strong，copy有何不同？實踐出真知！上代碼

strong

#import "ViewController.h"

@interface ViewController ()

@property (nonatomic,strong)NSString *name;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    NSMutableString *string = [[NSMutableString alloc] initWithString:@"張三"];
    self.name = string;
    NSLog(@"string:%@, 地址:%p", string, string);
    NSLog(@"self.name:%@, 地址%p", self.name, self.name);
    [string appendString:@"1"];
    NSLog(@"string:%@, 地址:%p", string, string);
    NSLog(@"self.name:%@, 地址%p", self.name, self.name);
}

@end

運行結果

2021-06-04 22:54:17.867578+0800 iOSDemo[1557:53589] string:張三, 地址:0x6000008aecd0
2021-06-04 22:54:17.867716+0800 iOSDemo[1557:53589] self.name:張三, 地址0x6000008aecd0
2021-06-04 22:54:17.867810+0800 iOSDemo[1557:53589] string:張三1, 地址:0x6000008aecd0
2021-06-04 22:54:17.867893+0800 iOSDemo[1557:53589] self.name:張三1, 地址0x6000008aecd0

copy

//
//  ViewController.m
//  iOSDemo
//
//  Created by 彭之耀 on 2021/6/4.
//

#import "ViewController.h"

@interface ViewController ()

@property (nonatomic,copy)NSString *name;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    NSMutableString *string = [[NSMutableString alloc] initWithString:@"張三"];
    self.name = string;
    NSLog(@"string:%@, 地址:%p", string, string);
    NSLog(@"self.name:%@, 地址%p", self.name, self.name);
    [string appendString:@"1"];
    NSLog(@"string:%@, 地址:%p", string, string);
    NSLog(@"self.name:%@, 地址%p", self.name, self.name);
    
}

@end

運行結果

2021-06-04 22:56:08.281057+0800 iOSDemo[1582:54701] string:張三, 地址:0x6000008c5110
2021-06-04 22:56:08.281187+0800 iOSDemo[1582:54701] self.name:張三, 地址0x6000006b6240
2021-06-04 22:56:08.281303+0800 iOSDemo[1582:54701] string:張三1, 地址:0x6000008c5110
2021-06-04 22:56:08.281394+0800 iOSDemo[1582:54701] self.name:張三, 地址0x6000006b6240

分析

• 當我們進行賦值的時候，strong 是直接將地址值傳遞給了對象的屬性，而 copy 是開辟了新的空間，并把值賦值給對象的屬性。
• NSString類型的屬性推薦使用 copy，因為NSMutableString是NSString的子類，若將NSMutableString類型的對象賦值給NSString類型的屬性，當對象變化時，屬性值會發(fā)生改變，所以NSString推薦使用copy修飾。（面試重點??）
• 深拷貝、淺拷貝的區(qū)別在上面也有體現(xiàn)，淺拷貝是拷貝地址，深拷貝是拷貝值。