Looking inside the runtime.h, I found the definition of the structure objc_class.
Among various members, We have this :-
struct objc_method_list **methodLists
We definitely need to know what all methods a class has,
But a list of methods should be fine, but why do we have "lists" ?
Why not just one list ?
Also, can anyone specify that, Are methods inherited from superclass part of that list or we get to them via superclass pointer that points to parent class's structure.
Here is my detail investigation into struct objc_method_list **methodLists : http://blog.csdn.net/jasonblog/article/details/7303618
And in short, methodLists stores SEL-IMP mapping of the instance methods by default. In this situation, it has only one list.
As the name 'methodLists' suggests, it can contain more than one list. If you add a category to a class, the runtime system will insert one more list into methodLists, which points to the method-list of the category.
I tried to answer this question several months ago, but at that time SO discard my answer due to network problem. Now I meet it again :)
The purpose is explained in objc-class.m, as linked by Georg:
cls->methodLists may be in one of three forms:
NULL: The class has no methods.
non-NULL, with CLS_NO_METHOD_ARRAY set: cls->methodLists points
to a single method list, which is the class's only method list.
non-NULL, with CLS_NO_METHOD_ARRAY clear: cls->methodLists points to
an array of method list pointers. The end of the array's block
is set to -1. If the actual number of method lists is smaller
than that, the rest of the array is NULL.
Attaching categories and adding and removing classes may change
the form of the class list. In addition, individual method lists
may be reallocated when fixed up.
Classes are initially read as #1 or #2. If a category is attached
or other methods added, the class is changed to #3. Once in form #3,
the class is never downgraded to #1 or #2, even if methods are removed.
Classes added with objc_addClass are initially either #1 or #3.
The short answer is therefore "because of categories." When a category is injected, rather than try to combine its method list with the one existing list, a new entry is simply added to methodLists, and set to the list coming from the category. This probably makes category injection faster, since it avoids (potential) large reallocations and copying.
I know this sort of question has been asked before, but I still feel that the answer is too ambiguous for me (and, by extension, some/most beginners) to grasp.
I have been trying to teach myself broader concepts of programming than procedural and basic OOP. I understand the concrete concepts of OOP (you make a class that has data (members) and functions (methods) and then instantiate that class at run time to actually do stuff, that kind of thing).
I think I have a handle on what a class is (sort of a design blueprint for an instance to be created in its likeness at compile time). But if that's the case, what is an object? I also know that in prototype based languages, this can muck things up even more, but perhaps this is why there needs to be a clear distinction between object and instance in my mind.
Beyond that, I struggle with the concepts of "object" and "instance". A lot of resources that I read (including answers at SO) say that they are largely the same and that the difference is in semantics. Other people say that there is a true conceptual difference between the two.
Can the experts here at SO help a beginner have that "aha" moment to move forward in the world of OOP?
Note: this isn't homework, I don't go to school - however, I think it would help people that are looking for homework help.
A blueprint for a house design is like a class description. All the houses built from that blueprint are objects of that class. A given house is an instance.
The truth is that object oriented programming often creates confusion by creating a disconnect between the philosophical side of development and the actual mechanical workings of the computer. I'll try to contrast the two for you:
The basic concept of OOP is this: Class >> Object >> Instance.
The class = the blue print.
The Object is an actual thing that is built based on the 'blue print' (like the house).
An instance is a virtual copy (but not a real copy) of the object.
The more technical explanation of an 'instance' is that it is a 'memory reference' or a reference variable. This means that an 'instance' is a variable in memory that only has a memory address of an object in it. The object it addresses is the same object the instance is said to be 'an instance of'. If you have many instances of an object, you really just have many variables in difference places in your memory that all have the same exact memory address in it - which are all the address of the same exact object. You can't ever 'change' an instance, although it looks like you can in your code. What you really do when you 'change' an instance is you change the original object directly. Electronically, the processor goes through one extra place in memory (the reference variable/instance) before it changes the data of the original object.
The process is: processor >> memory location of instance >> memory location of original object.
Note that it doesn't matter which instance you use - the end result will always be the same. ALL the instances will continue to maintain the same exact information in their memory locations - the object's memory address - and only the object will change.
The relationship between class and object is a bit more confusing, although philosophically its the easiest to understand (blue print >> house). If the object is actual data that is held somewhere in memory, what is 'class'? It turns out that mechanically the object is an exact copy of the class. So the class is just another variable somewhere else in memory that holds the same exact information that the object does. Note the difference between the relationships:
Object is a copy of the class.
Instance is a variable that holds the memory address of the object.
You can also have multiple objects of the same class and then multiple instances of each of those objects. In these cases, each object's set of instances are equivalent in value, but the instances between objects are not. For example:
Let Class A
From Class A let Object1, Object2, and Object3.
//Object1 has the same exact value as object2 and object3, but are in different places in memory.
from Object1>> let obj1_Instance1, obj1_Instace2 , obj1_Instance3
//all of these instances are also equivalent in value and in different places in memory. Their values = Object1.MemoryAddress.
etc.
Things get messier when you start introducing types. Here's an example using types from c#:
//assume class Person exists
Person john = new Person();
Actually, this code is easier to analyze if you break it down into two parts:
Person john;
john = new Person();
In technical speak, the first line 'declares a variable of type Person. But what does that mean?? The general explanation is that I now have an empty variable that can only hold a Person object. But wait a minute - its an empty variable! There is nothing in that variables memory location. It turns out that 'types' are mechanically meaningless. Types were originally invented as a way to manage data and nothing else. Even when you declare primitive types such as int, str, chr (w/o initializing it), nothing happens within the computer. This weird syntactical aspect of programming is part of where people get the idea that classes are the blueprint of objects. OOP's have gotten even more confusing with types with delegate types, event handlers, etc. I would try not focus on them too much and just remember that they are all a misnomer. Nothing changes with the variable until its either becomes an object or is set to a memory address of an object.
The second line is also a bit confusing because it does two things at once:
The right side "new Person()" is evaluated first. It creates a new copy of the Person class - that is, it creates a new object.
The left side "john =", is then evaluated after that. It turns john into a reference variable giving it the memory address of the object that was just created on the right side of the same line.
If you want to become a good developer, its important to understand that no computer environment ever works based on philosophic ideals. Computers aren't even that logical - they're really just a big collection of wires that are glued together using basic boolean circuits (mostly NAND and OR).
The word Class comes from Classification (A Category into which something is put), Now we have all heard that a Class is like a Blueprint,but what does this exactly mean ? It means that the Class holds a Description of a particular Category ,(I would like to show the difference between Class , Object and Instance with example using Java and I would request the readers to visualise it like a Story while reading it , and if you are not familiar with java doesn't matter) So let us start with make a Category called HumanBeing , so the Java program will expressed it as follows
class HumanBeing{
/*We will slowly build this category*/
}
Now what attributes does a HumanBeing have in general Name,Age,Height,Weight for now let us limit our self to these four attributes, let us add it to our Category
class HumanBeing{
private String Name;
private int Age;
private float Height;
private float Weight;
/*We still need to add methods*/
}
Now every category has a behaviour for example category Dog has a behaviour to bark,fetch,roll etc... , Similarly our category HumanBeing can also have certain behaviour,for example when we ask our HumanBeing what is your name/age/weight/height? It should give us its name/age/weight/height, so in java we do it as follows
class HumanBeing{
private String Name;
private int Age;
private float Height;
private float Weight;
public HumanBeing(String Name,int Age,float Height,float Weight){
this.Name = Name;
this.Age = Age;
this.Height = Height;
this.Weight = Weight;
}
public String getName(){
return this.Name;
}
public int getAge(){
return this.age;
}
public float getHeight(){
return this.Height;
}
public float getWeight(){
return this.Weight;
}
}
Now we have added behaviour to our category HumanBeing,so we can ask for its name ,age ,height ,weight but whom will you ask these details from , because class HumanBeing is just a category , it is a blueprint for example an Architect makes a blueprint on a paper of the building he wants to build , now we cannot go on live in the blueprint(its description of the building) we can only live in the building once it is built
So here we need to make a humanbeing from our category which we have described above , so how do we do that in Java
class Birth{
public static void main(String [] args){
HumanBeing firstHuman = new HumanBeing("Adam",25,6.2,90);
}
}
Now in the above example we have created our first human being with name age height weight , so what exactly is happening in the above code? . We are Instantiating our category HumanBeing i.e An Object of our class is created
Note : Object and Instance are not Synonyms In some cases it seems like Object and Instance are Synonyms but they are not, I will give both cases
Case 1: Object and Instance seems to be Synonyms
Let me elaborate a bit , when we say HumanBeing firstHuman = new HumanBeing("Adam",25,6.2,90); An Object of our category is created on the heap memory and some address is allocated to it , and firstHuman holds a reference to that address, now this Object is An Object of HumanBeing and also An Instance of HumanBeing.
Here it seems like Objects and Instance are Synonyms,I will repeat myself they are not synonyms
Let Us Resume our Story , we have created our firstHuman , now we can ask his name,age,height,weight , this is how we do it in Java
class Birth{
public static void main(String [] args){
HumanBeing firstHuman = new HumanBeing("Adam",25,6.2,90);
System.out.println(firstHuman.getName());
System.out.println(firstHuman.getAge());
...
...
}
}
so we have first human being and lets move feather by give our first human being some qualification ,let's make him a Doctor , so we need a category called Doctor and give our Doctor some behaviour ,so in java we do as follows
class Doctor extends HumanBeing{
public Doctor(String Name,int Age,float Height,float Weight){
super(Name,Age,Height,Weight);
}
public void doOperation(){
/* Do some Operation*/
}
public void doConsultation(){
/* Do so Consultation*/
}
}
Here we have used the concept of Inheritance which is bringing some reusability in the code , Every Doctor will always be a HumanBeing first , so A Doctor will have Name,Age,Weight,Height which will be Inherited from HumanBeing instead of writing it again , note that we have just written a description of a doctor we have not yet created one , so let us create a Doctor in our class Birth
class Birth{
public static void main(String [] args){
Doctor firstDoctor = new Doctor("Strange",40,6,80);
.......
.......
/*Assume some method calls , use of behaviour*/
.......
.......
}
}
Case 2: Object and Instance are not Synonyms
In the above code we can visualise that we are Instantiating our category Doctor and bringing it to life i.e we are simply creating an Object of the category Doctor , As we already know Object are created on Heap Memory and firstDoctor holds a reference to that Object on the heap ;
This particular Object firstDoctor is as follows (please note firstDoctor holds a reference to the object , it is not the object itself)
firstDoctor is An Object of class Doctor And An Instance of A class Doctor
firstDoctor is Not An Object of class HumanBeing But An Instance of class HumanBeing
So a particular Object can be an instance to a particular class but it need not be an object of that given class
Conclusion:
An Object is said to be an Instance of a particular Category if it satisfies all the characteristic of that particular Category
Real world example will be as follows , we are first born as Humans so image us as Object of Human , now when we grow up we take up responsibilities and learn new skills and play different roles in life example Son, brother, a daughter, father ,mother now What are we actually?We can say that we are Objects of Human But Instances of Brother,daughter,...,etc
I hope this helps
Thank You
Objects are things in memory while instances are things that reference to them. In the above pic:
std(instance) -> Student Object (right)
std1(instance) -> Student Object (left)
std2(instance) -> Student Object (left)
std3(instance) -> no object (null)
An object is an instance of a class (for class based languages).
I think this is the simplest explanation I can come up with.
A class defines an object. You can go even further in many languages and say an interface defines common attributes and methods between objects.
An object is something that can represent something in the real world. When you want the object to actually represent something in the real world that object must be instantiated. Instantiation means you must define the characteristics (attributes) of this specific object, usually through a constructor.
Once you have defined these characteristics you now have an instance of an object.
Hope this clears things up.
"A class describes a set of objects called its instances." - The Xerox learning Research Group, "The Smalltalk-80 System", Byte Magazine Volume 06 Number 08, p39, 1981.
What is an Object ?
An object is an instance of a class. Object can best be understood by finding real world examples around you. You desk, your laptop, your car all are good real world examples of an object.
Real world object share two characteristics, they all have state and behaviour. Humans are also a good example of an object, We humans have state/attributes - name, height, weight and behavior - walk, run, talk, sleep, code :P.
What is a Class ?
A class is a blueprint or a template that describes the details of an object. These details are viz
name
attributes/state
operations/methods
class Car
{
int speed = 0;
int gear = 1;
void changeGear(int newGear)
{
gear = newGear;
}
void speedUp(int increment)
{
speed = speed + increment;
}
void applyBrakes(int decrement)
{
speed = speed - decrement;
}
}
Consider the above example, the fields speed and gear will represent the state of the object, and methods changeGear, speedUp and applyBrakes define the behaviour of the Car object with the outside world.
References:
What is an Object ?
What is a Class ?
I think that it is important to point out that there are generally two things. The blueprint and the copies. People tend to name these different things; classes, objects, instances are just some of the names that people use for them. The important thing is that there is the blueprint and copies of it - regardless of the names for them. If you already have the understanding for these two, just avoid the other things that are confusing you.
Lets compare apples to apples. We all know what an apple is. What it looks like. What it tastes like. That is a class. It is the definition of a thing. It is what we know about a thing.
Now go find an apple. That is an instance. We can see it. We can taste it. We can do things with it. It is what we have.
Class = What we know about something. A definition.
Object/Instance = Something that fits that definition that we have and can do things with.
In some cases, the term "object" may be used to describe an instance, but in other cases it's used to describe a reference to an instance. The term "instance" only refers to the actual instance.
For example, a List may be described as a collection of objects, but what it actually holds are references to object instances.
I have always liked the idea that equals the definition of a class as that of an "Abstract Data Type". That is, when you defined a class you're are defining a new type of "something", his data type representation, in terms of primitives and other "somethings", and his behavior in terms of functions and/or methods. (Sorry for the generality and formalism)
Whenever you defined a class you open a new possibility for defining certain entities with its properties and behavior, when you instantiate and/or create a particular object out of it you're actually materializing that possibility.
Sometimes the terms object and instances are interchangeable. Some OOP purists will maintain that everything is an object, I will not complain, but in the real OOP world, we developers use two concepts:
Class: Abstract Data Type sample from which you can derive other ADT and create objects.
Objects: Also called instances, represents particular examples of the data structures and functions represented by a given Abstract Data Type.
Object Oriented Programming is a system metaphor that helps you organize the knowledge your program needs to handle, in a way that will make it easier for you to develop your program. When you choose to program using OOP you pick up your OOP-Googles, and you decide that you will see the problem of the real world as many objects collaborating between themselves, by sending messages. Instead of seeing a Guy driving a Car you see a Guy sending a message to the car indicating what he wants the car to do. The car is a big object, and will respond to that message by sending a message to it's engine or it's wheel to be able to respond properly to what the Driver told him to do in the message, etc...
After you've created your system metaphor, and you are seeing all the reality as objects sending messages, you decide to put all the things your are seeing that are relevant to your problem domain in the PC. There you notice that there are a lot of Guys driving different cards, and it's senseless to program the behavior of each one of them separately because they all behave in the same way... So you can say two things:
All those guys behave in the same way, so I'll create a class called
Driver that will specify who all the Drivers in the world behave,
because they all behave in the same way. (And your are using class based OOP)
Or your could say Hey! The second Driver behaves in the same way as the first Driver, except he likes going a little faster. And the third Driver behaves in the same way as the first Driver, except he likes zigzagging when he drives. (And you use prototype based OOP).
Then you start putting in the computer the information of how all the Drivers behave (or how the first driver behave, and how the second and third differ from that one), and after a while you have your program, and you use the code to create three drivers that are the model you are using inside that PC to refeer to the drivers you saw in the real world. Those 3 drivers that you created inside the PC are instances of either the prototype ( actually the first one is the prototype, the first one might be the prototype himself depending on how you model things) or the class that you created.
The difference between instance and object is that object is the metaphor you use in the real world. You choose to see the guy and the car as objects (It would be incorrect to say that you see them as instances) collaborating between themselves. And then you use it as inspiration to create your code. The instance only exists in your program, after you've created the prototype or the class. The "objects" exist outside the PC because its the mapping you use to unite the real world with the program. It unites the Guy with the instance of Driver you created in the PC. So object and instance are extremely related, but they are not exactly the same (an instance is a "leg" of an object in the program, and the other "leg" is in the real world).
I guess the best answer has already been given away.
Classes are blueprints, and objects are buildings or examples of that blueprint did the trick for me as well.
Sometimes, I'd like to think that classes are templates (like in MS Word), while objects are the documents that use the template.
Extending one of the earlier given examples in this thread...
Consider a scenario - There is a requirement that 5 houses need to be built in a neighbourhood for residential purposes. All 5 houses share a common construction architecture.
The construction architecture is a class.
House is an object.
Each house with people staying in it is an instance.
This might be an odd question, but it has actually caused me some headache.
In Object oriented programming, there are accepted names for key concepts. In our model, we have classes with methods and fields. Now, going to the data world:
An instance of a class is called an object.
An instance of a field is called... what?
A value? Isn't the term value a little broad for this? I have been offered "property" as well, but isn't property also part of the model and not the data?
(This is not purely academic, I am actually coding these concepts.)
Updated: Let me take an example. I have a class "Person" with a field "age". If I create 20 Person instances, each such instance is called an object. So far so good. But let's say I take Person "Igor", and set his age to 20. What is the storage location that contains the number 20 now called? Is it a field, or a value, or something else?
Another update: A quote from Pavel Feldman in this related question describes in different words what I tried to describe above:
"I'd say that in class-based OOP field belongs to class and does not have a value. It is so when you look at reflection in c# or java - class has fields, field has type, name, etc. And you can get value of the field from object. You declare field once, in class. You have many objects with same fields but different values."
A field can't be instantiated. A field can only contain a value. The value can be either a primitive/native type or a reference/pointer to an object instance.
As per your update: if the object represents a real world entitiy, then it's often called property. With a "real world entity" I mean something personal/human, e.g. Person, Product, Order, Car, etc. If the object does not represent something personal/human, e.g. List, String, Map, then it's more often called field. That's just what I've observed as far.
Agree with BalusC. However I think what you are asking is what to call the field of an instantiated object. Remember that an object contains both state (data) and operations (methods) you could refer to an object field as state
A field is a field weather you talk about it in the context of a class, or in the context of an object.
class C {
int i; // i is a field
}
and
obj = new C();
obj.i = 7; // obj.i is a field
As opposed to parameter vs argument there is no distinction in terminology for "instantiated" an "uninstantiated" fields.
An instance of a class is an object, a class may contain fields that point to other instantiated objects (or a null pointer). It makes no sense to say an instance of a field, but rather you might talk about the object to which a particular field points to, which may be different for different instances. Or you may talk about the type of a field (which class it belongs to)
Isn't the answer basically that we have no name for values of fields of an instance of a class (or object)?
It's like giving a name to the value returned by a method of an instance of a class...
I guess "state" is the best answer anyway as suggested "BalusC".
I have a simple question about object oriented design but I have some difficulties figuring out what is the best solution. Say that I have an object with some methods and a fairly large amount of properties, perhaps an Employee object. Properties, like FirstName, Address and so on, which indicates a data structure. Then there could be methods on the Employee object, like IsDueForPromotion(), that is more of OO nature.
Mixing this does not feel right to me, I would like to separate the two but I do not know how to do it in a good way. I have been thinking about putting all property data in a struct and have an internal struct object inside the employee object, private EmployeeStruct employeData ...
I am not sure this is a really good idea however, maybe I should just have all methods and proerties in the same class and go with that. Am I making things to complicated if I separate data from methods?
I would very much appreciate if someone have any ideas about this.
J
Wasn't the idea of OO-design to encapsulate data and the corresponding methods together?
The question here is how the Employee object could possibly know about begin due for promotion. I guess that method belongs somewhere else to a class which has the informations to decicde that. really stupid example Manager m = new Manager(); manager.IsDueForPromotion(employeeobject);
But other methods to access the fields of Employee belong to this class.
The question I raised about IsDueForPromotion depends on you application and if your Employee is a POJO or DTO only or if it can have more "intelligent" methods associated too.
if your data evolves slower than behaviour you may want to give a try to Visitor pattern:
class Employee {
String name;
String surName;
int age;
// blah blah
// ...getters
// ...setters
// other boilerplate
void accept(EmployeeVisitor visitor) {
visitor.visitName(name);
visitor.visitAge(age);
// ...
}
}
interface EmployeeVisitor {
void visitName(String name);
void visitAge(int age);
}
with this design you can add new operations without changing the Employee class.
Check also use the specification pattern.
Object operations (methods) are supposed to use the properties. So I feel its better to leave them together.
If it does not require properties, its a kind of utility method and should be defined else ware, may in some helper class.
Well, OO is a way of grouping data and functionality that belong together in the same location. I don't really see why you would make an exception 'when there is a lot of data'. The only reason I can think of is legibility.
Personally I think you would be making things needlessly complex by coming up with a separate struct to hold your data. I'm also conflicted as to wether this would be good practice. On the one hand, how a class implements it's functionality, or stores it's data is supposed to be hidden from the outside world. On the other hand, if data belongs to a class, it feels unnatural to store it in something like a struct.
It may be interesting to look at the data you have and see if it can be modeled into smaller domain objects. For example, have an Address object that holds a street, housenumber, state, zip, country, etc value. That way, your Employee object will just hold an Address object. The Address object could then be reused for your Company objects etc.
The basic principle of Object Oriented programming is grouping data such as FirstName and Address with the functionality that goes with it, such as IsDueForPromotion(). It doesn't matter how much data the object is holding, it will still hold that data. The only time you want to remove data from an object is if it has nothing to do with that object, like storing the company name in the Employee object when it should be stored in a company object.