iOS promotion path-strong and weak references

In the Objective-c arc mode, see the following code

ID OBJC =[[nsobject Alloc]init];

Here the ID is strongly referenced by default

In the case of a strong reference, there are times when a circular reference is required, and weak is needed to help.

int main (int argc, const char * argv[]) {

ID __weak obj0 = nil;

@autoreleasepool {

if (YES) {

ID obj1 = [[NSObject alloc]init];

Obj0 = obj1;

NSLog (@ "%@", obj0);

}

NSLog (@ "%@", obj0);

}

return 0;

}

Printing results:

2015-09-01 15:30:25.640 Strong Reference Testing [2166:153376] <NSObject:0x100211f20>

2015-09-01 15:30:25.642 Strong Reference Testing [2166:153376] (NULL)

因为obj1生成的默认的为强引用(__strong)，在超出if的作用域之后，obj1所持有的对象被释放， obj0为弱引用，所以obj0不持有对象，在obj1对象释放后，obj0自动的被赋值为nil 弱引用的特性是，不持有对象，即便是写成id __weak obj1 = [[NSObject alloc] init]; 此代码系统会给与警告，因为这里obj1被声明成弱引用，那么在赋值之后，alloc出来的对象会被立即释放。

iOS promotion path-strong and weak references