This question already has answers here:
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.
Related
I'm following one of the iOS tutorials from Ray Wenderlich (Scarybugs part 1). But I notice for each property in the model, he always "#synthesize" it in the implementation.
Here is the example of the models:
#import <Foundation/Foundation.h>
#interface RWTScaryBugData : NSObject
#property (strong) NSString *title;
#property (assign) float rating;
- (id)initWithTitle:(NSString*)title rating:(float)rating;
#end
--
#import "RWTScaryBugData.h"
#implementation RWTScaryBugData
#synthesize title = _title;
#synthesize rating = _rating;
- (id)initWithTitle:(NSString*)title rating:(float)rating {
if ((self = [super init])) {
self.title = title;
self.rating = rating;
}
return self;
}
#end
--
#import <Foundation/Foundation.h>
#class RWTScaryBugData;
#interface RWTScaryBugDoc : NSObject
#property (strong) RWTScaryBugData *data;
#property (strong) UIImage *thumbImage;
#property (strong) UIImage *fullImage;
- (id)initWithTitle:(NSString*)title rating:(float)rating thumbImage:(UIImage *)thumbImage fullImage:(UIImage *)fullImage;
#end
--
#import "RWTScaryBugDoc.h"
#import "RWTScaryBugData.h"
#implementation RWTScaryBugDoc
#synthesize data = _data;
#synthesize thumbImage = _thumbImage;
#synthesize fullImage = _fullImage;
- (id)initWithTitle:(NSString*)title rating:(float)rating thumbImage:(UIImage *)thumbImage fullImage:(UIImage *)fullImage {
if ((self = [super init])) {
self.data = [[RWTScaryBugData alloc] initWithTitle:title rating:rating];
self.thumbImage = thumbImage;
self.fullImage = fullImage;
}
return self;
}
#end
I know "#synthesize" is basically to allocate an instance variable for a property, but it has been taken care of by default for every "#property" in ".h file" (although not visible).
My questions is: is it necessary to "#synthesize" every "#property" we have in our public API? (I tried deleting all the "#synthesize" in the implementation, and it still worked)
#synthesize is no longer needed. The compiler will synthesize the getter and setter as required with an instance variable named as _<propertyName> automatically. It creates the instance variable but more importantly it creates the getter and setter methods (for readwrite properties).
If you've manually provided the getter/setter for a property, then an instance variable won't be automatically synthesized, and you'll need to add the #synthesize statement. From the docs:
Note: The compiler will automatically synthesize an instance variable in all situations where it’s also synthesizing at least one accessor method. If you implement both a getter and a setter for a readwrite property, or a getter for a readonly property, the compiler will assume that you are taking control over the property implementation and won’t synthesize an instance variable automatically.
If you still need an instance variable, you’ll need to request that one be synthesized:
#synthesize property = _property;
As noted in the Objective-C Feature Availability Index, automatic synthesis of property instance variables was introduced with Xcode 4.4 (LLVM Compiler 4.0) and requires the modern runtime (all code on iOS, 64-bit code on OS X).
So, the tutorial is a bit dated, that's all.
hope this will help little more.
#property(nonatomic) NSString *name;
the #property is an Objective-C directive which declares the property
-> The "`nonatomic`" in the parenthesis specifies that the property is non-atomic in nature.
-> and then we define the type and name of our property.
-> prototyping of getter and setter method
now go to .m file
previously we have synthesis this property by using #synthesis , now it also NOT required , it automatically done by IDE.
-> this #synthesis now generate the getter and setter(if not readonly) methods.
and Then why we even write #synthesis in our code if it always done by IDE .
one of the basic use is :-
what our IDE do internally
#synthesis name=_name;
we use _name to access particular property but now you want synthesis by some other way like
firstname you can do it like
#synthesis name= firstname
or just by name
#synthesis name=name
So from it you can access this property as you want.
This question already has an answer here:
Closed 10 years ago.
Possible Duplicate:
Properties and instance variable declaration
Whats the difference between the following two:
SomeClass.h
#interface SomeClass : NSObject {
NSString *someString;
}
#property (strong, nonatomic) NSString *someString;
#end
SomeClass.h
#interface SomeClass : NSObject
#property (strong, nonatomic) NSString *someString;
#end
I know whats the difference between the declaration inside the { } after the interface and a property is, but whats the difference between using both and using just a property?
Since the LLVM version 4.2 compiler there is no longer a difference. You no longer HAVE to declare a property variable inside the {}.
{
NSString *someString;
}
This is an ivar.
#property (strong, nonatomic) NSString *someString;
This is a property which creates setter and getter (accessors). Also one class instance with same name is created for you.
EDIT:
If you only use ivar, you cant use self.ivar name.
You have to use by _ivar, means directly to the ivar.
Inside { & } are protected. While #property are public.
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
This question already has answers here:
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.
I'm new to Objective-C and I have been working with simple programs, and I was wondering how you would synthesize an Objective-C object pointer. So if I had a simple header file like so:
//
// Rectangle.h
// Program 8
#import <Foundation/Foundation.h>
#import "XYPoint.h"
#interface Rectangle : NSObject
{
XYPoint *origin;
}
- (XYPoint *) origin; // getter
- (void) setOrigin: (XYPoint *)pt; // setter
#end
How do you synthesize the object pointer *origin with #property and #synthesize?
Like this in the .h interface:
#property (nonatomic, retain) XYPoint* origin;
Nonatomic is optional and can be removed, which would make the property thread-safe. Retain means that the reference count will be incremented, Copy is an alternative to this which copies and increments the reference count, Assign is an alternative which is not very safe for use with objects and more intended for primitive types.
In the .m implementation:
#synthesize origin;
or
#synthesize origin = _origin;
In case you were wondering what the _origin is doing, there is a great explanation of this in the answer to this question here.
Don't forget to release your properties in dealloc (or viewDidUnload - obviously not in the class you are writing here, but if working with a view controller).
Properties are used to avoid boilerplate code to get and set variables. In your case above you would not declare the getters and setter in the header, instead declare a property for origin and synthesize that in your implementation-file.
In interface (.h) use ONE of these, NOT all of them:
#property (retain) XYPoint* origin; // will increment the retain count (strong)
#property (copy) XYPoint* origin; // will make your own copy with retain count 1 (strong)
#property (assign) XYPoint* origin; // will not increase the retain count (not strong)
In implementation (.m):
#synthesize origin;
Depending on how you are going to use "origin", you may either retain it, copy it or only assign it (see above).
Depending on what OS and version you are targeting you may not need to declare the actual variable or synthesize it (if I'm not remembering wrong), the property itself is enough.