CODEWITHSUNDEEP
objective-cpropertiessynthesis
RichWalt
RichWalt

Reputation: 522

@property and @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? )

Upvotes: 9

Views: 7498

Answers (3)

yoAlex5
yoAlex5

Reputation: 34225

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

Upvotes: 0

Rob Napier
Rob Napier

Reputation: 299345

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).

Upvotes: 18

FluffulousChimp
FluffulousChimp

Reputation: 9185

@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.

Upvotes: 7

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