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I was wondering, if I define a new class variable, for example for the class MyClass, is the definition going to be in MyClass or in MyClass class? Does MyClass class even know about the new class variable?
Yes, class variables are shared with the class and the metaclass. They are also shared with all subclasses (and their metaclasses). A class variable is usually Capitalized, to convey better the idea of being shared in a scope broader than the class. You define class variables in the class (not the metaclass).
Class variables should not be confused with class instance variables, which are instance variables defined at the metaclass level, i.e., instance variables of the class object. This notion is somewhat obscure despite its simplicity (or because of it): instance variables are always defined in the class to define the shape (slots) of its instances. Thus, if we apply this definition to the metaclass, which is the class of the class, an instance variable defined here defines the shape of its instances, of which there is (usually) only one, the class.
Going back to class variables, you define them in the class (inst side) and initialize them in the metaclass (i.e., class side). Remember that these are (partial) globals in the sense that will be shared among instances, subinstances, subclasses and metaclasses and so they must be handled with the usual care we treat globals.
One more clarification
When we say that instance variables are shared among instances and subinstances, we mean their names (and positions in memory of the object slots); we don't mean their values (contents of said slots). Thus, two instances of the class C will share the name, say color, if the class defines the ivar color, but their values at each of the instances will be independent. In other words, what it is shared is the name, not the value.
With class variables what is shared is both the name and the value. It is actually the Association object, for example Theme -> aTheme, what's shared. In consequence, any modification to the value of a class variable affects all its references. This is not the case with class instance variables because they are nothing but instance variables, except that they shape the class and its subclasses, rather than regular instances and subinstances.
For more information on Smalltalk variables see https://stackoverflow.com/a/42460583/4081336
Just as a complement to Leandro's answer, here is the main Squeak implementation specific method that explains the sharing of class variables between instance side (class) and class side (metaclass):
Metaclass>>classPool
"Answer the dictionary of class variables."
^thisClass classPool
where thisClassis the unique instance of the Metaclass, that is the class itself...
There are high chances though to find similar implementation in most Smalltalk dialects.
The compiler will first try to resolve the variable as a method/block temporary (including methd/block parameters), then instance variables, then shared variables.
The classPool method is sent by the compiler in this last phase.
A Leandro did explain, the compiler either resolve the binding just as an offset that will be directly transcripted in the bytecode in case of instance variable slot or method temporary variable, or as a kind of Association for the shared variable case, this association being generally added to the CompiledMethod literals and effectively shared among all methods dealing with this variable (all methods points to the same Assocation object which is effectively shared).
The compiler part is much more dialect specific, in Squeak, it's this method which is used for resolving the binding of shared variables:
class>>bindingOf: varName environment: anEnvironment
"Answer the binding of some variable resolved in the scope of the receiver"
| aSymbol binding |
aSymbol := varName asSymbol.
"First look in local classVar dictionary."
binding := self classPool bindingOf: aSymbol.
binding ifNotNil:[^binding].
"Next look in local shared pools."
self sharedPools do:[:pool |
binding := pool bindingOf: aSymbol.
binding ifNotNil:[^binding].
].
"Next look into superclass pools"
superclass ifNotNil: [^ superclass bindingOf: aSymbol environment: anEnvironment].
"No more superclass... Last look in declared environment."
^anEnvironment bindingOf: aSymbol
This is to remind you that one of the most interesting part of Smalltalk is that you can dig into the implementation from within the IDE, Smalltalk is essentially written in Smalltalk!
I was looking through this example:
class SQLObject
def self.columns
return #columns if #columns
columns = DBConnection.execute2(<<-SQL).first
SELECT
"#{table_name}".*
FROM
"#{table_name}"
LIMIT
0
SQL
columns.map!(&:to_sym)
#columns = columns
end
def self.table_name
#table_name ||= self.name.underscore.pluralize
end
def insert
column_symbols = self.class.columns.drop(1)
column_names = column_symbols.map(&:to_s).join(", ")
question_marks = (['?'] * column_symbols.count).join(", ")
DBConnection.execute(<<-SQL, *attribute_values)
INSERT INTO
#{self.class.table_name} (#{column_names})
VALUES
(#{question_marks})
SQL
self.id = DBConnection.last_insert_row_id
end
end
And I was confused as to why it's ok to call the table_name method in the "self.columns" method as if it were an instance method. Isn't the "table_name" method a class method? Hence, shouldn't it be called as "self.class.table_name" in the "self.columns" method as well?
When inside an abstract class, the self refers to the proper class, not the object. That's why you can access the method without explicitly telling self
Short answer: self is the implicit receiver in Ruby. If you leave it out, the message will be sent to self. In other words, it is never necessary to say self.foo in a message send, because foo is always implicitly the same as self.foo.
Long answer: there is no such thing as a "class method" in Ruby. What you call a "class method" is actually nothing but a singleton method defined on an object which just happens to be an instance of the class Class.
Actually, there are no singleton methods in Ruby, either. A singleton method is really nothing but a regular old instance method which is defined on the singleton class of an object.
[Note: This is not to say that you should not use those terms. Saying "class method" is clearly simpler and communicates intent better than "instance method of the singleton class of an object of class Class". Also, note that there are methods such as Object#singleton_methods. Those terms clearly exist in the Ruby Community, but it is important to understand that these concepts do not exist in the Ruby Language. They are a tool for communication, not an actual Ruby concept.]
The singleton class is a class that is associated with an individual object. Each object has exactly one singleton class and that object is the only instance (the "singleton instance", hence the name) of its singleton class.
In fact, the inheritance hierarchy of an object always starts with its singleton class, i.e. the class pointer of an object always points to its singleton class, and the singleton class's superclass pointer then points to the class that created this object. You just don't see the singleton class in the inheritance chain, because methods like Object#class "skip over" the singleton class when calculating the inheritance chain. It is, however, always used during method lookup.
In fact, method lookup is actually very simple (the only exception is Module#prepend, which I am going to ignore for this explanation):
Dereference the receiver's class pointer.
If that class's method table has a method by the name of the message, invoke it. STOP.
If not, dereference the class's superclass pointer.
GOTO #2.
If you reach a class that doesn't have a superclass, restart the algorithm with the message method_missing(original_message_name, ...), unless the message name is already method_missing, in which case raise a NoMethodError.
So, it all comes down to two questions:
When defining a method, which class (or module) are you defining it in?
When sending a message, which object are you sending the message to?
When you define a method using def foo.bar, Ruby will first evaluate the expression foo and then define a method bar on the resulting object's singleton class (the definee). If you don't provide foo, i.e. you just say def bar, then bar is defined on the default definee, which is a little bit of a tricky concept. Normally, it is the closest lexically enclosing module definition. If there is no enclosing module definition, i.e. you are at the top-level, the default definee is Object and the method visibility is private.
When you send a message using foo.bar, Ruby will first evaluate the expression foo and then send the message bar to the resulting object (the receiver). If you don't provide foo, i.e. if you just say bar, then the default receiver is self.
Which brings us to the next question:
What is self?
Within a method definition body, self is the receiver of the message send that led to the invocation of this method. So, if you send the message foo.bar, then any reference to self in the definition of the method bar will, during this one execution of the method, evaluate to foo.
Within a module or class definition body, self is the class or method itself. (That is why defining a singleton method instance method of the singleton class using def self.bar works in a module or class definition body.)
Within a block or lambda literal, self is lexically captured, i.e. it is whatever self is at the point where the block or lambda literal is written. However, there are a couple of methods which change how self is evaluated, most notably the BasicObject#instance_eval, BasicObject#instance_exec, Module#module_eval, Module#module_exec, Module#class_eval, Module#class_exec, … family of methods.
If you are always aware of the answers to those three questions (where am I defining a method, what is the receiver of a method call, what is self) at any point in your code, then you have basically understood the most important parts of Ruby.
So, to put it all together:
At the point where you are defining the method using def self.columns, we are in a class definition body, so self refers to the class object itself (SQLObject).
The syntax def foo.bar defines a method bar on the singleton class of foo. In this case, you define the method columns on the singleton class of SQLObject. This means that the object SQLObject is the only object in the universe that will respond to the message columns.
Both of these also apply to the definition of self.table_name.
Inside of the method body of self.columns, self will dynamically refer to whatever the receiver is.
When you send the message columns to SQLObject, i.e. you write SQLObject.columns, then inside the body of the SQLObject::columns method, the receiver (i.e. self) will be set to SQLObject. (In this case, the receiver will always be SQLObject, since this is a method of a singleton class.)
So, inside the method body of self.columns, when you write the message send table_name, this is a message send with an implicit receiver. Since the implicit receiver is self, this is equivalent to self.table_name. self at this point is bound to SQLObject, so this is equivalent to SQLObject.table_name. In other words, the message is sent to SQLObject.
Now, per the algorithm I outlined above, the method lookup first retrieves the class pointer from SQLObject. As mentioned, the class pointer always points to the singleton class. So, we look inside the singleton class of SQLObject to see if we find a method named table_name, and we do!
Method lookup is finished, everything works.
Are they given assigned a value the moment they are declared (the interface) or are they assigned a value in the constructor of the class (the implementation)? If possible please give a brief example of how constant variables are assigned values in classes.
Objective-C does not support const instance variables. All instances variables are initialized to zero or nil when the class is instantiated.
How would you return an object from a method, so it is read-only for the caller?
Please note that this isn't a property that can be simply set to read-only when it's getter is declared
i.e #property(nonatomic,retain,readonly) NSDate* pub_date;
For example:
-(SomeClass*)getObject
{
SomeClass* object = [[SomeClass alloc] init];
//Don't allow writing to 'object'
return object;
}
Thanks.
Short answer: there's no simple way of doing this.
Longer answer: Apple's framework defines a sort of standard for its collection classes where the immutable collection is the base class and the mutable collection is the inheriting class. So, for example, NSMutableArray inherits from NSArray. You can follow that standard, and have methods that return MyClass to clients while using MyMutableClass inside them. Technically the client can still send the mutating messages, of course, but IMHO that's not a big risk (after all, the client doesn't know your implementation details).
There are other, more complicated options - you can use pointer swizzling, or subclass and override all mutating methods, or simply copy the mutable class into an immutable counterpart (that's not complicated but may incur a performance hit). But for best results you should probably follow Apple's example.
It depends what the object is. If it has a mutable / immutable pair (like NSString/ NSMutableString) then your getter method can return the immutable version.
Otherwise, you can't control the behaviour of other objects - once you've returned an object, there is no control over it from the object that originally provided it.
If you are concerned that another object may alter an object returned from a getter, and thereby amend the property held within the original object, then you should return a copy of the object instead.
Example:
Object A has a mutable string property, object B asks for this mutable string, the getter directly returns the instance variable backing the property.
Object B then changes the string - the property of object A has also been amended because both objects have a pointer to the same mutable string.
In this case, you would return a copy of the object rather than the object itself. If your object is a custom one, you must implement the NSCopying protocol to allow this.
A further note - declaring a property as read only simply means that no setter accessor will be generated - i.e. objectA.property = newValue; will result in a compiler error.
I've been hearing these two words used in Microsoft tutorials for VB.NET. What is the difference between these two words when used in reference to variables?
Value vis-a-vis Reference Types
Variables in C# are in 1 of 2 groups. Value types or Reference types. Types like int and DateTime are value types. In contrast, any class you create is a reference type. C# strings are also a reference type. Most things in the .NET framework are reference types.
Parts of a Variable
There is the variable name and its value. Two parts.
The variable's name is what you declare it to be. The value is what you assign to it.
Variables are Initialized
All variables are always given an initial value at the point the variable is declared. Thus all variables are initialized.
For value types, like int the compiler will give them a valid value if you do not do so explicitly. int's initialize to zero by default, DateTime's initialize to DateTime.MinValue by default.
Reference type variables initialize to the object you give it. The compiler will not assign an object (i.e. a valid value) if you don't. In this case the value is null - nothing. So we say that the reference is initialized to null.
Objects are Instantiated
Humans are born. Objects are instantiated. A baby is an instance of a Human, an object is an instance of some Class.
The act of creating an instance of a Class is called instantiation (Ta-Da!)
So declare, initialize, and instantiate come together like this
MyClass myClassyReference = new MyClass();
In the above, it is wrong to say "... creating an instance of an object..."
edit - inspired by comments discussion
Three distinct things are going on (above) using distinct terminology and that terminology is not interchangeable :
A reference variable is declared - MyClass myClassyReference
An object is instantiated (...from/of a given class, implied) - new MyClass()
The object is assigned to the variable. =.
Restating the facts:
A reference-type variable is also called simply "a reference". A "value-type variable" is not a reference.
This: "objectA is an instance of an object" is profoundly wrong. If objectA was "an instance of objectB" then it must be that objectA begins life with objectB's type - whatever that is - and current state - whatever that is. What about creating objects D, E, and F as objectB changes? Nay, nay! It is the conceptual and technical case the "objectA is an instance of a Class". "Instantiation" and "instance of" have precise meaning - an object gets its type, definitions, and values from a Class.
MyClass myClassyReference = null Generally we don't say "the variable is assigned to null" and we never say "the variable is referencing null", No. instead we say "the variable is null"; or "the variable is not referencing anything", or "the reference is null"
Practical Application:
I jab my finger at your code and say "this instance has an invalid property. Maybe that's why the loop fails. You gotta validate parameters during instantiation." (i.e. constructor arguments).
I see this in your code ,
MyClass myClassyReference;
myClassyReference.DoSomething();
"You declared the variable but never assigned it. it's null so it's not referencing anything. That's why the method call throws an exception."
end edit
The Unbearable Lightness of Being
A reference type variable's name and value exists independently. And I do mean independent.
An instantiated object may or may not have a reference to it.
An instantiated object may have many references to it.
A declared reference may or may not be pointing to an object.
A variable is initialized with a value. An object is instantiated when memory is allocated for it and it's constructor has been run.
For instance here is a variable:
Dim obj as Object
This variable has not been initialized. Once I assign a value to the obj variable, the variable will be initialized. Here are examples of initialization:
obj = 1
obj = "foo"
Instantiation is a very different thing but is related since instantiation is usually followed by initialization:
Dim obj As New Object()
In the preceding line of code, the obj variable is initialized with the reference to the new Object that was instantiated. We say that the new Object was instantiated because we have created a new instance of it.
Now I believe that VB.NET makes this a lot more confusing than C# because it is not clear that an assignment is taking place in the code above. In C# it is much clearer that there is both an instantiation of an instance and an initialization of a variable:
Object obj = new Object();
To initialize something is to set it to its initial value. To instantiate something is to create an instance of it.
Often this is the more or less same thing. This:
SqlConnection conn = new SqlConnection();
instantiates a SqlConnection object, and initializes the conn variable by setting it to the that instance.
Since an object's constructor also sets the object's properties to their default values, it's often correct to say that instantiating an object initializes it. (Misleading, if the object exposes a method that you have to explictly call to initialize it after it's instantiated, as is sometimes the case.)
*Instantiation means to create an instance for a class or object.Initialization means to *initiate the same object or class for any purpose.**
Instantiated means that an instance of the object has been created. Initiated means that that same object has done some initialization.
When you instantiate a class or object, you're creating a new instance of it, or allocating memory to "hold" one. Initializing that object would be the instructions that are performed during instantiation.
Instantiation is when you create an instance of a class. That instance is then an object, and you can set its properties, or call methods on it (tell it to do things).
Initiation is when you set up a set of initial conditions for something. That something might be an object, where you tell it to initiate itself, or just a variable to which you assign a value.
An object might initialise some other things, or even instantiate other objects as part of its initiation.
The difference is that instantiation is creation of a thing that can do stuff; initiation is stuff that gets done.
See the Java docs:
https://docs.oracle.com/javase/tutorial/java/javaOO/objectcreation.html
"Point originOne = new Point(23, 94);
Declaration: The code set in bold are all variable declarations that associate a variable name with an object type.
Instantiation: The new keyword is a Java operator that creates the object.
Initialization: The new operator is followed by a call to a constructor, which initializes the new object."
We can see it this way. For a line of code below:
var p = new Person();
The above line can be read as following two ways:
The variable p has been initialized as a person class
Person class has been instantiated in variable p
The subject of reference or context matters. When talking in terms of variable, we use the word initialize. When talking in terms of class/type, we use the word instantiate.
Instantiation refers to the allocation of memory to create an instance of a class whereas initialization refers to naming that instance by assigning the variable name to that instance.
Eg: SqlConnection conn = new SqlConnection();
Here new is a keyword which allocates memory for an instance and conn is a variable name assigned for that instance.
Others have explained the difference, so I wont go into detail. But there are cases where instantiation does not properly initialize an object. When you instantiate an object you also initialize it with some data. The class/type will have the initialization logic, whereas the instantiation logic is typically carried out by thenew keyword (basically memory allocation, reference copying etc). But instantiation need not necessarily result in a valid state for objects which is when we can say the object is uninitialzed. Here's a practical example where an object can be instantiated but not initialized (sorry e.g. in C#).
class P { string name = "Ralf"; }
WriteLine(new P().name); // "Ralf";
WriteLine((FormatterServices.GetUninitializedObject(typeof(P)) as P).name); // null
GetUninitializedObject doesn't call the constructor to instantiate object there (but some internal magic).
One could also argue value types are not instantiated but only initialized as it doesn't need new allocation when you do new.. but that's up to one's definition of instantiation.
In object-oriented parlance:
To instantiate means creating an object of some class, which initial state may be undefined.
The class is a blueprint which is used by the program to create objects. Objects created are compliant with the blueprint and can be manipulated by the program. E.g. variables current_client and previous_client can be assigned objects of class Customer. An instance of class X is an object instantiated from class X.
In the code the class is a permanent static description of what an object can do, but the objects themselves are temporary and dynamic. They have an individual state which can be changed (e.g. the Customer name, the associated orders). Instantiation can be done like this:
dim current_client as new Customer (VB)
Customer* current_client = new Customer() (C++)
current_client = Customer() (Python)
new Customer, new Customer() and Customer() are equivalent forms in different languages to trigger the instantiation.
In the end objects are destructed to release memory and other resources required for their existence and working.
To initialize means assigning an initial state to the object before it is used.
This initialization can be part of the instantiation process, in that case values are explicitly assigned to object attributes in the constructor of the object. Alternatively it can be left to the user who can decide whether it is required or not. The latter method allows faster instantiation, but requires the user's code to not read the value of any attribute before this code has explicitly assigned a value to this attribute. E.g. this code:
current_client.count = current_client.count + 1
is not allowed before the attribute count has been set by the user, since it can contain any initial value, including an invalid value which would trigger an execution error.