越来越容易的 Objective-c XCode4.5以后

How to Determine if ARC is Enabled in Xcode 5 Project

判断ARC是否开启的代码

Here’s a quick code check for ARC (Automatic Reference Counting):

#if __has_feature(objc_arc)
  // ARC
#else
  // No ARC
#endif

以前声明属性 要这样

.h

@property (nonatomic, strong) NSString * username
.m

 @synthesize username = _username;

现在只要

.h

@property (nonatomic, strong) NSString * username

以前 

NSNumber*number=[NSNumbernumberWithInt:1];

NSArray*array=[NSArrayarrayWithObjects:@"one",@"two",nil];

NSDictionary*dict=[NSDictionarydictionaryWithObjectsAndKeys:@"value1",@"key1",@"value2",@"key2",nil];

现在只要

NSNumber*number = @1;

NSArray*array = @[@"one",@"two"];

NSDictionary*dict = @{@"key1":@"value1",@"key2":@"value2"};

以前 

NSString *str= [arrayobjectAtIndex:0];

现在

 NSString *str = array[0];

这样也可以 array[1] = @”VW Jetta”;

上述初始化数组的代码默认创建不可变数组。如果需要可变数组，则还需要进一步调用mutableCopy方法，创建可变数组。也一样
NSMutableArray *carMakes = [@[@"Chevy", @"BMW", @"Toyota", @"Volvo", @"Smart"]mutableCopy];

NSDictionary也一样，包括可变

NSDictionary *bookListing = {key1 : object1, key2 : object2, key3 : object3};

和数组一样，下标可用来访问字典项，如下使用key1下标来访问bookListing字典对象：
bookObject = bookListing[key1];
相同地，如下代码可更新bookListing字典对象中key2键所对应的对象值：
bookListing[key2] = newBookObject;