I've done a lot of searching about casting an object in Ruby, and can only find examples of casting a string to an integer or vice versa. I'm curious how to cast other, more complex objects..
For Example with these classes...
class Animal
attr_accessor :weight, :name
end
class Duck < Animal
def speak
'Quack'
end
end
I would expect to be able to do something like the following.
irb> animal = Animal.new
irb> duck = (Duck)animal
irb> duck.speak
=> "Quack"
But I can't quite figure out how to do the casting as I would expect in Java or a similar language.
An object in Ruby is created with a particular class and it will be destroyed with the same class, it never changes. You can't cast, either, all you can do is call functions which convert, a process that involves creating a new object or using another as a proxy.
In your case you can't cast, but you can create a duck with identical properties:
class Animal
attr_accessor :weight, :name
def initialize(options)
# Allow calling new with either an options hash, or another
# animal that's intended to be cloned.
case (options)
when Animal
#weight = options.weight
#name = options.name
when Hash
#weight = options[:weight]
#name = options[:name]
end
end
end
class Duck < Animal
def speak(phrase)
'%s says %s' % [ name, phrase.inspect ]
end
end
animal = Animal.new(weight: 10, name: 'Bill')
duck = Duck.new(animal)
puts duck.speak('quack!')
A second option is to move the duck-like functionality to a separate module you can mix-in:
module ActsAsDuck
def speak
'Quack!'
end
end
animal.extend(ActsAsDuck)
puts animal.speak
That mixes in any methods in that module into your object instance.
One thing that you should understand about Ruby is that the concept of type is not related to the class of an object. An object can respond to multiple interfaces other than it's original class (for example, by including modules or defining singleton methods). So, it's not necessary to typecast an object to have access to a particular interface. This means that is not important what an object is, the really important thing its what it's does.
This concept applies to strings and integers too. You don't typecast a string to integer, you ask a string object to create a new integer that is its idea of itself in this form.
If you want a further understanding about this, I suggest you to read about a concept known as Duck typing
In your case you can just instantiate a Duck object directly like this:
irb> duck = Duck.new
irb> duck.quack
=> "Quack"
As far as I know you can't cast an object to a subclass in java. You can only cast an object to its superclass or implemented interfaces. In ruby you don't need this casting. If you have an instance of Duck you can treat as an Animal.
Is this close enough?
class Animal
def subclass klass
klass.new
end
end
class Duck < Animal
def speak
'Quack'
end
end
animal = Animal.new
duck = animal.subclass(Duck)
duck.speak
#=> "Quack"
I am using super to pass arguments to the parent initialize method, which is not called by default. This is what it looks like. (Notice the use of super on the last two arguments)
module Pet
def initialize name, is_pet
#is_pet = is_pet
if is_pet
#name = name
else
#name = "Unnamed"
end
end
def pet?
return #is_pet
end
def get_name
return #name
end
end
class Dog
include Pet
def initialize tricks, name, is_pet
#tricks = tricks
super name, is_pet
end
def get_tricks
return #tricks
end
end
Here's what I can do with it:
d = Dog.new ["roll", "speak", "play dead"], "Spots", true
d.pet? #=> true
d.get_tricks #=> ["roll", "speak", "play dead"]
d.get_name #=> "Spots"
It works fine, but I'm just wondering if there's a better way to do this.
It is not a good programming practice to hard code a fixed string like "Unnamed" as the value for #name. In such case, you should assign nil, and do whatever modification to it when you print it. Suppose you do this.
Then is_pet can be deduced from whether name is nil or not, so it is redundant to have that as an instance variable. You can simply apply !! to name in order to get is_pet. Therefore, you should get rid of such instance variable.
You have get_ prefixes as getter methods, but in Ruby, it is a better practice to have the same name as the instance variables (without the atmark) as the getter name.
This will give you:
module Pet
attr_reader :name
def initialize name; #name = name end
end
class Dog
include Pet
attr_reader :tricks
def initialize tricks, name
#tricks = tricks
super(name)
end
end
d = Dog.new ["roll", "speak", "play dead"], "Spots"
d.tricks #=> ["roll", "speak", "play dead"]
d.name #=> "Spots"
!!d.name #=> true (= `is_pet`)
Do not write code that calls super to get into an included module. Don't write modules that will expect children to call super. That's not the point of modules.
It's good object oriented style to not ask about what things are. Look up "tell, don't ask" and duck typing in general.
If you want to provide a default initialize method, you probably want inheritance. But there are occasionally valid use cases for overriding initialize in a module. The idiomatic thing to do here is a hook method:
module Pet
def initialize(options = {})
#name = options[:name]
post_initialize(options)
end
def post_initialize(options = {})
# can be overridden in including modules
end
end
class Dog
include Pet
def post_initialize(options = {})
#tricks = options[:tricks]
end
end
dog = Dog.new(name: "Fido", tricks: ["play dead", "roll over"])
A module is for including some shared behavior among many different things. It's good to consider it like an adjective describing what you might do with a class that includes it. Words that end in "-able" (like Enumerable or Comparable), describing a receiving class, or "-or" (Iterator, Interactor), describing a doing class, are good candidates for being modules.
Here's the code:
class Dungeon
attr_accessor :player
def initialize(player_name)
#player = Player.new(player_name)
end
end
Now, if I write:
dungeon = Dungeon.new("Robert")
puts dungeon.player.name
it's obviously going to spit out Robert.
I'm a little new to Ruby, so forgive me if this question is obvious to you all. I'm struggling to wrap my head around it still. I am learning about "instance", "class", and global variables and my question is: How does Ruby know that :player in the code above refers to #player? Why isn't the code written, instead, :#player?
Does that make any sense?
attr_accessor is just a method that defines a public getter/setter. #player is the actual instance variable that stores values. The getter it generates is based on the symbol name, so :player creates these methods:
def player
#player
end
def player=(player)
#player = player
end
If you only want the getter you can use attr_reader. If you only want the setter you can use attr_writer. attr_accessor combines these two methods and gives you both methods.
One thing that is easy to overlook when first learning Ruby is that the construct
attr_accessor :property_name
is just a shorter way of writing getter and setter methods for a class's instance variables:
# getter
def property_name
#property_name
end
# setter
def property_name=( some_value )
#property_name = some_value
end
It's basically an example of the "metaprogramming" support in Ruby, but it feels like something more mysterious.
Also, you do not have to use the symbol style, you could instead have:
attr_accessor "property_name"
The symbol version just "reads better" for most people.
I am having a hard time understanding attr_accessor in Ruby.
Can someone explain this to me?
Let's say you have a class Person.
class Person
end
person = Person.new
person.name # => no method error
Obviously we never defined method name. Let's do that.
class Person
def name
#name # simply returning an instance variable #name
end
end
person = Person.new
person.name # => nil
person.name = "Dennis" # => no method error
Aha, we can read the name, but that doesn't mean we can assign the name. Those are two different methods. The former is called reader and latter is called writer. We didn't create the writer yet so let's do that.
class Person
def name
#name
end
def name=(str)
#name = str
end
end
person = Person.new
person.name = 'Dennis'
person.name # => "Dennis"
Awesome. Now we can write and read instance variable #name using reader and writer methods. Except, this is done so frequently, why waste time writing these methods every time? We can do it easier.
class Person
attr_reader :name
attr_writer :name
end
Even this can get repetitive. When you want both reader and writer just use accessor!
class Person
attr_accessor :name
end
person = Person.new
person.name = "Dennis"
person.name # => "Dennis"
Works the same way! And guess what: the instance variable #name in our person object will be set just like when we did it manually, so you can use it in other methods.
class Person
attr_accessor :name
def greeting
"Hello #{#name}"
end
end
person = Person.new
person.name = "Dennis"
person.greeting # => "Hello Dennis"
That's it. In order to understand how attr_reader, attr_writer, and attr_accessor methods actually generate methods for you, read other answers, books, ruby docs.
attr_accessor is just a method. (The link should provide more insight with how it works - look at the pairs of methods generated, and a tutorial should show you how to use it.)
The trick is that class is not a definition in Ruby (it is "just a definition" in languages like C++ and Java), but it is an expression that evaluates. It is during this evaluation when the attr_accessor method is invoked which in turn modifies the current class - remember the implicit receiver: self.attr_accessor, where self is the "open" class object at this point.
The need for attr_accessor and friends, is, well:
Ruby, like Smalltalk, does not allow instance variables to be accessed outside of methods1 for that object. That is, instance variables cannot be accessed in the x.y form as is common in say, Java or even Python. In Ruby y is always taken as a message to send (or "method to call"). Thus the attr_* methods create wrappers which proxy the instance #variable access through dynamically created methods.
Boilerplate sucks
Hope this clarifies some of the little details. Happy coding.
1 This isn't strictly true and there are some "techniques" around this, but there is no syntax support for "public instance variable" access.
attr_accessor is (as #pst stated) just a method. What it does is create more methods for you.
So this code here:
class Foo
attr_accessor :bar
end
is equivalent to this code:
class Foo
def bar
#bar
end
def bar=( new_value )
#bar = new_value
end
end
You can write this sort of method yourself in Ruby:
class Module
def var( method_name )
inst_variable_name = "##{method_name}".to_sym
define_method method_name do
instance_variable_get inst_variable_name
end
define_method "#{method_name}=" do |new_value|
instance_variable_set inst_variable_name, new_value
end
end
end
class Foo
var :bar
end
f = Foo.new
p f.bar #=> nil
f.bar = 42
p f.bar #=> 42
attr_accessor is very simple:
attr_accessor :foo
is a shortcut for:
def foo=(val)
#foo = val
end
def foo
#foo
end
it is nothing more than a getter/setter for an object
Basically they fake publicly accessible data attributes, which Ruby doesn't have.
It is just a method that defines getter and setter methods for instance variables. An example implementation would be:
def self.attr_accessor(*names)
names.each do |name|
define_method(name) {instance_variable_get("##{name}")} # This is the getter
define_method("#{name}=") {|arg| instance_variable_set("##{name}", arg)} # This is the setter
end
end
If you are familiar with OOP concept, You must familiar with getter and setter method.
attr_accessor does the same in Ruby.
Getter and Setter in General Way
class Person
def name
#name
end
def name=(str)
#name = str
end
end
person = Person.new
person.name = 'Eshaan'
person.name # => "Eshaan"
Setter Method
def name=(val)
#name = val
end
Getter method
def name
#name
end
Getter and Setter method in Ruby
class Person
attr_accessor :name
end
person = Person.new
person.name = "Eshaan"
person.name # => "Eshaan"
Simple Explanation Without Any Code
Most of the above answers use code. This explanation attempts to answer it without using any, via an analogy/story:
Outside parties cannot access internal CIA secrets
Let's imagine a really secret place: the CIA. Nobody knows what's happening in the CIA apart from the people inside the CIA. In other words, external people cannot access any information in the CIA. But because it's no good having an organisation that is completely secret, certain information is made available to the outside world - only things that the CIA wants everyone to know about of course: e.g. the Director of the CIA, how environmentally friendly this department is compared to all other government departments etc. Other information: e.g. who are its covert operatives in Iraq or Afghanistan - these types of things will probably remain a secret for the next 150 years.
If you're outside the CIA you can only access the information that it has made available to the public. Or to use CIA parlance you can only access information that is "cleared".
The information that the CIA wants to make available to the general public outside the CIA are called: attributes.
The meaning of read and write attributes:
In the case of the CIA, most attributes are "read only". This means if you are a party external to the CIA, you can ask: "who is the director of the CIA?" and you will get a straight answer. But what you cannot do with "read only" attributes is to make changes changes in the CIA. e.g. you cannot make a phone call and suddenly decide that you want Kim Kardashian to be the Director, or that you want Paris Hilton to be the Commander in Chief.
If the attributes gave you "write" access, then you could make changes if you want to, even if you were outside. Otherwise, the only thing you can do is read.
In other words accessors allow you to make inquiries, or to make changes, to organisations that otherwise do not let external people in, depending on whether the accessors are read or write accessors.
Objects inside a class can easily access each other
On the other hand, if you were already inside the CIA, then you could easily call up your CIA operative in Kabul because this information is easily accessible given you are already inside. But if you're outside the CIA, you simply will not be given access: you will not be able to know who they are (read access), and you will not be able to change their mission (write access).
Exact same thing with classes and your ability to access variables, properties and methods within them. HTH! Any questions, please ask and I hope i can clarify.
I faced this problem as well and wrote a somewhat lengthy answer to this question. There are some great answers on this already, but anyone looking for more clarification, I hope my answer can help
Initialize Method
Initialize allows you to set data to an instance of an object upon creation of the instance rather than having to set them on a separate line in your code each time you create a new instance of the class.
class Person
def initialize(name)
#name = name
end
def greeting
"Hello #{#name}"
end
end
person = Person.new("Denis")
puts person.greeting
In the code above we are setting the name “Denis” using the initialize method by passing Dennis through the parameter in Initialize. If we wanted to set the name without the initialize method we could do so like this:
class Person
attr_accessor :name
# def initialize(name)
# #name = name
# end
def greeting
"Hello #{name}"
end
end
person = Person.new
person.name = "Dennis"
puts person.greeting
In the code above, we set the name by calling on the attr_accessor setter method using person.name, rather than setting the values upon initialization of the object.
Both “methods” of doing this work, but initialize saves us time and lines of code.
This is the only job of initialize. You cannot call on initialize as a method. To actually get the values of an instance object you need to use getters and setters (attr_reader (get), attr_writer(set), and attr_accessor(both)). See below for more detail on those.
Getters, Setters (attr_reader, attr_writer, attr_accessor)
Getters, attr_reader: The entire purpose of a getter is to return the value of a particular instance variable. Visit the sample code below for a breakdown on this.
class Item
def initialize(item_name, quantity)
#item_name = item_name
#quantity = quantity
end
def item_name
#item_name
end
def quantity
#quantity
end
end
example = Item.new("TV",2)
puts example.item_name
puts example.quantity
In the code above you are calling the methods “item_name” and “quantity” on the instance of Item “example”. The “puts example.item_name” and “example.quantity” will return (or “get”) the value for the parameters that were passed into the “example” and display them to the screen.
Luckily in Ruby there is an inherent method that allows us to write this code more succinctly; the attr_reader method. See the code below;
class Item
attr_reader :item_name, :quantity
def initialize(item_name, quantity)
#item_name = item_name
#quantity = quantity
end
end
item = Item.new("TV",2)
puts item.item_name
puts item.quantity
This syntax works exactly the same way, only it saves us six lines of code. Imagine if you had 5 more state attributable to the Item class? The code would get long quickly.
Setters, attr_writer: What crossed me up at first with setter methods is that in my eyes it seemed to perform an identical function to the initialize method. Below I explain the difference based on my understanding;
As stated before, the initialize method allows you to set the values for an instance of an object upon object creation.
But what if you wanted to set the values later, after the instance was created, or change them after they have been initialized? This would be a scenario where you would use a setter method. THAT IS THE DIFFERENCE. You don’t have to “set” a particular state when you are using the attr_writer method initially.
The code below is an example of using a setter method to declare the value item_name for this instance of the Item class. Notice that we continue to use the getter method attr_reader so that we can get the values and print them to the screen, just in case you want to test the code on your own.
class Item
attr_reader :item_name
def item_name=(str)
#item_name = (str)
end
end
The code below is an example of using attr_writer to once again shorten our code and save us time.
class Item
attr_reader :item_name
attr_writer :item_name
end
item = Item.new
puts item.item_name = "TV"
The code below is a reiteration of the initialize example above of where we are using initialize to set the objects value of item_name upon creation.
class Item
attr_reader :item_name
def initialize(item_name)
#item_name = item_name
end
end
item = Item.new("TV")
puts item.item_name
attr_accessor: Performs the functions of both attr_reader and attr_writer, saving you one more line of code.
I think part of what confuses new Rubyists/programmers (like myself) is:
"Why can't I just tell the instance it has any given attribute (e.g., name) and give that attribute a value all in one swoop?"
A little more generalized, but this is how it clicked for me:
Given:
class Person
end
We haven't defined Person as something that can have a name or any other attributes for that matter.
So if we then:
baby = Person.new
...and try to give them a name...
baby.name = "Ruth"
We get an error because, in Rubyland, a Person class of object is not something that is associated with or capable of having a "name" ... yet!
BUT we can use any of the given methods (see previous answers) as a way to say, "An instance of a Person class (baby) can now have an attribute called 'name', therefore we not only have a syntactical way of getting and setting that name, but it makes sense for us to do so."
Again, hitting this question from a slightly different and more general angle, but I hope this helps the next instance of class Person who finds their way to this thread.
Simply put it will define a setter and getter for the class.
Note that
attr_reader :v is equivalant to
def v
#v
end
attr_writer :v is equivalant to
def v=(value)
#v=value
end
So
attr_accessor :v which means
attr_reader :v; attr_writer :v
are equivalant to define a setter and getter for the class.
Simply attr-accessor creates the getter and setter methods for the specified attributes
Another way to understand it is to figure out what error code it eliminates by having attr_accessor.
Example:
class BankAccount
def initialize( account_owner )
#owner = account_owner
#balance = 0
end
def deposit( amount )
#balance = #balance + amount
end
def withdraw( amount )
#balance = #balance - amount
end
end
The following methods are available:
$ bankie = BankAccout.new("Iggy")
$ bankie
$ bankie.deposit(100)
$ bankie.withdraw(5)
The following methods throws error:
$ bankie.owner #undefined method `owner'...
$ bankie.balance #undefined method `balance'...
owner and balance are not, technically, a method, but an attribute. BankAccount class does not have def owner and def balance. If it does, then you can use the two commands below. But those two methods aren't there. However, you can access attributes as if you'd access a method via attr_accessor!! Hence the word attr_accessor. Attribute. Accessor. It accesses attributes like you would access a method.
Adding attr_accessor :balance, :owner allows you to read and write balance and owner "method". Now you can use the last 2 methods.
$ bankie.balance
$ bankie.owner
Despite the large number of existing answers, none of them seems to me to explain the actual mechanism involved here. It's metaprogramming; it takes advantage of the following two facts:
You can modify a module / class on the fly
A module / class declaration is itself executable code
Okay, so imagine the following:
class Nameable
def self.named(whatvalue)
define_method :name do whatvalue end
end
end
We are declaring a class method named which, when called with a value, creates an instance method called name which returns that value. That is the metaprogramming part.
Now we'll subclass that class:
class Dog < Nameable
named "Fido"
end
What on earth did we just do? Well, in the class declaration, executable code executes with reference to the class. So the bare word named is actually a call to the class method named, which we inherited from Nameable; and we are passing the string "Fido" as the argument.
And what does the class method named do? It creates an instance method called name, which returns that value. So now, behind the scenes, Dog has a method that looks like this:
def name
"Fido"
end
Don't believe me? Then watch this little move:
puts Dog.new.name #=> Fido
Why did I tell you all that? Because what I just did with named for Nameable is almost exactly what attr_accessor does for Module. When you say attr_accessor you are calling a class method (inherited from Module) that creates instance methods. In particular, it creates a getter and setter method for the instance property whose name you provide as argument, so that you don't have to write those getter and setter methods yourself.
Defines a named attribute for this module, where the name is symbol.id2name, creating an instance variable (#name) and a corresponding access method to read it. Also creates a method called name= to set the attribute.
module Mod
attr_accessor(:one, :two)
end
Mod.instance_methods.sort #=> [:one, :one=, :two, :two=]
To summarize an attribute accessor aka attr_accessor gives you two free methods.
Like in Java they get called getters and setters.
Many answers have shown good examples so I'm just going to be brief.
#the_attribute
and
#the_attribute=
In the old ruby docs a hash tag # means a method.
It could also include a class name prefix...
MyClass#my_method
I am new to ruby and had to just deal with understanding the following weirdness. Might help out someone else in the future. In the end it is as was mentioned above, where 2 functions (def myvar, def myvar=) both get implicitly for accessing #myvar, but these methods can be overridden by local declarations.
class Foo
attr_accessor 'myvar'
def initialize
#myvar = "A"
myvar = "B"
puts #myvar # A
puts myvar # B - myvar declared above overrides myvar method
end
def test
puts #myvar # A
puts myvar # A - coming from myvar accessor
myvar = "C" # local myvar overrides accessor
puts #myvar # A
puts myvar # C
send "myvar=", "E" # not running "myvar =", but instead calls setter for #myvar
puts #myvar # E
puts myvar # C
end
end
Attributes and accessor methods
Attributes are class components that can be accessed from outside the object. They are known as properties in many other programming languages. Their values are accessible by using the "dot notation", as in object_name.attribute_name. Unlike Python and a few other languages, Ruby does not allow instance variables to be accessed directly from outside the object.
class Car
def initialize
#wheels = 4 # This is an instance variable
end
end
c = Car.new
c.wheels # Output: NoMethodError: undefined method `wheels' for #<Car:0x00000000d43500>
In the above example, c is an instance (object) of the Car class. We tried unsuccessfully to read the value of the wheels instance variable from outside the object. What happened is that Ruby attempted to call a method named wheels within the c object, but no such method was defined. In short, object_name.attribute_name tries to call a method named attribute_name within the object. To access the value of the wheels variable from the outside, we need to implement an instance method by that name, which will return the value of that variable when called. That's called an accessor method. In the general programming context, the usual way to access an instance variable from outside the object is to implement accessor methods, also known as getter and setter methods. A getter allows the value of a variable defined within a class to be read from the outside and a setter allows it to be written from the outside.
In the following example, we have added getter and setter methods to the Car class to access the wheels variable from outside the object. This is not the "Ruby way" of defining getters and setters; it serves only to illustrate what getter and setter methods do.
class Car
def wheels # getter method
#wheels
end
def wheels=(val) # setter method
#wheels = val
end
end
f = Car.new
f.wheels = 4 # The setter method was invoked
f.wheels # The getter method was invoked
# Output: => 4
The above example works and similar code is commonly used to create getter and setter methods in other languages. However, Ruby provides a simpler way to do this: three built-in methods called attr_reader, attr_writer and attr_acessor. The attr_reader method makes an instance variable readable from the outside, attr_writer makes it writeable, and attr_acessor makes it readable and writeable.
The above example can be rewritten like this.
class Car
attr_accessor :wheels
end
f = Car.new
f.wheels = 4
f.wheels # Output: => 4
In the above example, the wheels attribute will be readable and writable from outside the object. If instead of attr_accessor, we used attr_reader, it would be read-only. If we used attr_writer, it would be write-only. Those three methods are not getters and setters in themselves but, when called, they create getter and setter methods for us. They are methods that dynamically (programmatically) generate other methods; that's called metaprogramming.
The first (longer) example, which does not employ Ruby's built-in methods, should only be used when additional code is required in the getter and setter methods. For instance, a setter method may need to validate data or do some calculation before assigning a value to an instance variable.
It is possible to access (read and write) instance variables from outside the object, by using the instance_variable_get and instance_variable_set built-in methods. However, this is rarely justifiable and usually a bad idea, as bypassing encapsulation tends to wreak all sorts of havoc.
Hmmm. Lots of good answers. Here is my few cents on it.
attr_accessor is a simple method that helps us in cleaning(DRY-ing) up the repeating getter and setter methods.
So that we can focus more on writing business logic and not worry about the setters and getters.
The main functionality of attr_accessor over the other ones is the capability of accessing data from other files.
So you usually would have attr_reader or attr_writer but the good news is that Ruby lets you combine these two together with attr_accessor. I think of it as my to go method because it is more well rounded or versatile.
Also, peep in mind that in Rails, this is eliminated because it does it for you in the back end. So in other words: you are better off using attr_acessor over the other two because you don't have to worry about being to specific, the accessor covers it all. I know this is more of a general explanation but it helped me as a beginner.
Hope this helped!