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Closed 11 years ago.
Possible Duplicate:
Properties and Instance Variables in Objective-C 2.0
I'm confused by these two code segments:
First:
//.h
#interface Student : NSObject {
}
#property (nonautomic, copy) NSString *name;
#property (nonautomic, retain) NSNumber *age;
#end
//.m
#implementation Student
#synthesize name;
#synthesize age;
#end
Second:
//.h
#interface Student : NSObject {
NSString *name; // <<============ difference
NSNumber *age; // <<============ difference
}
#property (nonautomic, copy) NSString *name;
#property (nonautomic, retain) NSNumber *age;
#end
//.m
#implementation Student
#synthesize name;
#synthesize age;
#end
Both of these can work. So is it necessary to declare variables in the {}?
Starting with the modern runtime (x86_64 and ARM6...and iOS Simulator) you no longer need to declare synthesized ivars. In the first example #synthesize is adding the instance variable for you.
Agree with #Joshua. I too was confused with this in the beginning. It's basically old convention vs new convention after the runtime updates. I think Apple realized that declaring ivars was redundant when you're gonna declare #property, so why not let the #synthesize take care of it when it creates the setters and getters. One less statement for us to write, yay!
(Some of these convention changes were explained in one of the earlier WWDC videos... i think)
The Objective-C Programming Language: Property Implementation Directives
There are differences in the behavior of accessor synthesis that depend on the runtime (see also “Runtime Difference”):
For the legacy runtimes, instance variables must already be declared in the #interface block of the current class. If an instance variable of the same name as the property exists, and if its type is compatible with the property’s type, it is used—otherwise, you get a compiler error.
For the modern runtimes (see “Runtime Versions and Platforms” in Objective-C Runtime Programming Guide), instance variables are synthesized as needed. If an instance variable of the same name already exists, it is used.
Related
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Adding a getter makes using an underscore incorrect syntax
(3 answers)
Closed 7 years ago.
#import "AppDelegate.h"
#interface AppDelegate ()
#property (strong, readwrite, nonatomic) NSNumber *currentNumber;
#end
#implementation AppDelegate
- (NSNumber *)currentNumber {
}
- (void)setCurrentNumber:(NSNumber *)currentNumber {
}
Why I can't access _currentNumber in currentNumber?
If I will remove setCurrentNumber then I can access _currentNumber in currentNumber?
The #property does not cause the ivar generation; rather, it's the implicit #synthesize. However, when you implement both the getter and setter, there is nothing to (implicitly) synthesize, so clang doesn't bother. If you add an explicit #synthesize statement, you'll get your ivar.
Like Avi mentioned, there is #synthesize keyword. If you just declare property(without implementing getter and setter for it) the corresponding ivar will be generated. Its name will be [underscore][property_name].For example declaring #property currentNumber; leads to implicit applying `#synthesize currentNumber = _currentNumber;
But if you want to assign ivar with another name to your property you can declare this ivar and synthesize the property with it:
#interface AppDelegate ()
{
NSNumber *_anotherIvar;
}
#property (strong, readwrite, nonatomic) NSNumber *currentNumber;
#end
#implementation AppDelegate
#synthesize currentNumber = _anotherIvar;
#end
#synthesize tells the compiler to create corresponding getter and setter which are backed by ivar on the right side of assignment. And if you provide your own setter - you prevent compiler from creating implicit synthesize(and generating ivar), so you need to provide ivar by yourself somehow(for example use explicit synthesize).
Here is good explanation about #synthesize.
I'm fairly new to objective-c, and I want to write a model class in iOS 7 using ARC, but I'm not sure whether I need the synthesize in the .m file. Could any one give me some advice on this?
//user.h
#interface User : NSObject
#property(nonatomic, assign) NSInteger age;
#property(nonatomic, copy) NSString *firstname;
#property(nonatomic, copy) NSString *lastname;
#end
//user.m
#implementation User
#synthesize age, firstname, lastname;
#end
Now comes my question:
Is the user.m file's synthesize necessary or not if in another class (e.g. ViewContoller class), I want to read & set the User class's age property, or firstname/lastname properties?
Not needed . Modern objective C compiler will auto synthesize the properties for you. Its a compiler feature started with the LLVM 4.0 comes with Xcode 4.4. If you just want the default behaviour of property no need of manual #synthesize.
I'm very new to Objective C. (Two days now). When read about #synthesize, it seemed to overlap with my understanding #property (which I thought I understood) ... So, some details need to be ironed out in my mind ... it's bugging me.
Please correct me if I'm wrong about differences of #property and #synthesize:
If you declare a #property in your #interface, then you're telling the world that users can expect to use standard getters and setters for that property. Futhermore, XCode will make generic getters and setters for you. ... BUT, To what degree does that happen with the #property declaration? ( I.E. does that mean "completely" ... like unseen declarations for it in your #interface, and also unseen code in your #interface?
-Or-
Does #property take care of the unseen code declarations in your #interface only - whereas #synthesize takes care of the unseen code implementation in your #implementation section? )
First, note that the latest version of Xcode does not require #synthesize at all anymore. You can (and should) just omit it. That said, here's what the pieces do.
#property is a declaration of accessors. It is just a declaration. There is very little difference between the following:
#property (nonatomic, readwrite, strong) NSString *something;
vs.
- (NSString *)something;
- (void)setSomething:(NSString)aSomething;
The main difference is that declaring these methods using #property lets the compiler automatically generate (synthesize) the implementations for you. There is no requirement that you let the compiler do it for you. You are absolutely free to implement something and setSomething: by hand, and it is common to do. But, if you don't implement them by hand, the compiler will automatically create an ivar for you called _something and create a reasonable implementation for the getter and setter.
In older versions of Xcode, you had to explicitly request the auto-generation using the #synthesize keyword. But that is no longer required. Today, the only reason to use #synthesize is if you want the ivar to have a non-standard name (never do that).
A key point here is that the methods something and setSomething: are just methods. There is nothing magical about them. They're not special "property methods." They're just methods that by convention access a piece of state. That piece of state is often stored in an ivar, but does not need to be.
To be even more clear: object.something does not mean "return the ivar named _something from object." It means "return the result of [object something], whatever that does." It is common for that to return the value of an ivar.
You should declare all of your state (internal and external) using #property declarations, and you should avoid directly declaring ivars. You should also always access your properties via their accessors (self.something), except in the init and dealloc methods. In init and dealloc, you should directly use the ivar (_something).
#property declares a property on your class with whatever atomicity and setter semantics you provide.
With Xcode 4.4, autosynthesis is available wherein you are provided with a backing ivar from your property without declaring it in #synthesize. This ivar has the form of _propertyName where your property name is propertyName.
Objective-C #property and #synthesize
#property
generates get/set method
today(from Xcode v4.4 with the LLVM v4.0) #property additionally uses #synthesize inside
#synthesize propertyName = _propertyName
#synthesize:
generates a new iVar or link with existing iVar
generates an implementation of the get/set method with an appropriate iVar
[Case when #synthesize can be used]
#property
#interface SomeClass : NSObject
#property NSString *foo;
#end
//generated code
#interface SomeClass : NSObject
- (NSString *)foo;
- (void)setFoo:(NSString)newFoo;
#end
#synthesize pattern
#synthesize <property_name> = <variable_name>;
//Using
//1. Specify a variable. New variable(variableName) will be generated/linked with existing
#synthesize propertyName = variableName
//if variableName is not exist it generates:
//NSString *variableName;
//read access
NSString *temp = variableName;
//2. Default. New variable(propertyName - the same name as a property) will be generated/linked with existing
#synthesize propertyName
//is the same as
//#synthesize propertyName = propertyName
//if propertyName is not exist it generates:
//NSString *propertyName;
//read access
NSString *temp = propertyName;
//if you specify not-existing <property_name> you get
//Property implementation must have its declaration in interface '<class_name>' or one of its extensions
previously you had to use next syntax:
#interface SomeClass : NSObject
{
//1. declare variable
NSString *_foo;
}
//2. create property
#property NSString *foo;
#end
#implementation SomeClass
//3. link property and iVar
#synthesize foo = _foo;
#end
But today you can use next syntax
#interface SomeClass : NSObject
//1. create property
#property NSString *foo;
#end
Next, the same code, will be generated for both cases
#interface SomeClass : NSObject
{
//variable
NSString *_foo;
}
//getter/setter
- (void)setFoo:(NSString *)newFoo;
- (NSString *)foo;
#end
#implementation SomeClass
- (void)setFoo:(NSString *)newFoo
{
_foo = newFoo;
}
- (NSString *)foo
{
return _foo;
}
#end
Here are 2 methods to assign property in Objective-C :
METHOD 1
// in header
#interface Book : NSObject {
NSString *_title;
}
#property (nonatomic, retain) NSString *title;
// in implementation
#implementation Book
#synthesize title = _title;
METHOD 2
// in header
#interface Book : NSObject {
NSString *title;
}
#property (nonatomic, retain) NSString *title;
// in implementation
#implementation Book
#synthesize title;
What are the difference? I use Method 1 recently, as more tutorials recommend Method 1, but nobody explains why.
The difference is the names. In #2 the property and instance field have the same name. In #1 they have different names.
The advantage to #1 is that it's difficult to accidentally reference the property when you mean the instance field or vice-versa. Referencing the wrong one can lead to having a object retained twice or not retained at all.
The advantage to #2 is that it's marginally simpler, and it works fine if you're careful and a bit formal in your use of things.
[And, I see, one flavor specifies assign and the other retain, which is a whole different lecture. You'd not normally use assign with an object pointer.]
Firstly, I recommend you use copy instead of retain(and assign) for NSString type of instance. If it's Mutable, then it gets copied; If not, then it just gets retained.
Maybe you'll like THIS DISCUSSION.
And for you question, the difference is that first one use the same name and the second one use the different name for iVar & property.
Actually, you have a METHOD 3 to use:
// in header
#interface Book : NSObject {
}
#property (nonatomic, copy) NSString *title;
// in implementation
#implementation Book
#synthesize title;
For #synthesize to work in the legacy runtime, you must either provide an instance variable with the same name and compatible type of the property or specify another existing instance variable in the #synthesize statement. With the modern runtime, if you do not provide an instance variable, the compiler adds one for you. For example, given the following class declaration and implementation.
Here is a sample code of official doc, you can make it clear( it includes difference between your METHOD 1 & METHOD 2):
#interface MyClass : NSObject {
float sameName;
float otherName;
}
#property float sameName;
#property float differentName;
#property float noDeclaredIvar;
#end
#implementation MyClass
#synthesize sameName;
#synthesize differentName=otherName;
#synthesize noDeclaredIvar;
#end
The compiler for the legacy runtime would generate an error at #synthesize noDeclaredIvar; whereas the compiler for the modern runtime would add an instance variable to represent noDeclaredIvar.
Note: iPhone applications and 64-bit programs on Mac OS X v10.5 and later use the modern version of the runtime. Other programs (32-bit programs on Mac OS X desktop) use the legacy version of the runtime. You can refer it HERE).
However, I suggest to use METHOD 1 or METHOD 3. As you can just use self.title in code, the property will help you manage the alloc & release. If you use METHOD 2, you may mix title with self.title(but _title is more clear, uh?). :)
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
Properties and Instance Variables in Objective-C 2.0
Objective-C Properties with or without instance variables
What is the difference between the following two pieces of code? Both are compilable and I don't know which is "correct".
#interface JTPlayer : NSObject {
NSString *userId;
NSString *name;
BOOL inBattle;
}
#property (nonatomic, copy) NSString *userId;
#property (nonatomic, copy) NSString *name;
#property (nonatomic, assign) BOOL inBattle;
#end
and
#interface JTPlayer : NSObject
#property (nonatomic, copy) NSString *userId;
#property (nonatomic, copy) NSString *name;
#property (nonatomic, assign) BOOL inBattle;
#end
One is the previous version of declaring properties. As you can see, you needed to declare variables by hand, and then declare properties applied to those variables.
The second is the newer version that manages the rest for you, declaration of variables and correspondences with properties.
Both blocks are correct, but the first one requires more keystrokes. This code (both versions) is supposed to belong to a header file, accompanied with a source file with .m extension that contains the implementation. This implementation will contain #synthesize instructions, that generate the getter and setter methods for you.
For more information, you really should read the Apple Guide to Objective-C. Also check out http://www.raywenderlich.com .
The current version of the Objective-C runtime does not require you to specify you instance variables for properties. #synthesize will add them for you automatically.
Check out this article that I put up awhile ago. It explains about instance variables and properties.
Objective-C Properties with or without instance variables