
# #<BaseA:0x000000027df9e0>
# BaseA
# 2
BaseB.new.base_temp
# #<BaseB:0x000000027e38b0>
# BaseB
# 3
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Let's consider this:
class Container
def function_one
...
end
def function_two
...
end
def function_three
...
end
attr_accessor :result_from_function_one
attr_accessor :result_from_function_two
attr_accessor :result_from_function_three
end
Since I can't create separate algorithm body for other classes, I created four separate classes. When I need to run algorithm one, I create a class with function one, and so on:
class Container
...
end
class ContainerWithFunctionOne < Container
def function_one
...
end
attr_accessor :result_from_function_one
end
class ContainerWithFunctionTwo < Container
def function_two
...
end
attr_accessor :result_from_function_two
end
class ContainerWithFunctionThree < Container
def function_three
...
end
attr_accessor :result_from_function_three
end
But when I combine function_one with function_two, I have an issue because they need to use the same data structure. So I was thinking about dividing the class Container into modules:
module FunctionOne
class Container
def function_one
...
end
attr_accessor :result_from_function_one
end
end
module FunctionTwo
class Container
def function_two
...
end
attr_accessor :result_from_function_two
end
end
module FunctionThree
class Container
def function_three
...
end
attr_accessor :result_from_function_three
end
end
But when I try to run it:
require_relative 'FunctionOne'
require_relative 'FunctionTwo'
require_relative 'FunctionThree'
containter = Container.new
container.function_one
container.function_two
container.function_three
it gave a run time error:
in `<top (required)>': uninitialized constant Container (NameError)
and I don't know how to fix this problem.
You can try
container = FunctionOne::Container.new
to create a new container
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i have simple, how to return array with class instances ? I'm trying to return the array, but this variable return an empty array.
For example :
class Library
def initialize
##books = []
end
def all
##books
end
def add_book(arg = {})
#book = Book.new(arg)
##books << #book
end
end
class Book
attr_accessor :name, :year, :author, :content
def initialize( arg = {})
#name = arg[:name]
#year = arg[:year]
#author = arg[:author]
#content = arg[:content]
end
end
##books is a Library class variable. I am using method add_book to put books into #books, but how can i return array of these instances ? Sorry for bad english.
Thanks in advance !
When you call the method new to create a new object, ruby runs the initialize method. Since the initialize method sets ##books to an empty array, of course Library.new.all will return an empty array.
Class variables are shared by all instances of a class, so it doesn't make sense to be resetting it when you initialize a new Library as you'd be zeroing out the books stored in all other Library instances. From your usage it looks like you want a plain instance variable:
class Library
def initialize
#books = []
end
# you could replace this method with a `attr_reader :books`
def all
#books
end
# consider changing the method signature to accept a Book instance
def add_book(arg = {})
#books << Book.new(arg)
end
end