I'm having trouble initializing class variables in Pharo. I started by creating a class with a single class variable:
Object subclass: #ClassVariableTestBehavior
instanceVariableNames: ''
classVariableNames: 'test'
package: 'DummyPackage'
And then on the class side I created an initialize message and set the variable to nil.
ClassVariableTestBehavior class >>> initialize
test := nil
I saved and then created an instance method:
ClassVariableTestBehavior >>> test
^ test
and went back again and changed the class method to be:
ClassVariableTestBehavior class >>> initialize
test := 34
In the playground I then printed the result of the following:
ClassVariableTestBehavior new test.
Which was nil. Why hasn't the value of the class variable updated to be 34?
The class>>initialize method is used only once, when the code is initially loaded from an external file. It does not get run every time the method is edited. (If you modified a comment would you want the data to get wiped out!?) In fact, the nil value didn't come from your method but was just the initial default value.
A common convention is to add a comment to the initialize method with a line of code that could be executed.
"
ClassVariableTestBehavior initialize.
"
I think you are confused about initialize. Based on your comment you want to create a constant value shared among the instances.
The initialize is usually a method for defining values for instance of an object you are creating. The issue with Smalltalk is that not every Smalltalk implementation runs initialize when creating an instance; so watch out for this one.
If you want to share value among instances you can create a class method like this:
ClassVariableTestBehavior class >> #test
Where:
test
"Class method returning always the same value"
^ 34
When you want to access the value you simply do:
ClassVariableTestBehavior test.
If you want to access it when you have your own instance create it you can do it like this:
| instance |
instance := ClassVariableTestBehavior new.
instance class test. "This accesses the class variable"
Edit - due to excellent comment by Leandro, which I forgot to mention.
You should use capital letter for the class variable! (credit to Leandro)
If you would like to have a value stored in a class variable instead of just constant value in a method as I have shown above. You can do it using class variable.
If you define class variable as Test, then you need to have a getter and setter for it.
You would have:
ClassVariableTestBehavior class >> #test
test
"returns value of class variable"
^ Test
ClassVariableTestBehavior class >> #test:
test: aNumber
"sets the value of class variable"
Test := aNumber
You would access the value the same way as above (just before the edit).
How can we initialize a class variable in Visualworks Smalltalk and destroy it after its use?
I want to know about ClassVariables. NOT ClassInstanceVariables.
I am implementing Singleton pattern and here is my code
MyClass class>> aClasMethod
aClassVariable isNil ifTrue:[
aClassVariable := 'I am a variable'.
]
^aClassVariable.
Once the variable is created, I am not able to destroy it.
i.e Next time I run my code, I see that the class variable is retaining its previous value.
How can I avoid this?
I tried this:
MyClass allInstances do: [:inst |
inst become: nil
].
But of no use.
The best way is simply to add a class method to set the class variable to nil and then call it whenever it's an appropriate time to clear it. I do this all the time with the Singleton pattern.
i think we usually use singleton pattern so that we only have one object. I don't know why do you want to store a string value in the class variable. Try to store a object so you will have only one copy of that specific object and store values in the instance variables. so that specific object has only some specific values. Then in future if u need to update the values stored in instance variable then make one more method updateValues and do everything in that.
In my code I have a class called 'ProfileShareViewController', In which I have imported another class I have created called 'OwnProfileData', And I have also created an Instance of that class (class = OwnProfileData) as a property Of 'ProfileShareViewController' and synthesized it (instance called 'OwnProfile').
In another class I have called 'EditProfileViewController', I have imported the 'ProfileShareViewController', and now I am trying to change a property of the OwnProfile object from the ProfileShareViewController within the EditProfileViewController class.
For some reason that doesn't work. I have Tried typing:
[[ProfileShareViewController ownProfile] setName:#"Ido"];
(The property I am trying to set is Name, and as it is synthesized in OwnProfileData, I am using 'setName').
This doesn't work and I get the warning: "No known class method for selector 'ownMethod'.
Any Idea as for why that might happen and how I can fix this?
Thanks for your comments! Any support is highly appreciated!
You need an instance of ProfileShareViewController, because ownProfile is an instance property, not an class method. Read about the differences between classes and instances.
Or did I misunderstood smth?
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.
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.