what is the difference between a readwrite property and a nonatomic assign property?

Question

I have seen readwrite on int, BOOL etc same as nonatomic, assign.

I am some what confused on this. I do know that on non native objects, we typically do nonatomic, retain.

Answer 1

Here's the short answer:

atomic vs nonatomic primarily ensures that complete values are returned from synthesized getters and that complete values are written by synthesized setters (atomic is default.)

readwrite vs readonly determines whether a synthesized property has a synthesized accessor or not (readwrite has a setter and is the default, readonly does not).

assign vs retain vs copy determines how the synthesized accessors interact with the Objective-C memory management scheme:

• assign is the default and simply performs a variable assignment
• retain specifies the new value should be sent -retain on assignment and the old value sent -release
• copy specifies the new value should be sent -copy on assignment and the old value sent -release.

Answer 2

After reading so many Articles, SO posts and made demo apps to check Variable property attributes, I decided to put all the attributes information together

1. atomic //default
2. nonatomic
3. strong=retain //default
4. weak= unsafe_unretained
5. retain
6. assign //default
7. unsafe_unretained
8. copy
9. readonly
10. readwrite //default

so below is the detailed article link where you can find above mentioned all attributes, that will defiantly help you. Many thanks to all the people who give best answers here!!

Variable property attributes or Modifiers in iOS

1. retain = strong
   • it is retained, old value is released and it is assigned
   • retain specifies the new value should be sent -retain on assignment and the old value sent -release
   • retain is the same as strong.
   • apple says if you write retain it will auto converted/work like strong only.
   • methods like "alloc" include an implicit "retain"

Example:

@property (nonatomic, retain) NSString *name;

@synthesize name;

2. assign
   • assign is the default and simply performs a variable assignment
   • assign is a property attribute that tells the compiler how to synthesize the property's setter implementation
   • I would use assign for C primitive properties and weak for weak references to Objective-C objects.

Example:

@property (nonatomic, assign) NSString *address;

@synthesize address;

9. readonly

   • declaring your property as readonly you tell compiler to not generate setter method automatically.
   • Indicates that the property is read-only.
   • If you specify readonly, only a getter method is required in the @implementation block. If you use the @synthesize directive in the @implementation block, only the getter method is synthesized. Moreover, if you attempt to assign a value using the dot syntax, you get a compiler error.

Example:

@property (nonatomic, readonly) NSString *name;

@synthesize name;

10. readwrite
    • setter and getter generated.
    • Indicates that the property should be treated as read/write.
    • This attribute is the default.
    • Both a getter and setter method are required in the @implementation block. If you use the @synthesize directive in the implementation block, the getter and setter methods are synthesized.

Example:

@property (nonatomic, readwrite) NSString *name;

@synthesize name;

Answer 3

readwrite means that both a getter and a setter exist; the opposite is readonly. Normally the only time you'd explicitly declare a property readwrite is in a class extension for a class where the public interface declares the property readonly — so that it's publicly read-only, but internally you can both get and set.