1. @property (nonatomic, strong) NSString *string1;
2. @property (nonatomic, strong) NSString *string2;

1. @synthesize string1;
2. @synthesize string2;

1. self.string1 = @"String 1";
2. self.string2 = self.string1;
3. self.string1 = nil;
4. NSLog(@"String 2 = %@", self.string2);

1. @property (nonatomic, strong) NSString *string1;
2. @property (nonatomic, weak) NSString *string2;

1. @synthesize string1;
2. @synthesize string2;

1. self.string1 = @"String 1";
2. self.string2 = self.string1;
3. self.string1 = nil;
4. NSLog(@"String 2 = %@", self.string2);

1. @property (nonatomic, strong) NSString *string1;
2. @property (nonatomic, unsafe_unretained) NSString *string2;

1. self.string1 = @"String 1";
2. self.string2 = self.string1;
3. self.string1 = nil;
4. NSLog(@"String 2 = %@", self.string2);

1. __strong NSString *yourString = @"Your String";
2. __weak  NSString *myString = yourString;
3. yourString = nil;
4. __unsafe_unretained NSString *theirString = myString;
5. //现在所有的指针都为nil

1. __strong NSString *yourString = @"Your String";
2. __weak  NSString *myString = yourString;
3. __unsafe_unretained NSString *theirString = myString;
4. yourString = nil;
5. //现在yourString与myString的指针都为nil,而theirString不为nil,但是是野指针。

1. - (void) generateErrorInVariable:(__autoreleasing NSError **)paramError{
2. NSArray *objects = [[NSArray alloc] initWithObjects:@"A simple error", nil];
3. NSArray *keys = [[NSArray alloc] initWithObjects:NSLocalizedDescriptionKey, nil];
4. NSDictionary *errorDictionary = [[NSDictionary alloc] initWithObjects:objects forKeys:keys];
5. *paramError = [[NSError alloc] initWithDomain:@"MyApp" code:1 userInfo:errorDictionary];
6. }
7. -(void)test
8. {
9. NSError *error = nil;
10. [self generateErrorInVariable:&error];
11. NSLog(@"Error = %@", error);
12. }

1. -(NSString *)stringTest
2. {
3. NSString *retStr = [NSString stringWithString:@"test"];
4. return [[retStr retain] autorelease];
5. }

1. -(NSString *)stringTest
2. {
3. __autoreleasing NSString *retStr = [NSString alloc] initWithString:@"test"];
4. return retStr;
5. }

Setter Semantics

These attributes specify the semantics of a set accessor. They are mutually exclusive.

strong

Specifies that there is a strong (owning) relationship to the destination object.

weak

Specifies that there is a weak (non-owning) relationship to the destination object.

If the destination object is deallocated, the property value is automatically set to nil.

(Weak properties are not supported on OS X v10.6 and iOS 4; use assigninstead.)

copy

Specifies that a copy of the object should be used for assignment.

The previous value is sent a release message.

The copy is made by invoking the copy method. This attribute is valid only for object types, which must implement the NSCopying protocol.

assign

Specifies that the setter uses simple assignment. This attribute is the default.

You use this attribute for scalar types such as NSInteger and CGRect.

retain

Specifies that retain should be invoked on the object upon assignment.

The previous value is sent a release message.

In OS X v10.6 and later, you can use the __attribute__ keyword to specify that a Core Foundation property should be treated like an Objective-C object for memory management:

@property(retain) __attribute__((NSObject)) CFDictionaryRef myDictionary;