I have the following code:
class Bike
attr_reader :chain
def initialize
#chain = default_chain
end
def default_chain
raise 'SomeError'
end
end
class MountainBike < Bike
def initialize
super
end
def default_chain
4
end
end
mb = MountainBike.new
p mb.chain
When in the initialization we call super, I would expect the default_chain of the super class to be called, and therefore the Exception to be launched. However, it seems that the Bike class actually goes and calls the default_chain method of the original caller. Why is that?
As said #BorisStitnicky you need to use singleton_class. For more understanding, in this post you may find info: Understanding the singleton class when aliasing a instance method
The point of putting a same-named method in a sub-class is to overload that method.
ie to replace it with your new version.
The code will look at the calling sub-class's version of any method - before it starts looking up the ancestry chain for a method, because the sub-class is the context of the current code.
So even though the method is referred-to in the superclass... the ruby interpreter will still first look at your sub-class's set of method as the main context.
If you also want to call the parent's one - you must use super in your default_chain method too.
Alternatively rename the one in the parent class to something else and call it from your superclass only.
In object oriented programming, which method is called is solely determined by the receiver and the method name.
Method name initialize is called on the receiver: MountainBike instance, for which MountainBike#initialize matches, and is executed.
Within the execution, super is called, for which Bike#initialize matches, and is executed.
Within the execution, method name default_chain is called on the implicit receiver self, which is the MountainBike instance. MountainBike#default_chain matches, and is executed.
If Bike#default_chain were to be called in the last step on the receiver (MountainBike instance), it would mean that method calling would depend not only on the receiver and the method name, but also on the context. That would make it too complicated, and would defeat the purpose of object oriented programming. Again, the idea of object oriented programming is that method call is solely determined by the receiver and the method name, not the context.
Related
I've seen this a lot in Ruby (started using Ruby for the first time a couple years ago):
class MyAwesomeClass
another_method
def initialize; end
end
And I can't get my head around what type of method another_method is...
Looking at examples, it's sometimes declared as a class method, but usually you call class methods in your code like this:
AnotherClass.another_method
And most importantly you call your methods regardless if they are from another class, module, or inherited class inside another method of your class you use them. For example:
module SomeModule
def some_method; end
end
class AnotherClass
private
def another_method; end
end
class YetAnotherClass
self.yet_another_method; end
end
class MyAwesomeClass < AnotherClass
include SomeModule
def initialize
some_method # method from my included module
another_method # method inherited from AnotherClass
YetAnotherClass.yet_another_method # class method called directly
end
end
So what are these methods that can be called at the top of a class and NOT called inside of another method as my examples above?
Are you able to just call class methods inside another class (that it inherits from) outside of your method calls?
I can't get my head around what type of method another_method is...
There is only one type of method in Ruby: every method in Ruby is an instance method of some module. There is no other type of method.
We may sometimes talk about "class methods" or "singleton methods", but that is just a convenient way of talking about a particular way to use methods. The methods themselves are no different.
Every method in Ruby is an instance method of some module. The question is simply: which module?
We are sometimes talking about class methods, but class methods don't really exist. Class methods are simply singleton methods of an object that happens to be an instance of the Class class.
There is absolutely no difference between
Foo = ''
def Foo.bar; end
and
class Foo; end
def Foo.bar; end
and
module Foo; end
def Foo.bar; end
We call the first one a "singleton method", the second one a "class method", and the third one a "module function", but there is actually no difference between the three. The only difference is that the first object is an instance of the String class, the second object is an instance of the Class class, and the third object is an instance of the Module class, that's it.
Actually, I must confess something: I lied. I wrote that class methods are simply singleton methods of an object that happens to be an instance of the Class class, but in reality … singleton methods don't exist either.
A singleton method is simply a boring old normal instance method of the singleton class.
There is no difference between
foo = ''
def foo.bar; end
and
foo = ''
class << foo
def bar; end
end
There you have it: singleton methods are actually just instance methods of the singleton class, class methods are actually just instance methods of the singleton class of an object that is an instance of the Class class, module functions are actually just instance methods of the singleton class of an object that is an instance of the Module class.
It's just that "instance method of the singleton class of an object that is an instance of the Class class" is annoying to say and write all the time, so we say "class method" instead, knowing full well that there is no such thing.
Looking at examples, it's sometimes declared as a class method, but usually you call class methods in your code like this:
AnotherClass.another_method
Again, there is no such thing as a class method. There is exactly one kind of method, and they are always called the same way:
receiver.method(args)
If the receiver is self, then you can leave it out, and if you have no arguments, you can leave them out, too.
And most importantly you call your methods regardless if they are from another class, module, or inherited class inside another method of your class you use them.
That is not true.
In fact, in your own example, you are calling two methods outside of a method body: Module#private and Module#include, and you seem to have no trouble with those. Other methods that I am sure you have already called outside of a method body are Module#attr_accessor, Kernel#require, or Kernel#puts. In fact, in simple scripts, there is often not a single method definition body at all.
So what are these methods that can be called at the top of a class and NOT called inside of another method as my examples above?
They are instance methods just like any other instance methods, there is absolutely nothing special about them.
Are you able to just call class methods inside another class (that it inherits from) outside of your method calls?
Since class methods don't exist, and these are simply instance methods just like every other instance method, the method lookup algorithm is of course also just the method lookup algorithm for instance methods:
Start with the private internal hidden __klass__ pointer of the receiver. If you can't find the method there …
Get the private internal hidden __superklass__ pointer and repeat.
And that's it. (Yes, okay, there is a tiny bit more to it: if you come to a point where there is no __superklass__ pointer, then you try again with method_missing and the name of the method as an argument, and if you also cannot find that, then you raise a NoMethodError.)
So, let's try that in your example:
class MyAwesomeClass
another_method
end
Okay, first off: what is the receiver? Well, if no receiver is given, then the receiver is always implicitly self. Now, what is self here?
A ha! That is actually the most important question in Ruby, always. At any point in Ruby, you must always know what self is.
Well, we know that inside of a method body, self is the receiver of the message send. So, we can guess: what would make the most sense for self inside of a class definition. The class itself, of course! Let's test that:
what_is_self = class MyAwesomeClass
self
end
p what_is_self
# MyAwesomeClass
Well, looks like we're right. Okay, we know the receiver of the message send is MyAwesomeClass, so what is the first step in our algorithm? We get the private internal hidden __klass__ pointer.
We cannot actually do that in Ruby (it is a private internal hidden implementation detail of the Ruby implementation). But let me assure you that is pretty much always going to be the singleton class. (There are some objects which cannot have singleton classes, and some Ruby implementations optimize the creation of singleton classes so that they are only created when needed, but for all intents and purposes, we can assume that the __klass__ pointer is always the singleton class.)
We look inside the singleton class, and we find no definition of the method another_method. So, we move to the second step of the algorithm: we get the __superklass__ of the singleton class.
The __superklass__ of the singleton class is usually going to be the class of the object. So, in this case, the object is MyAwesomeClass, which is a class, so the class of MyAwesomeClass and the __superklass__ of the singleton class is going to be Class.
Again, we look inside Class and we don't find another_method. Ergo, we get Class's __superklass__, which is Module. We also don't find another_method here, so we move on to Object, which also doesn't have another_method.
Now, it gets interesting again, because Object's __superklass__ pointer is actually Kernel and not BasicObject as you might have thought. (More precisely, it is an include class which shares its method table with the Kernel module, since Kernel is not a class at all and thus cannot be a superclass.)
Kernel also doesn't have a method named another_method, so we get Kernel's __superklass__ (again, technically we are talking about an include class that proxies Kernel and not Kernel itself), which is BasicObject. Finally, BasicObject does not have a __superklass__.
Which means we start aaaaaaaaaaaall the way back from step #1 again, but this time as-if you had written
class MyAwesomeClass
method_missing(:another_method)
end
We do the whole song-and-dance again, and we also never find a method until we get to the very top, to BasicObject, which has a method_missing that roughly looks like this:
class BasicObject
def method_missing(meth, *args)
raise NoMethodError, "undefined method `#{meth}' for #{inspect}", meth, args, receiver: self
end
end
So, if you want your call to another_method to not fail, it needs to be defined anywhere along the whole chain we just walked up:
In MyAwesomeClass's singleton class
In a module that MyAwesomeClass extends
In Class
In a module that Class includes
Or in a module included by that module
In Module
In a module that Module includes
Or in a module included by that module
In Object
In Kernel
Or another module that Object includes
Or in a module that Kernel includes
In BasicObject
In a module that BasicObject includes
Or in a module included by that module
Or maybe the Ruby implementation you are using has an implementation-specific superclass of BasicObject (e.g. MacRuby has Objective-C's NSObject as the superclass of BasicObject)
To illustrate difference between different type of methods, consider this example:
class Foo
puts 'method called outside of method'
def initialize
puts 'initialize called'
end
def instanse_method
puts 'instance method called'
end
def self.clazz_method
puts 'class method called'
end
end
Foo
foo = Foo.new
foo.instanse_method
Foo.clazz_method
What will be the output?
method called outside of method
initialize called
instance method called
class method called
So what are these methods that can be called at the top of a class and NOT called inside of another method as my examples above?
As you can see, any method can be called before initialize method. It's executed when class is loaded.
Just calling
Foo
would be sufficient enough to output:
method called outside of method
(but notice that it was not called multiple times)
Looking at examples, it's sometimes declared as a class method, but usually you call class methods in your code like this: AnotherClass.another_method
It's like static function in PHP. In my example, it's Foo#clazz_method.
And most importantly you call your methods regardless if they are from another class, module, or inherited class inside another method of your class you use them
Usually, you have to include a file that define another class before using its method. But most frameworks like Ruby on Rails, have already built-in autoloading system, so it looks like it's working magically without explicit require or include. But there are times when RoR does not know where your custom classes are. In this case, you have to require the file explicitly before you use it. Well, usually in the beginning of the file.
It'll be easier to understand by looking at it in action.
# Step 1
# Run this in IRB
class MyClass
def self.another_method
puts "Executed when class is loaded to memory"
end
end
# Step 2
# Now when you run the following in IRB after that, its going to execute the
# class method which was defined in its parent class (MyClass)
class MyAwesomeClass < MyClass
another_method # Executed ONCE when the class is loaded to memory for the first
def initialize; end
end
# Now if you instantiate MyAwesomeClass though, it will not print the same as of
# Step 2 as class method :another_method already has been executed when class
# was loaded
awesome1 = MyAwesomeClass.new
The body of a class will be interpreted and executed sequentially & behaves much like you'd expect it inside an instance method. Try putting a puts "Hello World" in the body of your class definition like so:
class MyClass
# Executes when the class is defined in IRB(Loaded to memory)
puts "Hello World"
end
I was working on a coding problem at codewars.com. This is the answer to said problem:
class Conjurer
def self.conjure(name, lambda)
define_method name, lambda
end
end
I would like to articulate the way I am reading it, and receive criticism that helps me better understand the meaning of this code.
The class Conjurer is created.
An instance method of the class is called, representing the class itself.
It is given permanent parameters name and lambda.
Unsure what define_method name, lambda means.
define_method makes a method named name and passes it a block lambda. So if you call the method name, it runs the block represented by the variable lamba.
The self.conjure is allowing you do define a method within the class: the name parameter becomes the name of the method, the lambda becomes the body of the method itself.
The class Conjurer is created.
Yes. More accurately, the Conjurer class is opened, meaning that it's being opened for modification. In this case, the conjure method is being added.
In Ruby, you can open a class many times:
class Conjurer
def self.something
# code
end
end
class Conjurer
def self.other_thing
# code
end
end
This is the same as doing
class Conjurer
def self.something
# code
end
def self.other_thing
# code
end
end
An instance method of the class is called, representing the class itself.
No. It's not a method call, it's a method definition. The def keyword defines a new method. The def self. defines a method for the Conjurer class rather than a Conjurer.new instance.
It is given permanent parameters name and lambda.
Almost. The parameters are name and lambda, but they're not arguments of a call (see here for the difference between argument and parameter), because that isn't a method call, is a method definition.
Unsure what define_method name, lambda means.
define_method is part of the Ruby language. It does what is says, it defines a method with the name name that when called, executes lambda. See the docs here.
In Ruby, when making a new class, we will define the constructor method like so:
class Thing
def initialize
do_stuff
end
end
However, when actually making an instance of the object, we find ourselves not calling initialize on the instance but new on the class.
That being the case, why don't we instead define ::new?
class Thing
def self.new
do_stuff
end
end
Is there something ::new does beind the scenes that initalize doesn't define? Are those two different at all? Would defining ::new work? Or is it just that def initialize is shorter (not) than def self.new?
I'm thinking there must be a good reason for the disparity.
New allocates space for the new object and creates it. It then calls the Objects initialize method to create a new Object using the allocated memory. Usually, the only part you want to customize is the actual creation, and are happy to leave the behind memory allocation to the Object.new method, so you write an initialize method. What new is doing under the hood looks something like this (except in C):
class Object
def self.new(*args, &block)
object = allocate
object.send(:initialize, *args, &block)
return object
end
end
So when you call Object.new, what actually happens is:
1) Memory is allocated
2) The objects initialize method is called.
To provide access to instance variables.
Instance variables, such as #value, are accessible from instances only but not from within a class method. This differs from languages such as Java where private instance variables have class rather than instance scope and are thus accessible from the static constructor.
class Thing
def initialize
#value = 42
end
end
class Thing
def self.new
# no way to set the value of #value !!!!!!!!
end
end
For those interested in the history of Ruby, the object model with instance-private instance variables goes back to Smalltalk. And you'll find the same pattern in modern Smalltalk dialect like Pharo, new is implemented in Object to call self initialize such that subclasses can easily initialize instance variables.
To elaborate on Abraham's point, in effect, that is reversing the wrapping relation. If you had allocate as a primitive and usually define new, then you would always have to do common things like calling allocate and returning the object at the end, which is a redundant thing to do. By having new and initialize, the former will wrap the latter, so you only need to define what is wrapped, not the wrapper.
new is a class method, so when you define that, you do not have access the instance methods and instance variables by default, and you need to rely on accessors. On the other hand, initialize is an instance method, so it will be easier to work with instance variables and instance methods.
Is there a way to get a true proc from a method in Ruby?
An UnboundMethod obtained via instance_method does not fit the bill because I can only bind it to an object of the class that declared the method. I can't reinterpret self inside the method body the way I could in a proc (using instance_exec).
Similarly, a Method obtained via method is not okay, because self is bound to the receiver of method and I cannot change it.
Edit (Clarification):
What I'm trying to do is to take a method defined in one class and transfer it to another class. This means I need to be able to reinterpret the meaning of self within the method. For procs, this is possible via instance_exec and instance_eval, but not for methods.
Why I am trying to move methods from one class to another? Long story short, to implement a form of namespacing, as I am most displeased with the visibility control provided by Ruby (there is no way to hide a module member to an including class). This is however far beyond the scope of this question.
Maybe to_proc from Method can help you:
class A
def test
puts 'this is a test'
end
end
m = A.new.method(:test)
m.to_proc.call #=> this is a test
UPDATE: Just an idea
By using sourcify gem convert proc from first object to source, and then evaulate it in the context of second object
I have been a Java programmer for a while and I am trying to switch to ruby for a while. I was just trying to develop a small test program in ruby and my intention is something like following.
I want to create a simple linked list type of an object in ruby; where an instance variable in class points to another instance of same type.
I want to populate and link all nodes; before the constructor is called and only once. Something that we'd usually do in Java Static block.
Initialize method is a constructor signature in ruby. Are there any rules around them? Like in Java you cannot call another constructor from a constructor if its not the first line (or after calling the class code?)
Thanks for the help.
-Priyank
I want to create a simple linked list type of an object in ruby; where an instance variable in class points to another instance of same type.
Just a quick note: the word type is a very dangerous word in Ruby, especially if you come from Java. Due to an historic accident, the word is used both in dynamic typing and in static typing to mean two only superficially related, but very different things.
In dynamic typing, a type is a label that gets attached to a value (not a reference).
Also, in Ruby the concept of type is much broader than in Java. In Java programmer's minds, "type" means the same thing as "class" (although that's not true, since Interfaces and primitives are also types). In Ruby, "type" means "what can I do with it".
Example: in Java, when I say something is of type String, I mean it is a direct instance of the String class. In Ruby, when I say something is of type String, I mean it is either
a direct instance of the String class or
an instance of a subclass of the String class or
an object which responds to the #to_str method or
an object which behaves indistinguishably from a String.
I want to populate and link all nodes; before the constructor is called and only once. Something that we'd usually do in Java Static block.
In Ruby, everything is executable. In particular, there is no such thing as a "class declaration": a class body is just exectuable code, just like any other. If you have a list of method definitions in your class body, those are not declarations that are read by the compiler and then turned into a class object. Those are expressions that get executed by the evaluator one by one.
So, you can put any code you like into a class body, and that code will be evaluated when the class is created. Within the context of a class body, self is bound to the class (remember, classes are just objects like any other).
Initialize method is a constructor signature in ruby. Are there any rules around them? Like in Java you cannot call another constructor from a constructor if its not the first line (or after calling the class code?)
Ruby doesn't have constructors. Constructors are just factory methods (with stupid restrictions); there is no reason to have them in a well-designed language, if you can just use a (more powerful) factory method instead.
Object construction in Ruby works like this: object construction is split into two phases, allocation and initialization. Allocation is done by a public class method called allocate, which is defined as an instance method of class Class and is generally never overriden. It just allocates the memory space for the object and sets up a few pointers, however, the object is not really usable at this point.
That's where the initializer comes in: it is an instance method called initialize, which sets up the object's internal state and brings it into a consistent, fully defined state which can be used by other objects.
So, in order to fully create a new object, what you need to do is this:
x = X.allocate
x.initialize
[Note: Objective-C programmers may recognize this.]
However, because it is too easy to forget to call initialize and as a general rule an object should be fully valid after construction, there is a convenience factory method called Class#new, which does all that work for you and looks something like this:
class Class
def new(*args, &block)
obj = alloc
obj.initialize(*args, &block)
return obj
end
end
[Note: actually, initialize is private, so reflection has to be used to circumvent the access restrictions like this: obj.send(:initialize, *args, &block)]
That, by the way, is the reason why to construct an object you call a public class method Foo.new but you implement a private instance method Foo#initialize, which seems to trip up a lot of newcomers.
To answer your question: since an initializer method is just a method like any other, there are absolutely no restrictions as to what you can do whithin an initializer, in particular you can call super whenever, wherever, however and how often you want.
BTW: since initialize and new are just normal methods, there is no reason why they need to be called initialize and new. That's only a convention, although a pretty strong one, since it is embodied in the core library. In your case, you want to write a collection class, and it is quite customary for a collection class to offer an alternative factory method called [], so that I can call List[1, 2, 3] instead of List.new(1, 2, 3).
Just as a side note: one obvious advantage of using normal methods for object construction is that you can construct instances of anonymous classes. This is not possible in Java, for absolutely no sensible reason whatsoever. The only reason why it doesn't work is that the constructor has the same name as the class, and anonymous classes don't have a name, ergo there cannot be a constructor.
Although I am not quite sure why you would need to run anything before object creation. Unless I am missing something, shouldn't a list basically be
class List
def initialize(head=nil, *tail)
#head = head
#tail = List.new(*tail) unless tail.empty?
end
end
for a Lisp-style cons-list or
class List
def initialize(*elems)
elems.map! {|el| Element.new(el)}
elems.zip(elems.drop(1)) {|prv, nxt| prv.instance_variable_set(:#next, nxt)}
#head = elems.first
end
class Element
def initialize(this)
#this = this
end
end
end
for a simple linked list?
You can simply initialize your class variables in the class body, outside of any method declaration. It will behave like a static initializer in Java:
class Klass
##foo = "bar"
def sayFoo
puts ##foo
end
def self.sayFoo
puts ##foo
end
end
The class field ##foo is here initialized to "bar".
In ruby object creation works like this
class Class
def new(*args)
obj= self.allocate # get some memory
obj.send(:initialize) # call the private method initialize
end
end
Object#initialize is just an ordinary private method.
If you wan't something to happen before Object#initialize you have to write your own Class#new. But I see no reason why you would want to do that.
This is basically the same answer paradigmatic gave back in '09.
Here I want to illustrate that the "static initializer" can call other code. I'm simulating a scenario of loading a special user once, upon class initialization.
class Foo
def user
"Thomas"
end
end
class Bar
##my_user = Foo.new.user
def my_statically_defined_user
##my_user
end
end
b = Bar.new
puts b.my_statically_defined_user # ==> Thomas