With properties in obj-c, do we need to declare the instance variables?

Question

Eg if my .h file looks like this:

@interface MyClass : NSObject {
}

@property (nonatomic, retain) NSNumber *someId;
@property (nonatomic, retain) NSDate *expires;
@property (nonatomic, retain) NSString *someString;

...

Is that fine? It compiles and works, is it the 'done thing' ? Or should i really be doing the below:

@interface MyClass : NSObject {
    NSNumber *someId;
    NSDate *expires;
    NSString *someString;
}

@property (nonatomic, retain) NSNumber *someId;
@property (nonatomic, retain) NSDate *expires;
@property (nonatomic, retain) NSString *someString;

...

Is there any difference in either of the above ways, if i plan to always use the property accessors?

Does the @synthesize take care of creating the instance vars for me?

Please [search before posting](http://stackoverflow.com/search?q=%5Bobjc%5D+property+declare+ivar) [1](http://stackoverflow.com/questions/5031230/) [2](http://stackoverflow.com/questions/3336922/) [3](http://stackoverflow.com/questions/4299487/) [4](http://stackoverflow.com/questions/4333797/) [5](http://stackoverflow.com/questions/4706816/) — jscs, May 03 '11 at 02:38

gcamp · Accepted Answer · 2011-05-03T02:04:15.863

5

You understood correctly, @synthesize takes care of creating ivars for you. (That's a new feature of the runtime that's compatible with 64 bits Macs and iOS devices.)

What's nice with that, is that you can @property in your class extension and hide your ivars completely from the .h, separating completely the interface from the implementation.

You can also remove completely the '{}' from the .h, making it really clean.

edited May 03 '11 at 02:04

answered May 03 '11 at 01:57

gcamp

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If i do that, will it prevent me from accessing the ivar in my .m file? I would like to be forced to use the property eg 'self.blah' – Chris May 03 '11 at 02:05
No, you can still access the raw ivar if you feel the need. – Joshua Weinberg May 03 '11 at 02:23
2

You can still rename the iVar with a statement like @synthesize myProp = _myProp, to prevent accidental use without the getter. The iVar will be named _myProp. – Kendall Helmstetter Gelner May 03 '11 at 03:22

score 1 · Answer 2 · answered May 03 '11 at 02:06

1

Found this in the apple docs:

With the modern runtime, if you do not provide an instance variable, the compiler adds one for you

answered May 03 '11 at 02:06

Chris

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With properties in obj-c, do we need to declare the instance variables?

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