@interface ViewController ()

// Strong
@property (nonatomic, strong) NSString *sString;
@property (nonatomic, strong) NSArray *sArray;
// Copy
@property (nonatomic, copy) NSString *cString;
@property (nonatomic, copy) NSArray *cArray;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    // 使用strong时，如果这个属性指向一个可变对象，修改可变对象时，这个属性值也会被修改
    NSMutableString *mString = [NSMutableString stringWithString:@"strong"];
    self.sString = mString;
    self.cString = mString;
    [mString appendString:@"_change"];
    NSLog(@"\n sString = %@ \n cString = %@", self.sString, self.cString);
    
    // NString，NSArray，NSDictionary都有自己对应的子类：NSMutableString，NSMutableArray，NSMutableDictionary，而父类指针可以指向子类对象，使用copy可以让本对象不受外界（子对象）影响，无论给我传入的是一个可变对象还是一个不可变对象，都能保证自身持有的是一个不可变副本
    NSMutableArray *mArray = [NSMutableArray arrayWithObjects:@1, @2, @3,  nil];
    self.sArray = mArray;
    self.cArray = mArray;
    [mArray addObject:@4];
    NSLog(@"\n sArray = %@ \n cArray = %@", self.sArray, self.cArray);
}


@end

日志输出

  使用copy能保证属性不被子类对象修改时同时被修改。
  sString = strong_change 
  cString = strong
  sArray = (
    1,
    2,
    3,
    4
  ) 
  cArray = (
    1,
    2,
    3
  )

Copy 和 MutableCopy

// 浅拷贝：地址的拷贝
    // 深拷贝：内容的拷贝
    
    // 非集合对象的copy
    NSString *string = @"string";
    NSString *stringCopy = [string copy];
    NSString *stringMCopy = [string mutableCopy];
    NSLog(@"\nstring: \n%p \nstringCopy: \n%p \nstringMCopy: \n%p", string, stringCopy, stringMCopy);

string: 
0x100270088 
stringCopy: 
0x100270088 
stringMCopy: 
0x1c425a490
    
    // 非集合对象的mutableCopy
    NSMutableString *mString = [NSMutableString stringWithString:@"mString"];
    NSString *mStringCopy = [mString copy];
    NSString *mStringMCopy = [mString mutableCopy];
    NSLog(@"\nmString: \n%p \nmStringCopy: \n%p \nmStringMCopy: \n%p", mString, mStringCopy, mStringMCopy);

mString: 
0x1c425a2e0 
mStringCopy: 
0xa676e697274536d7 
mStringMCopy: 
0x1c425a070
    
    // 集合对象的copy NSArray，NSDictionary，NSSet
    NSArray *array = @[@1, @2, @3];
    NSArray *arrayCopy = [array copy];
    NSMutableArray *arrayMCopy = [array mutableCopy];
    NSLog(@"\narray: \n%p \narrayCopy: \n%p \narrayMCopy: \n%p", array, arrayCopy, arrayMCopy);

array: 
0x1c425a580 
arrayCopy: 
0x1c425a580 
arrayMCopy: 
0x1c425a0a0
    
    // 集合对象的mutableCopy
    NSMutableArray *mArray = [NSMutableArray arrayWithObjects:@1, @2, @3, nil];
    NSArray *mArrayCopy = [mArray copy];
    NSMutableArray *mArrayMCopy = [mArray mutableCopy];
    NSLog(@"\nmArray: \n%p \nmArrayCopy: \n%p \nmArrayMCopy: \n%p", mArray, mArrayCopy, mArrayMCopy);

mArray: 
0x1c425a640 
mArrayCopy: 
0x1c425a1f0 
mArrayMCopy: 
0x1c425a3d0

[immutableObject copy]是浅拷贝
[immutableObject mutableCopy]是深拷贝
[mutableObject copy]是深拷贝
[mutableObject mutableCopy]是深拷贝

集合对象的深拷贝只是对象本身，而集合对象的元素还是地址的拷贝，即单层深拷贝(浅拷贝)

block为什么要使用copy

block使用copy是在MRC中延续下来的，在MRC下，方法内部的block是存放在栈区，使用copy会将block拷贝到堆区。
在ARC下编译器会自动对block进行copy，因此我们使用copy或者strong的效果是一样的。但是我们在ARC下继续使用copy可以提醒我们编译器会自动帮我们实现copy的操作。

参考文章