Property vs Instance Variable (iVar) in Objective-c

This post is deprecated. Refer to Propety, Instance Variable and @synthesize.

Description

To declare a variable in a objective-c class, we have the following two ways:

  • Property
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@interface Photo : NSObject
@property (nonatomic, strong) NSString *photographer;
@end
  • Instance Variable (iVar)
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@interface Photo : NSObject {
    NSString *photographer;
}
@end


Then, what’s the difference?

Differences

private vs public

For a private/protected variable, use iVar; for a public variable, use property. If you want to use the benifit of property attributes for a private variable, like retain, nonatomic etc., declare the property in the implementation file as a private property.

For an iVar, you can use @private, @protected and @public. But these attributes only influence the access of its subclass, and has nothing to do with the access ability of its instances. Go to here for reference.

usage

Directly use an iVar inside the class, for example, photographer. But use self. for a property, for example, self.photographer.

performance

iVar is faster than property, as property will call the getter or setter functions. When you declare a property, the compiler will add getter and setter functions for the property.

@synthesize for property

UPDATE：Just read this paragraph, omit the paragraphs behind.

@synthesize is only for property, not iVar. It will help the property generate an instance variable, the getter and setter accessors for the property and use the generated instance variable in the getter and setter accessors. For every property, complier will automatically synthesize it using _propertyName. The auto synthesize is the same as you add @synthesize propertyName = _propertyName. If you only add @synthesize propertyName, it is the same with @synthesize propertyName = propertyName, and if you define an iVar with the name propertyName, they will be synthesized. That is to say, in the accessors (getter and setter) of property propertyName, it will use iVar propertyName.

Note for property inheritance: If the super class has a property declared as public, the subclass can inherit the property. But the auto synthesize mechnism will not sythesize the property automatically, and you can only use the getter and setter accessors of the superclass by using dot operation. If you want to synthesize manually, Be Carefull! You should avoid using the same iVar as them in the superclass. This will lead to an error message “property ‘propertyName’ atemping to use instance variable ‘InstanceVariableName’ declared in super class ‘SuperClassName’”.

If you inherit a property from a protocol, you mush synthesize it, as the protocol only declares it, not synthesize it.

Omit the following paragraphs in this section

If you add @synthesize photographer in the implementation, compiler will automatically add an iVar photographer and _photographer to the class. You can directly use photographer or _photographer instead of self.photographer to get or set the value. The iVar method is faster, but keep in mind that it will not call the getter or setter method.

If you declare the class like this and don’t @synthesize photographer:

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@interface Photo : NSObject {
  NSString *photographer;
}
@property (nonatomic, strong) NSString *photographer;
@end

There are actually two photographer variables in the class: when you use photographer directly, you are using the iVar, and when you use self.photographer, you are using the property.

However, when you use @synthesize photographer in the implementation file, the compoler will add photographer variable for the property. That is to say, photographer will be the property, and the iVar will not be usable.

References

  1. A more detailed description is here.
  2. reference, answer by Rachel Henderson
  3. Property session of the reference. This post is old fashioned.
  4. Here has a good explaination.
  5. 如何正确使用@synthesize（在有了自动合成属性实例变量之后，@synthesize还有哪些使用场景？）得出了一个结论：@synthesize只需要用于声明在protocol中的property。