Classes and Objects Flashcards
How to see all instance variables for the instance of a class?
.instance_variables
class Hello
def initialize
@sample = 2
end
end
Hello.new.instance_variables #=> [@sample]
What is the difference between supper and super() ?
super -> call superclass method with params from the method by default
super() -> calls methods without params
How to see all possible methods of the object?
.methods
class Hello
def hello
end
end
Hello.new.methods #=> [:hello, …]
What is self?
will show the same object
It’s a current object. An instance of the class inside the clas
class MyClass
def first_method
puts “Current instance within first_method: #{self}”
end
end
my_object = MyClass.new
puts “my_object refers to this object: #{my_object}”
my_object.first_method
When we must use the method self explicitly inside the class?
When you try to assign local values with the same name as a method
class MyClass
def first_method
a = 10
# self.a #=>’hello’
end
def a
‘hello’
end
end
my_object = MyClass.new
my_object.first_method
# => 10
How to see all the ancestors of the class in the right order?
=> [Class, Module, Object, Kernel, BasicObject]
Class.ancestors
How to make an existing method private?
private :method_name
How to make an existing class method private?
private_class_method :new
What is the base ruby class?
class Object < BasicObject
include Kernel
end
What is the class of class?
something like this
Class.class # => Class
class Class
end
Example = Class.new
instance_of_class = Example.new
instance_of_class.class # => Example
How to find what modules are included in a class?
Class.included_modules
# [Kernel]
What returns .singleton_methods on the class?
This method returns all class methods
class Animal
def self.test
end
end
p Animal.singleton_methods
# [:test]
The singleton class is like an “empty shell”, used only for the purpose of holding onto methods, it lives between the object itself & the object’s class.
What are the differences between prepend and include module?
Prepend overrides class methods
module Hello
def hello
p ‘from module’
end
end
class Name
prepend Hello
def hello
p ‘from class’
end
end
Name.new.hello
# from module
What happens if you define the same class twice but with different methods?
class Hello
def x
‘x’
end
end
class Hello
def y
‘y’
end
end
hello = Hello.new
p hello.x # x
p hello.y # y
They extend each other
Is it possible to assign class name to variables?
class Hello
def say
p ‘a’
end
end
hello = Hello
a = hello.new
a.say
What is the second way to define class? and with Inheritance?
Inheritance
hello = Class.new do
def say
p ‘say’
end
end
hello.new.say # say
# Class.new(Array)
Is it possible to create an instance of Module class?
yes
What is the second way to create module?
$sample = Module.new do
def hello
p ‘hello’
end
end
class Hello
include $sample
end
Hello.new.hello
What are the differences between variables and constant?
- The scope ( variables don’t available everywhere )
What’s the class of Object? BasicObject? Class? Module?
=> Class
p Object.class
p BasicObject.class
p Class.class
p Module.class
What’s the superclass of Module?
p Module.superclass
# Object
Is it possible to call a private method with an explicit receiver? (self.method)
Private methods are governed by a single simple rule: you cannot call a private method with an explicit receiver. In other words, every time you call a private method, it must be on the implicit receiver—self.
class C
def public_method
self.private_method
end
private
def private_method; end
end
C.new.public_method
# raised error
What does Refinement mean? How to use it?
refine to define and using to add
Refinements are similar to Monkeypatches, but they’re not global. A Refinement is active in only two places: the refine block itself and the code starting from the place where you call using until the end of the module (if you’re in a module definition) or the end of the file (if you’re at the top level)
module StringExtension
refine String do
def hello
self * 10
end
end
end
using StringExtension
p ‘s’.hello
What are three places where a program leaves the previous scope behind and opens a new one?
• Class definitions ( class keywords)
• Module definitions ( module keywords)
• Methods ( def keywords)
Is it possible to define method inside another method?
Yes
def hello
def hello2
p ‘Hello’
end
hello2
end
hello # ‘Hello’
What are the differences between Class Variables, Class Instance variable and instance variable of class object?
Class Variable—they belong to class hierarchies ( something like global value)
class MyClass
@@my_var = 1 # Class Variable
@my_var = 1 # Class Instance Varible
def self.read; @my_var; end
def write; @my_var = 2; end # Intance variable of class object
def read; @my_var; end
end
obj = MyClass.new
obj.read # => nil
obj.write
obj.read # => 2
MyClass.read # => 1
What is SIngleton Method?
str = ‘dd’
def str.hello
self.upcase
end
p str.hello # DD
A method like this one, which is specific to a single object, is called a Singleton Method.
How to create your own attr_reader ?
1. define macros
# 2. dynamically create method inside
# 3. get variable
class Hello
def self.my_attr_reader(method)
define_method method do
instance_variable_get(“@#{method}”)
end
end
def initialize
@hello = ‘Sample’
@hello2 = ‘SSS’
end
my_attr_reader :hello
my_attr_reader :hello2
end
p Hello.new.hello
p Hello.new.hello2
How extend module work inside?
module MyModule
def my_method; ‘hello’; end
end
class MyClass
class «_space;self
include MyModule
end
end
p MyClass.my_method
How to get 1+ 1 == 3?
alias do not override both methods
class Integer
alias_method :original_plus, :+
def +(value)
self.original_plus(value).original_plus(1)
end
end
p 1+ 1
What is class macros?
attr_reader
attr_accessor
and methods something like this
How to disable a ruby method?
Ruby provides a special keyword which is known as undef keyword. This keyword used to avoid the current working class from responding to calls to the specified named methods or variable.
def hello
p ‘a’
end
undef hello
# or remove_method hello ( which will remove from the class but not from parrents)
Is it possible to get access inside the class/module/method to the local variable outside?
Yes, if class was created via block. The same for module and define_method
a = 10
Hello = Class.new do
p a
end
Hello.new
# 10
# method
a = 10
define_method(:hello) do
p a
end
hello
Can you tell me how Constant are inherited for classes?
1
class C
Hello = 2
end
module A
Hello = 1
class B < C
p Hello
end
end
How to create class methods? 4 options
class Hello
def self.method_1
p “method 1”
end
class «_space;self
def method_2
p “method 1”
end
end
end
def Hello.method_3
p ‘method 3’
end
class «_space;Hello
def method_4
p ‘method 4’
end
end
Hello.method_1
Hello.method_2
Hello.method_3
Hello.method_4
How to check if a new class has been instantiated or new module has been included?
def self.inherited(subclass)
end
def self.included(klass)
puts “Hey, I’ve been included in #{klass}”
end
