Strong and weak

The strong and weak in property

Strong keyword and retain, using it, reference counting automatic +1, with an example to explain everything

@property (nonatomic, strong) NSString *string1;

@property (nonatomic, strong) NSString *string2;

There are two of these properties,

@synthesize string1;

@synthesize string2;

Guess what results The following code will output?

Self.string1 = @"String 1";

Self.string2 = self.string1;

Self.string1 = nil;

NSLog (@"String 2 =%@", self.string2);

The result is: string 2 = string 1

Since string2 is a strong-defined property, the reference count of +1 makes them point to the value @ "String 1", which is not difficult to understand if you are familiar with retain.

Then we look at the Weak keyword:

If this declares two properties:

@property (nonatomic, strong) NSString *string1;

@property (nonatomic, weak) NSString *string2;

and define

@synthesize string1;

@synthesize string2;

Let's guess, what is the output below?

Self.string1 = @"String 1";

Self.string2 = self.string1;

Self.string1 = nil;

NSLog (@"String 2 =%@", self.string2);

The result is: String 2 = null

Analysis, because Self.string1 and Self.string2 point to the same address, and string2 no retain memory address, and Self.string1=nil freed memory, so string1 is nil. A pointer that is declared as weak, the pointer to the address once released, these pointers will be assigned to nil. Such a benefit can effectively prevent the wild hands. Why does Daniel say that when the pointer space is released, the pointer will be NULL when it is developed in C + +. Here we do this step with the weak keyword.

Strong and weak