I understand that local variables are limited to the scope they were declared in and instance variables exist as long as the class exists, but what happens if you declare a local variable in the class scope without prefixing it with #? Doesn't that implicitly it is an instance variable, even though you didn't use an # to declare it as one?
instance variables exist as long as the class exists
They exist as long as the object exist. Instance variables are per-object, not per-class.
what happens if you declare a local variable in the class scope without prefixing it with #?
Then the variable is in scope within the class definition, but not within any defs inside that class definition as those introduce a new scope.
Doesn't that implicitly make it an instance variable, even though you didn't use an # to declare it as one?
No.
If you use define_method instead of def to create methods, the local variable will be accessible within the methods, but since the variable only exists once (not once per object), they'd act more like class variables than instance variables in that case. I also can't think of a good reason why you'd use them that way.
Using #makes it an instance variable for an object that you create. When you are doing things with that object you can set local variables but they disappear after use. Instance variables will stay around as long as there is an object.
Related
Ruby has 5 variable scopes:
Local Variables: these are the normal variables, example x = 25, y = gaurish where x and y are local variables.
Instance Variables: these are denoted with # symbol infront of the actual variable name. mainly used with classes, so that each instance/object of the class has a different/separate value. example. #employee.name = 'Alex'
Class Variables: denoted with ## symbols in front of variable name. class variable, I think have same value accos all instances/object.
Global variables: they start with $ symbol and are accessible everywhere. example $LOAD_PATH
Constants: Must start with Capital letter but by convention written in ALL_CAPS. although, it is a constant but its value its not constant and can be changed(ruby will throw a warning, though). so in the sense, this also acts like a variable.
As you may notice,all of the above are variables which store some value of some type and their value can be changed. But, each scope does something little bit different. Having 5 different types of variable scopes is confuses hell out of me. Mainly, I have difficulty deciding under what case, I should be using a particular scope in my code. so I have some questions in my mind. please answer:
I notice that local variables and class variables stay same for all objects/instances, unlike instance variables. so what difference between Local variables and Class variables?
Can local variables be used in place of class variables? or vice-versa. And if yes, then why, and if no, then why not?
Global variables in ruby remind me of the evil global $x variables in PHP. Are global variables in ruby also considered evil and therefore should not be used. OR, there are specific cases where it makes sense to use global variables in ruby?
Why constants are not constants and allow their value to be changed? A constant's value by definition should be constant right? else, we can just use another variable and don't change its value. would that be equivalent to a CONSTANT in ruby?
Any page/resource/link which explains the difference between 5 different variable scopes in ruby? I like to keep one handy for reference.
Under what use-case, I should be using a particular variable scope in my code. so one would explain all 5 cases with can example that would be cool, this is my main reason for confusion.
is there a de facto choice like public in java? Which would be the safe bet in most use-cases?
Thanks for taking time to read and answer question
Class variables are the same for all instances, because they're class variables–associated with the class. Everything access the same variable, including each instance.
No. Local variables are just that–local. They may be local to a function, or local to the class declaration, which is different than being a class variable. Locals in a class declaration go out of scope when the class declaration ends.
That's because they're exactly the same–they're global. Global state is always evil; this is not a property of the language or environment. That said, some global state may be required–that's just the way it is. It makes sense to use global state when there's global state. The trick is to use global state properly, which is sometimes a non-trivial endeavor.
That's just how Ruby is.
One has already been given by Chris.
I would think this question would be largely self-answering. Global when the entire world needs access. Instance when it's specific to a class instance. Local when it's only required in a local scope (e.g., a method, a block (note differences between 1.8 and 1.9 with regard to block scope), etc.) Constant when the variable isn't supposed to change. A class variable when it's something that either every instance needs, or if exposed via a class method, something tightly associated with a class.
There is no "most use-cases", it totally depends on what you're doing with the variable. And public isn't the de facto choice in Java–it depends on the entity in question. Default Java scope is package-private (methods, properties). Which to use in Ruby depends entirely upon the use-case, noting that as with Java, and even more easily in Ruby, things can be circumvented.
Local variables are not equivalent to class ones, they are tied to the block you are in. Any local declared in a block can be used there. Class variables are tied to the class structure you are in. Defining a class is itself a block, so you might be able to access local variables similarly, but you'll find if you refer to your class in a different context you cannot refer to the local variable in the same fashion.
Global variables are considered bad form, if abused. OOP should help you structure your programs such that constants on the entire program aren't needed. Think of them as actually being global: if you can't vouch for its consistency to the entire world, like the value of PI or whether or not you love your wife, you probably shouldn't be making promises.
Once you get the hang of them, I find "Ruby Variable Scope" to be a good reference.
I found a good and in-depth explanation regarding different scopes and their visibilities in Ruby in the following link .Scopes and Visibilities with Examples in detail.
According to it ,
Class variable (##a_variable): Available from the class definition and any sub-classes. Not available from anywhere outside.
Instance variable (#a_variable): Available only within a specific object, across all methods in a class instance. Not available directly from class definitions.
Global variable ($a_variable): Available everywhere within your Ruby script.
Local variable (a_variable): It is available only in the particular method or block in ruby script.
Some more explanation regarding Instance variables is : You can access instance variables in any method of that particular class. While local variables can't be accessed outside of the method. Below is an example from ruby monk which explains this concept thoroughly.
class Item
def initialize(item_name, quantity)
#item_name = item_name
#quantity = quantity
end
def show
puts #item_name
puts #quantity
supplier = "Acme corp"
puts supplier
end
end
Item.new("tv",1).show
Here supplier is a local variable which can be accessed from show method only in our example.Just try to declare the supplier variable in the initialize method and printing it out in show method. It will give an error as undefined variable supplier.
i hope it helps. :)
Here is my two cents for this and is writing for c++/java/c# programmers:
A Ruby local/$Global variable is a bit like c++/java/c#'s local/Global variable.
A Ruby #instance variable it's like c++/java/c#'s member variable /class property which can be accessible in any member function/class method.Though Ruby instance can be applied not only to class but also to Ruby Module.
A Ruby ##class variable is like as c++/java's static member variable which is share between all instances of the same class.
I have a number of questions regarding ruby classes and modules. I have written a number of test files to kind of explore them further that are too long to post here (https://github.com/Senjai/Learning-Ruby/blob/master/class_test.rb and https://github.com/Senjai/Learning-Ruby/blob/master/mixin-instance-var-conflict.rb)
First three questions are about variables.
So far that I understand it, instance variables are prefixed with #, and are unique to each instance of the class. They can only be accessed with accessor methods (like attr_accessor)
Second, class variables, they're prefixed by ##.
How come class variables can't be used with attr_accessor/reader?
What's the difference between class.class_variable and instance.class_variable? If an instance modifies a class variable from within the class (e.g in an instance method) does that affect the static variable in the class definition?
Thirdly,
Why do instance/class variables need to be prefixed with #? What happens when they aren't prefixed at all? Like in the example below:
module Test
State = {}
def state=(value)
State[object_id] = value
end
def state
State[object_id]
end
end
For class methods, I understand that instance methods are defined as normal functions, and class methods have to be prefixed with self. Why can't you use class methods in an instantiated object? Why does self make it so special?
Lastly, for the scope resolution operator, I've only found that it works for constants and class methods, what's the point of having a scope resolution operator in the first place if they can be accessed in dot notation?
So I want a module with a variable and access methods.
My code looks something like this
module Certificates
module Defaults
class << self
attr_accessor :address
def get_defaults
address = "something"
make_root_cert
end
def make_root_cert
blub = address
# do somthing
end
end
end
I inspected it with pry.
The result is
Certificates::Defaults has methods called address and address=.
If I call address in the get_defaults method it returns "something" as expected
If I call it in make_root_cert it returns nil
I used this way of attr_accessor creation in another module and it worked fine. I hope I'm just misunderstanding the way ruby works and somebody can explain why this example doesn't work. Maybe using the implementation details of the ruby object model.
Jeremy is right.
My findings
This seems inconsistent to me.
If you use the expression "address" and the instance variable has not been set it returns the local variable
If the instance variable has been set and the local variable not it returns the instance variable.
If both have been set it returns the local variable.
On the other hand address="test" always sets the local variable.
In your get_defaults methods, address is a local variable. To use the setter, you have to type this:
self.address = "something"
That will properly call the address= method.
This rather confusing behavior occurs because the Ruby interpreter places local variable definition at a higher precedence than method calls. There is consistency here, but unless you know how it works in advance it can be hard to see clearly.
Given that so many things in Ruby are objects and method calls, it might be natural to assume that variable definition was some sort of a method called on something (like Kernel or main or the object in which it was defined or whatever) and that the resulting variable was some sort of object. If this was the case, you would guess that the interpreter would resolve name conflicts between variable definitions and other methods according to the rules of method lookup, and would only define a new variable if it didn't find a method with the same name as the potential variable definition first.
However, variable definition is not a method call, and variables are not objects. Instead, variables are just references to objects, and variable definition is something the interpreter keeps track of below the surface of the language. This is why Kernel.local_variables returns an array of symbols, and there's no way to get an array of some sort of local variable objects.
So, Ruby needs a special set of rules to handle name conflicts between variables and methods. Non-local variables have a special prefix denoting their scope ($, #, etc.) which fixes this, but not so for local variables. If Ruby required parens after methods, that would also address this problem, but we are given the luxury of not having to do that. To get the convenience of referencing local variables without a prefix and invoking methods without parens, the language just defaults to assuming you want the local variable whenever it's in scope. It could have been designed the other way, but then you would have weird situations where you defined a local variable and it was instantly eclipsed by some faraway method with the same name halfway across the program, so it's probably better like this.
The Ruby Programming Language, p. 88, has this to say:
"...local variables don't have a punctuation character as a prefix. This means that local variable references look just like method invocation expressions. If the Ruby interpreter
has seen an assignment to a local variable, it knows it is a variable and not a method, and it can return the value of the variable. If there has been no assignment, then Ruby treats the expression as a method invocation. If no method by that name exists, Ruby raises a NameError."
It goes on to explain why you were getting nil when calling address in make_root_cert:
"In general, therefor, attempting to use a local variable before it has been initialized results in an error. There is one quirk--a variable comes into existence when the Ruby interpreter sees an assignment expression for that variable. This is the case even if that assignment is not actually executed. A variable that exists but has not been assigned a value is given the default value nil. For example:
a = 0.0 if false # This assignment is never executed
print a # Prints nil: the variable exists but is not assigned
print b # NameError: no variable or method named b exists"
The setter method you get with attr_accessor leads the interpreter to create a variable before the setter method is ever called, but it has to be called to assign that variable a value other than nil. address = "something" in get_defaults defines a local variable within that method called address that goes out of scope at the end of the method. When you call make_root_cert, there's no local variable called address, so the getter method address that you got with attr_accessor is called and returns nil because the setter method hasn't been called to give it some other value. self.address= lets the interpreter know that you want the class method address= instead of a new local variable, resolving the ambiguity.
Ruby has 5 variable scopes:
Local Variables: these are the normal variables, example x = 25, y = gaurish where x and y are local variables.
Instance Variables: these are denoted with # symbol infront of the actual variable name. mainly used with classes, so that each instance/object of the class has a different/separate value. example. #employee.name = 'Alex'
Class Variables: denoted with ## symbols in front of variable name. class variable, I think have same value accos all instances/object.
Global variables: they start with $ symbol and are accessible everywhere. example $LOAD_PATH
Constants: Must start with Capital letter but by convention written in ALL_CAPS. although, it is a constant but its value its not constant and can be changed(ruby will throw a warning, though). so in the sense, this also acts like a variable.
As you may notice,all of the above are variables which store some value of some type and their value can be changed. But, each scope does something little bit different. Having 5 different types of variable scopes is confuses hell out of me. Mainly, I have difficulty deciding under what case, I should be using a particular scope in my code. so I have some questions in my mind. please answer:
I notice that local variables and class variables stay same for all objects/instances, unlike instance variables. so what difference between Local variables and Class variables?
Can local variables be used in place of class variables? or vice-versa. And if yes, then why, and if no, then why not?
Global variables in ruby remind me of the evil global $x variables in PHP. Are global variables in ruby also considered evil and therefore should not be used. OR, there are specific cases where it makes sense to use global variables in ruby?
Why constants are not constants and allow their value to be changed? A constant's value by definition should be constant right? else, we can just use another variable and don't change its value. would that be equivalent to a CONSTANT in ruby?
Any page/resource/link which explains the difference between 5 different variable scopes in ruby? I like to keep one handy for reference.
Under what use-case, I should be using a particular variable scope in my code. so one would explain all 5 cases with can example that would be cool, this is my main reason for confusion.
is there a de facto choice like public in java? Which would be the safe bet in most use-cases?
Thanks for taking time to read and answer question
Class variables are the same for all instances, because they're class variables–associated with the class. Everything access the same variable, including each instance.
No. Local variables are just that–local. They may be local to a function, or local to the class declaration, which is different than being a class variable. Locals in a class declaration go out of scope when the class declaration ends.
That's because they're exactly the same–they're global. Global state is always evil; this is not a property of the language or environment. That said, some global state may be required–that's just the way it is. It makes sense to use global state when there's global state. The trick is to use global state properly, which is sometimes a non-trivial endeavor.
That's just how Ruby is.
One has already been given by Chris.
I would think this question would be largely self-answering. Global when the entire world needs access. Instance when it's specific to a class instance. Local when it's only required in a local scope (e.g., a method, a block (note differences between 1.8 and 1.9 with regard to block scope), etc.) Constant when the variable isn't supposed to change. A class variable when it's something that either every instance needs, or if exposed via a class method, something tightly associated with a class.
There is no "most use-cases", it totally depends on what you're doing with the variable. And public isn't the de facto choice in Java–it depends on the entity in question. Default Java scope is package-private (methods, properties). Which to use in Ruby depends entirely upon the use-case, noting that as with Java, and even more easily in Ruby, things can be circumvented.
Local variables are not equivalent to class ones, they are tied to the block you are in. Any local declared in a block can be used there. Class variables are tied to the class structure you are in. Defining a class is itself a block, so you might be able to access local variables similarly, but you'll find if you refer to your class in a different context you cannot refer to the local variable in the same fashion.
Global variables are considered bad form, if abused. OOP should help you structure your programs such that constants on the entire program aren't needed. Think of them as actually being global: if you can't vouch for its consistency to the entire world, like the value of PI or whether or not you love your wife, you probably shouldn't be making promises.
Once you get the hang of them, I find "Ruby Variable Scope" to be a good reference.
I found a good and in-depth explanation regarding different scopes and their visibilities in Ruby in the following link .Scopes and Visibilities with Examples in detail.
According to it ,
Class variable (##a_variable): Available from the class definition and any sub-classes. Not available from anywhere outside.
Instance variable (#a_variable): Available only within a specific object, across all methods in a class instance. Not available directly from class definitions.
Global variable ($a_variable): Available everywhere within your Ruby script.
Local variable (a_variable): It is available only in the particular method or block in ruby script.
Some more explanation regarding Instance variables is : You can access instance variables in any method of that particular class. While local variables can't be accessed outside of the method. Below is an example from ruby monk which explains this concept thoroughly.
class Item
def initialize(item_name, quantity)
#item_name = item_name
#quantity = quantity
end
def show
puts #item_name
puts #quantity
supplier = "Acme corp"
puts supplier
end
end
Item.new("tv",1).show
Here supplier is a local variable which can be accessed from show method only in our example.Just try to declare the supplier variable in the initialize method and printing it out in show method. It will give an error as undefined variable supplier.
i hope it helps. :)
Here is my two cents for this and is writing for c++/java/c# programmers:
A Ruby local/$Global variable is a bit like c++/java/c#'s local/Global variable.
A Ruby #instance variable it's like c++/java/c#'s member variable /class property which can be accessible in any member function/class method.Though Ruby instance can be applied not only to class but also to Ruby Module.
A Ruby ##class variable is like as c++/java's static member variable which is share between all instances of the same class.
I have a class something like below, and I used instance variables (array) to avoid using lots of method parameters.
It works as I expected but is that a good practice?
Actually I wouldn't expect that worked, but I guess class methods are not working as static methods in other languages.
class DummyClass
def self.dummy_method1
#arr = []
# Play with that array
end
def self.dummy_method2
# use #arr for something else
end
end
The reason instance variables work on classes in Ruby is that Ruby classes are instances themselves (instances of class Class). Try it for yourself by inspecting DummyClass.class. There are no "static methods" in the C# sense in Ruby because every method is defined on (or inherited into) some instance and invoked on some instance. Accordingly, they can access whatever instance variables happen to be available on the callee.
Since DummyClass is an instance, it can have its own instance variables just fine. You can even access those instance variables so long as you have a reference to the class (which should be always because class names are constants). At any point, you would be able to call ::DummyClass.instance_variable_get(:#arr) and get the current value of that instance variable.
As for whether it's a good thing to do, it depends on the methods.
If #arr is logically the "state" of the instance/class DummyClass, then store it in instance variable. If #arr is only being used in dummy_method2 as an operational shortcut, then pass it as an argument. To give an example where the instance variable approach is used, consider ActiveRecord in Rails. It allows you to do this:
u = User.new
u.name = "foobar"
u.save
Here, the name that has been assigned to the user is data that is legitimately on the user. If, before the #save call, one were to ask "what is the name of the user at this point", you would answer "foobar". If you dig far enough into the internals (you'll dig very far and into a lot of metaprogramming, you'll find that they use instance variables for exactly this).
The example I've used contains two separate public invocations. To see a case where instance variables are still used despite only one call being made, look at the ActiveRecord implementation of #update_attributes. The method body is simply load(attributes, false) && save. Why does #save not get passed any arguments (like the new name) even though it is going to be in the body of save where something like UPDATE users SET name='foobar' WHERE id=1;? It's because stuff like the name is information that belongs on the instance.
Conversely, we can look at a case where instance variables would make no sense to use. Look at the implementation of #link_to_if, a method that accepts a boolean-ish argument (usually an expression in the source code) alongside arguments that are ordinarily accepted by #link_to such as the URL to link to. When the boolean condition is truthy, it needs to pass the rest of the arguments to #link_to and invoke it. It wouldn't make much sense to assign instance variables here because you would not say that the invoking context here (the renderer) contains that information in the instance. The renderer itself does not have a "URL to link to", and consequently, it should not be buried in an instance variable.
Those are class instance variables and are a perfectly legitimate things in ruby: classes are objects too (instances of Class) and so have instance variables.
One thing to look out for is that each subclass will have its own set of class instance variables (after all these are different objects): If you subclassed DummyClass, class methods on the subclass would not be able to see #arr.
Class variables (##foo) are of course the other way round: the entire class hierarchy shares the same class variables.