I am very much confused between instance variables and property. I have read number of posts regarding this but still i am not clear about it.
I am from JAVA background and what i infer from objective C documentation is that a property is similar to JAVA BEAN CLASS (one having getter and setter of instance varibles). A property can accessed from other classes through its getter and setter methods while an instance variable is private and cannot be accessed from other classes.
Am i right in thinking in this direction ?
The parallel with Java is very good. The only difference is that Objective C provides a way to access a property as if it were a variable, and Java does not. The other difference is that in Objective C you can synthesize properties, while in Java you need to write your getters and setters manually.
Property is a "syntactic sugar" over a getter method or a pair of a getter and a setter methods. Properties are often (but not always) backed by an instance variable, but they can be implemented in any way that you can implement a parameterless instance method.
Ok, instance variable and property is far away from each other. instance variable is a state of object and property is a assecor method(getter/setter) of that state(instance variable).
So whenever you create an property in header file. compiler convert those property in to accessor method. suppose you declared property - #property(nonatomic, assign, readwrite) NSString *name;
So compiler will be converted those in to
-(NSString *)name;
-(void)setName:(NSString *)name;
And then for definition for accessor method there is two way.
manually - use dynamic in implementation file(.m) and then give the definition of accessor method by doing this you won't get any warning.
Let compiler do the job - this can be done by synthesizing property e.g synthesize name;. so now compiler will generate the definition for the accessor method for you.
Hope it helps ;)
I know this subject has been beat to death here ... but some seem to be focusing on the technical details, whereas I wanted mention something along the lines of the BIG PICTURE ...
Think of properties as kind of first-class ivars. Both properties and ivars may model attributes of an object ... but an ivar gets special attention if you go ahead and set it up as a property. Basically, you should an attribute as a property (as opposed to an ivar) if you feel it needs getter / setter methods. Dot notation makes for very readable code. This may help in deciding when to declare a variable as a property as opposed to simply using a regular ivar.
A property in objective c is in fact the setter and getter methods that make it possible to access an attribute in a class from outside of it. So when you declare for example
#interface example:NSObject{
NSString *variable;
}
#property(readwrite, assign) NSString *variable;
#end
#implementation
#synthesize variable;
#end
You are in fact declaring the methods
-(NSString *)getVariable;
-(void)setVariable(NSString *)value;
And you can access then by using the point notation and the name of the property, like
instance.variable = something;
something = instance.variable;
The primary difference between instance variable and property is that for properties, the compiler will automatically generate a getter/setter method pair. For instance:
#property (nonatomic) int value;
will generate:
-(void)setValue:(int)i
{
value = i;
}
-(int)value
{
return self->value;
}
given #synthesized.
If you crab a book on Objective-C 1.0, you'll notice that this feature isn't available. This is a new feature in 2.0, also known as the dotted syntax. It's introduced mainly because the complicated getter/setter syntax.
The benefit of this feature is that even though you have the compiler automatically declared the pair for you, you can still manage to override it. For instance, you can still have -(void)setValue:(int)i declared as a method of your class, and override the behavior. This is useful in scenarios of validation, such as you want to put a limit on the range of value.
As far as Java is concerned, Objective-C actually do have #public instance variable syntax, but it's a habit not to use it. It's sort of similar to Java's concept of protecting a private variable through getter/setter. But its primary objective-c is to override getter/setter and minimize syntax.
Now this is just a preview, refer to http://cocoacast.com/?q=node/103 or some objective-c 2.0 books if you wanted to know more.
Well, maybe it was not clear that a property does not need an instance variable.
You can define a read-only property based on any calculation on instance variables or any other variables in the scope. The issue here is that you must manually code the getter.
#interface person:NSObject{
NSDate *birthDate;
}
#property(readonly) int age;
#end
#implementation
-(int) age{
// return calculated age based on birthDate and today;
}
#end
The name of the property does not need to be the same as the instance variable.
#synthesize myProperty = myVar;
I found this amazing thread which clearly explains each and evrything about properties.
http://www.iphonedevsdk.com/forum/iphone-sdk-tutorials/7295-getters-setters-properties-newbie.html
Thank you all for your responses.
Related
I'm not familiar with the program language which has both property and instance variable.
So I don't know good ways to use property and instance variable.
Now I use only properties in my Objective-C code and I don't use any instance variables.
Do I need to use instance variable?
Or using only property is the best practice for Objective-C?
#interface ViewController : UIViewController
{
// instance variable
#public
int a;
}
// property
#property(nonatomic, strong) NSString *b;
#end
The basic difference between variable and property is that, you can give attributes to property. You can not give any attributes to variable.
So, if you wish to have any specific behavior like retaining the variable, accessing it atomically, have access out side the class, you should go for the properties.
If you simply want to access the variable with in the class and no special behavior is attached to that variable, no need to access it via property. You can directly use variable itself. It will improve the performance also.
There are some advantages a #property has over an ivar:
Memory management : Behind the scenes it will create a setter which creates the variable with correct memory management. It will save you some headaches because you can easily see how the memory management is done (strong/weak and retain/copy/assign).
Accessibility from other classes: if you declare your #property in the .h and #synthesize it in the .m you ivar will be public readable and writeable. You can prevent this with a privat class extension. You even can declare a #property public readonly and declare them internally readwrite via a privat class extension.
Eg: a private property
// [In the implementation file]
#interface MyClass ()
#property (nonatomic, retain) NSMutableArray* someData; // private!!
#end
#implementation MyClass #synthesize someData
#end
Custom getter and setter: If you like you can still write custom getter and setters and you can even just write a getter or setter and let the other one automatically #synthesize. And you can write custom logic into such a getter and setter e.g. you can reload a tableview after a #property has changed.
Automatic Key-Value-Observing (KVO) compliant: If you use or planning to use KVO you get it basically for free by just declaring the property. Nothing else need to be done!
Dot notation: You can access getter and setter via dot notation if you have the #property.
self.myIvar = (id) someThing;
[array addObject:self.myIvar];
If you need you iVar to be public it is simpler to write one #property than writing a getter and setter for a iVar
With a #property you do not need to declare in iVar (in iOS and 64bit Mac Os X applications). You can do it via the #synthesize:
#synthesize myiVar = _myIvar;
Use properties everywhere. Don't even declare instance variables, but synthesize them like this: #synthesize myProperty = _myProperty in order to differentiate them from property names. Properties are good way to cope with memory management as well. The only place you must use the synthesized instance variable is in the dealloc method.
The advantages of the properties are a lot:
- The accessor methods define how will you get and set the value of your instance variable.
- You can customize the accessor methods (for example to lazy instantiate an ivar or do something when a setting a new value like setNeedsDisplay.
- You don't cope with memory management when setting a new value - the setter takes care for releasing/retaining (depending how have you declared the property - retain/copy/assign/strong.
- Some multithreading stuff with the atomic/nonatomic attributes
- You can take advantage of the KVO, when using properties
- And least, but not last - don't worry about performance issues if you have concernes that every time a getter or a setter is called...
A #property is an instance variable that has had some semantic sugar applied to it, to help expose it to the outside world (usually), and to help avoid writing boilerplate code for getting and setting it.
though properties are made generally when you need to access some variable outside of the class, mean getter n setter, but in objective C, an additional need to make property is that the memory management goes on compiler ends, so if you are using some object, not primitive data types, then you should use property and synthesize it, and then release in dealloc if you are using manual reference counting. but again the main objective to make properties it to access some iVar outside the class like passing parameters from one class to other etc.
If you #synthesize a #property, you will have access to a get and a set method, which are very convenient. You can also define how the setter will behave (retain, assign, copy) if it's nonatomic or atomic and if it's read only. If you don't specify anything (aka you don't synthesize the property) the ivar won't be visible outside of the class by default, but you can achieve this by using #public. Logically you can also define them as #private or #protected .
Normally I #synthesize a #property because I want to have access to the ivar from the outside of the class and also because I want a getter and setter methods.
The general opinion is that you should use properties whenever possible. If you're still in doubt, here is Apple's recommendation:
In general, you should use accessor methods or dot syntax for property access even if you’re accessing an object’s properties from within its own implementation, in which case you should use self.
...
The exception to this rule is when writing initialization, deallocation or custom accessor methods
...
You should always access the instance variables directly from within an initialization method because at the time a property is set, the rest of the object may not yet be completely initialized
Read the whole document here for a better understanding of the subject.
As for performance issues, for most apps the gain is insignificant. Read this for a very detailed description.
I see some examples sometimes would declare a property as well as variable other times they do not .e.g. some time I see code like this
#interface Test : NSObject
{
UIProgressView* _progressView;
}
#property (nonatomic,retain)UIProgressView* progressView;
#end
at other times I will come across
#interface Test : NSObject
#property (nonatomic,retain)UIProgressView* progressView;
#end
Why what are the reasons ? I am learning and almost always use property and variable both.
I have used UIProgressView just as example.
Using ivars instead properties is only useful if you want #protected access (access from subclasses only), or support the old runtime (which required both).
It depends whether the property is synthesized against an iVar or derived in some other way (or against another iVar).
IF we have an instance of the class - i.e:
Test *myTest = [[Test alloc] init];
Then basically the property declaration
#property (nonatomic,retain)UIProgressView* progressView;
is telling anyone interested in using the interface that they can access the following two functions on an instance of this class:
[myTest progressBar];
[myTest setProgressBar:aProgressBar];
And objective C also lets you use shorthand notation:
myTest.progressBar =
xxx = myTest.progressBar
which does exactly the same thing.
It is not necessary for these two methods to be implemented via an iVar of the same name as the property, or even via an iVar at all (they could do even do a database fetch or derive the value).
If you #synthesize the property (which means you want the precompiler to generate the above methods for you) and don't explicitly specify an iVar on the #synthesize directive, then the methods described above will automatically be generated (due to the synthesize method) to set or get the value to/from an iVar of the same name as the property (and the implementation will include retain/release logic depending on the property directive.
If you don't #synthesize the property then you provide your own implementation and it can be anything you want. It is also possible to #synthesize the property but include a directive to use a different iVar in the method definition:
#synthesize progressBar=someOtheriVar;
in which case you will not see an iVar of the same name as the property in the header file either.
I am puzzled by how is variable declared in objective C.
1: I see #property and #synthesize statement being used. My question regarding that is, what are these 2 statement for? Why are they always used together? I guess #synthesize is a shortcut to create the getter and setter?
2:Say, I want to declare an NSMutableArray that would be only be accessible inside the class that was declared in. I have to perform myArray = [[NSMutableArray alloc] init] before using the addObject method to write something to it. When do I release the array then?
3:Is there a different way to declaring a variable that is only accessible only at the class it was declared to being accessible at all classes?
4:Similar to question 2, but now the variable is an NSString. Why I don't have to alloc & init it to share the same variable within its own class? Whats the different between self.myString = #""; to myString = #"";
Thanks a lot.
For your first question:
#property (nonatomic, retain) NSString * someProperty;
This declares a property of the class. It becomes part of the public contract of the class but still lacks something important - actual implementation
#synthesize someProperty;
This is compiler sugar, its creates a getter and setter method for your property. To wit, this is the implementation that is needed in order to actually use your property in your class and from other classes.
You will in almost all situations, always have a #synthesize for each #property you declare.
For your second question:
You are correct about how to initialize your array property. In order to release it you would do the following in your classes dealloc method:
- (void) dealloc {
self.myarray = nil;
[super dealloc];
}
This effectively releases the array (assuming you declared your property with the keyword retain).
And for your last question:
Properties of a class are always available from other classes. In order to create a globally accessible variable you would declare it as static.
Ad 1: a property is a construct to control access an ivar (usually private) by getters and setters. Actually, a property doesn't even have to have a supporting ivar. Yes, #synthesize generates getter and setter (and ivar).
Ad 2: You release it when you don't need it anymore. When that is depends on the logic of your code.
Ad 3: If I understand that correcttly, you want #private ivars. Normally, ivars are protected, i.e. only accessible inside the class or in derived classes. Private ivars are only accessible inside the class itself. Properties are publicly accessible.
Ad 4: myString = #"" writes to the ivar directly, while self.myString = #"" uses the property setter.
You need to get a text on Objective-C or find an online tutorial -- it's a sufficiently arcane language that you can't hope to pick it up in dribs and drabs.
There are variables and there are properties -- two different things that intersect somewhat.
You can declare plain old instance variables the same way as in C/C++, more or less:
NSArray* myArray;
eg, placed in the {} enclosed section of the #interface.
But you can also have a PROPERTY, which you declare by saying #property in the #interface declaration (after the closing }). A property has a getter method -- by default called myProperty -- and a putter method -- by default called setMyProperty. If myProperty is the same name as one of your instance variables then you can use #synthesize to automatically create these methods.
Note that properties may be automatically retained when the default setter method is used. This is fairly convenient in terms of managing storage.
But managing storage is a big topic, one that you MUST read some good tutorial on -- we can't explain it in a few paragraphs.
1) #property declares a publicly accessible variable and associated getter and setter. #synthesize causes the compiler to automatically generate the definition (code) of the getter and setter.
2) You would declare the NSMutableArray in your class declaration, in the header file. You would initialize the variable in your init method, and you would release the variable in your dealloc method.
3) The variables created using #property are public. The variables defined in your class declaration (using #interface in the header file) can be declared as private to that class, using the #private keyword.
John, these questions are pretty basic. You would probably get a lot out of the Objective-C Programming Intro here ( http://developer.apple.com/library/ios/#documentation/Cocoa/Conceptual/ObjectiveC/Introduction/introObjectiveC.html#//apple_ref/doc/uid/TP30001163 ).
First off, please forgive the stupidness of this question but Im not from a C/C++ background. I'm a little unclear about what the difference in roles between the .h and .m files when it comes to properties.
I understand the concept of interfaces, and I see that in part the .h file is an interface for the implementation, but what I am not clear on is this:
Why are properties/methods defined outside of the {} braces?
What am i defining in the braces when I write something like this:
IBOutlet UITextField *numberField;
Is this a field definition in an interface?
When I am adding the #Property lines to the .h files are these actual implementations of a n auto property or just an interface blueprint? If so is the #syntesis the actual implementation?
I guess my biggest confusion seems to be that if I want a property I'm defining what I need in three different places (1) in the interfaces braces, (2) as #property outside the braces and (3) with #synthesis in the .m file. This seems long winded, but its fine if I can work out what these three parts do.
Cheers, Chris.
I'll answer your questions below, but perhaps the best way to learn this stuff is to read some user-friendly notes intended for folks new to the language, such as the Learn Objective-C tutorial over at cocoadevcentral.
An example
I'd like to help answer your questions with an example (I love learning by example). Let's say you're a teacher writing a program that asks students a particular yes/no question, and keeps track of how many get it correct and how many students it has asked.
Here is a possible interface for this class:
#interface Question : NSObject {
NSString* questionStr;
int numTimesAsked;
int numCorrectAnswers;
}
#property (nonatomic, retain) NSString* questionStr;
#property (nonatomic, readonly) int numTimesAsked;
#property (nonatomic) int numCorrectAnswers;
#property (nonatomic) int numWrongAnswers;
- addAnswerWithTruthValue: (BOOL) isCorrect;
#end
The three variables inside the braces are instance variables, and every instance of your class will have its own values for each of those variables. Everything outside the braces but before #end is a declaration of a method (including the #property declarations).
(Side note: for many objects, it's useful to have retain properties, since you want to avoid the overhead of copying the object, and make sure it isn't released while you're using it. It's legal to retain an NSString as in this example, but it is often considered good practice to use copy instead of retain since an NSString* might actually point to an NSMutableString object, which may later change when your code expects it to stay the same.)
What #property does
When you declare a #property, you're doing two things:
Declaring a setter and getter method in the class's interface, and
Indicating how the setter and getter behave.
For the first one, it's enough to know that this line:
#property (nonatomic, retain) NSString* questionStr;
is basically the same as this:
- (NSString*) questionStr; // getter
- (void) setQuestionStr: (NSString) newQuestionStr; // setter
in the header. You literally are declaring those two methods; you can call them directly, or use the dot notation as a shortcut to call them for you.
The "basically" part in "basically the same" is the extra info given by keywords like nonatomic and retain.
The nonatomic keyword indicates that they're not necessarily thread-safe. The common retain keyword indicates that the object retains any value that's set, and releases previous values as they're let go.
For example:
// The correct answer to both questions is objectively YES.
Question* myQuestion = [[Question alloc] init];
NSString* question1 = [[NSString alloc] initWithString:#"Is pizza tasty?"];
// question1 has retain count of 1, from the call to alloc
myQuestion.questionStr = question1;
// question1 now has a retain count of 2
NSString* question2 = [[NSString alloc] initWithString:#"Free iPhone?"];
myQuestion.questionStr = question2;
// question1 has a retain count of 1, and question2 has retain count of 2
If the #property declaration for questionStr had been assign instead, then all the myQuestion.questionStr = statements would not have made any changes at all to the retain counts.
You can read a little more about properties here.
What IBOutlet and IBAction do
These are basically no-op words which act only as a way to tell Interface Builder which pieces of the header file to pay attention to. IBOutlet literally becomes an empty string when the compiler looks at it, and IBAction becomes the void return value. We do need them to work with Interface Builder, though, so they are important -- just not to the compiler.
Quick note on C structs and arrow vs dot notation
By the way, the data part of an Objective-C object is very similar to a C struct. If you have a pointer to a C struct, you can use arrow notation -> to refer to a specific part of the struct, like this:
struct MyStructType {
int i;
BOOL b;
};
struct MyStructType* myStruct;
myStruct->i = 3;
myStruct->b = TRUE; // or YES in Objective-C.
This same syntax works the same way in Objective-C:
Question* question = [[Question alloc] init];
question->questionStr = #"Is this a long answer?"; // YES
But when you do this, there is no method call happening behind the scenes, unlike the dot notation. With the dot notation, you're calling the setter (or getter if there's no = afterwards), and these two lines are the same:
question.questionStr = #"Chocolate?";
[question setQuestionStr:#"Chocolate?"];
It's often a good idea to avoid the arrow notation in favor of the dot notation, since the dot notation lets you enforce valid state -- for example, that the pointers your class has are always retained. You can even disallow others from using the arrow notation by declaring your instance variables as #private; they can still use the getter and setter to access it, if you declare a #property for it.
What #synthesize does
Now, when you get around to actually implementing your class, #synthesize says something like "make sure the getter and setter get implemented for this property." It does not say "implement both of these for me," because the compiler is polite enough to check for your own implementation first, and only fill in the pieces you've missed. You don't have to use #synthesize at all, even if you use #property out the wazoo -- you could always just provide your implementations for your setters and getters, if you're into that sort of thing.
You probably noticed in the Question interface above that there's a property which is not an instance variable (numWrongAnswers), which is fine because you're just declaring methods. In the example code here, you can see how this actually works:
#implementation Question
#synthesize questionStr, numTimesAsked, numCorrectAnswers;
- (void) setNumCorrectAnswers: (int) newCorrectAnswers {
// We assume the # increases, and represents new answers.
int numNew = newCorrectAnswers - numCorrectAnswers;
numTimesAsked += numNew;
numCorrectAnswers = newCorrectAnswers;
}
- (int) numWrongAnswers {
return numTimesAsked - numCorrectAnswers;
}
- (void) setNumWrongAnswers: (int) newWrongAnswers {
int numNew = newWrongAnswers - self.numWrongAnswers;
numTimesAsked += numNew;
}
- (void) addAnswerWithTruthValue: (BOOL) isCorrect {
if (isCorrect) {
self.numCorrectAnswers++;
} else {
self.numWrongAnswers++;
}
}
#end
One thing that's happening here is we're faking an instance variable called numWrongAnswers, which would be redundant information if we stored it in the class. Since we know numWrongAnswers + numCorrectAnswers = numTimesAsked at all times, we only need to store any two of these three data points, and we can always think in terms of the other one by using the two values we do know. The point here is to understand that a #property declaration is really just about declaring a setter and getter method, which usually corresponds to an actual instance variable -- but not always. The #synthesize keyword by default does correspond to an actual instance variable, so that it's easy for the compiler to fill in the implementation for you.
Reasons to have separate .h and .m files
By the way, the whole point of declaring methods in one file (the .h header file) and defining their implementation in another (the .m or methods file) is to help decouple the code. For example, if you only update one .m file in your project, you don't have to recompile the other .m files, since their object code will remain the same -- this saves time. Another advantage is that you can use a library that includes only header files and pre-compiled object code, or even dynamic libraries where you need the header file so the compiler is aware of which methods exist, but those methods aren't even linked in with your executable file. These advantages are hard to appreciate when you first start coding, but just the logical breakdown and encapsulation of implementation becomes useful after a short while.
I hope that's helpful!
methods are defined outside of the braces since the braces are meant to encapsulate the state of the object which can be argued does not include the instance or class methods.
What you are defining in the braces are instance variables that can be referenced as self.ivar
The #property and #synthesize directives simply setup accessors for you instance variables so you can set them by doing self.ivar = someVar. So in other words it sets up the "dot syntax" for you to use.
and to answer your finale question: To define a property or instance variable simply declare it in your .h file as a variable inside the braces. To setup accessor methods on that same property you need to do BOTH #property and #synthesize.
Well that is just Objective C syntax, methods and #property outside {} and variables inside {}.
#property is the way of telling that you are going to write getter and setters (kind of enforcing it), but you can write getter/setter without setting them #property. #property is in .h file because its declaration. And why it is outside {}, well as i said before its just the syntax, what we can do?
#synthesis will in actual implement getter and setters, if you dont synthesis but you have set them #property, you have to implement those getter and setters by your hand. And #synthesis is in .m file because its implementation.
Something more for you to read on this subject can be find here.
http://theocacao.com/document.page/510
The variables inside the brackets define the physical structure of your class. Those are the actual instance variables that store information.
The stuff outside the brackets make up the class's interface — methods and properties. A property in and of itself does not reserve any storage space or affect any variable — it just declares a generic interface for accessing something. Remember that a property doesn't have to have an underlying instance variable — for example, the totalPrice property in a ShoppingCart class might dynamically sum the prices of all the items in the cart.
Inside the implementation file, you tell the class how to actually do its work. For methods, obviously, you just supply an implementation. For a property, you can either provide accessor implementations yourself or ask it to synthesize accessors for an instance variable.
Whats the difference between a property and an instance variable in Objective-C. I need to understand this in OOP terms. Is a property declaration just a convenience wrapper (with #synthesize in the implementation) for accessing instance variables?
thanks,
codecowboy.
Properties and ivars are two completely different things.
And instance variable is a variable stored inside the object, so each instance has its own. It is referenced by pointer addition relative to the object pointer/self (slightly indirected for the modern runtime, but functionally equivalent). ivars are generally internal to a class, and by default can only be accessed by the class and its descendents (#protected). Within methods they are available with no qualification, otherwise they can (but rarely are, ad usuaually should not) be accessed via indirection, eg obj->ivar.
A property defines a getter and setter (the setter is optional) interface. That's all it does. It defines two public methods:
- (TYPE) propname;
- (void) setPropname: (TYPE) newPropname;
These are defined as methods exactly as if you declared them like that, no more, no less. These methods are called either with the normal syntax ([obj propname] and [obj setPropname:n] or using the modern dot notation (obj.propname or obj.propname = n). These two options are syntactically different only, they behave identically, and you can use dot notation whether the methods are declared with #property or declared manually as above.
You must then implement the methods in the implementation, either by writing the methods yourself, by using #synthesize, or by handling the missing method dynamically.
Properties may be backed by an ivar (named the same or named differently (my preference to avoid confusion)), or they may not. They may store their value elsewhere, or they may calculate it from other data.
For example, you might have:
#property (nonatomic, readonly) NSString* fullname;
and then implement - (NSString*) fullname to return the concatenation of firstname and lastname.
I think you are pretty much there. The #property and #synthesize make the accessor declarations and implementation for the already declared ivar. You have various attributes you can define on the #property too giving you control over how it is generated to make it appropriate for the ivar
Have a look at "Objective C 2.0 Declared Properties"
The difference between Property and Instance ivar is, the variable which make as Property that can be visible in another Class whereas for accessing the iVar or instance you need to create the Object of that class and then you can access.
and With use of #synthesize compiler will generate the setter and getter for that property.
-(TYPE)name;-getter Method
-(void)setName:(TYPE)aName; setter Method