1、浅拷贝
浅拷贝就是对内存地址的复制,让目标对象指针和源对象指向同一片内存空间,当内存销毁的时候,指向这片内存的几个指针需要重新定义才可以使用,要不然会成为野指针。
浅拷贝就是拷贝指向原来对象的指针,使原对象的引用计数+1,可以理解为创建了一个指向原对象的新指针而已,并没有创建一个全新的对象。
2、深拷贝
深拷贝是指拷贝对象的具体内容,而内存地址是自主分配的,拷贝结束之后,两个对象虽然存的值是相同的,但是内存地址不一样,两个对象也互不影响,互不干涉。
深拷贝就是拷贝出和原来仅仅是值一样,但是内存地址完全不一样的新的对象,创建后和原对象没有任何关系。
3、总结:
深拷贝就是内容拷贝,浅拷贝就是指针拷贝。本质区别在于:
在iOS中深拷贝与浅拷贝要更加的复杂,涉及到容器与非容器、可变与不可变对象的copy与mutableCopy。下面用示例逐一分析:
1、非集合对象的copy与mutableCopy
1.1 不可变对象NSString
- (void) noMutableNSStringTest { NSString *str1 = @"test001"; NSMutableString *str2 = [str1 copy]; //copy返回的是不可变对象,str2不能被修改,因此会发生崩溃 //[str2 appendString:@"test"]; NSMutableString *str3 = [str1 mutableCopy]; [str3 appendString:@"modify"]; NSLog(@"str1:%p - %@ \r\n",str1,str1); NSLog(@"str2:%p - %@ \r\n",str2,str2); NSLog(@"str3:%p - %@ \r\n",str3,str3); }
打印结果:
2017-07-20 18:02:10.642 beck.wang[1306:169414] str1:0x106abdbd0 - test001 2017-07-20 18:02:10.643 beck.wang[1306:169414] str2:0x106abdbd0 - test001 2017-07-20 18:02:10.643 beck.wang[1306:169414] str3:0x608000260940 - test001modify
分析:str1、str2地址相同并且与str3地址不同,NSString的copy是浅拷贝,且copy返回的对象是不可变对象;mutableCopy是深拷贝。
1.2 可变对象NSMutableString
- (void) mutableNSStringTest { NSMutableString *mstr1 = [NSMutableString stringWithString:@"test002"]; NSMutableString *mstr2 = [mstr1 copy]; //copy返回的是不可变对象,mstr2不能被修改,因此会发生崩溃 //[str2 appendString:@"test"]; NSMutableString *mstr3 = [mstr1 mutableCopy]; [mstr3 appendString:@"modify"]; NSLog(@"mstr1:%p - %@ \r\n",mstr1,mstr1); NSLog(@"mstr2:%p - %@ \r\n",mstr2,mstr2); NSLog(@"mstr3:%p - %@ \r\n",mstr3,mstr3); }
打印结果:
2017-07-20 18:14:35.789 beck.wang[1433:180881] mstr1:0x610000075e40 - test002 2017-07-20 18:14:35.790 beck.wang[1433:180881] mstr2:0xa323030747365747 - test002 2017-07-20 18:14:35.790 beck.wang[1433:180881] mstr3:0x610000074480 - test002modify
分析:mstr1、mstr2、mstr3 地址都不同,NSMutableString对象copy与mutableCopy都是深拷贝,且copy返回的对象是不可变对象。
2、集合对象的copy与mutableCopy
2.1 不可变对象NSArray
- (void) mutableNSArrayTest { NSArray *arry1 = [[NSArray alloc] initWithObjects:@"value1", @"value2",nil]; NSArray *arry2 = [arry1 copy]; NSArray *arry3 = [arry1 mutableCopy]; NSLog(@"arry1:%p - %@ \r\n",arry1,arry1); NSLog(@"arry2:%p - %@ \r\n",arry2,arry2); NSLog(@"arry3:%p - %@ \r\n",arry3,arry3); }
打印结果:
2017-07-20 18:33:53.707 beck.wang[1502:194476] arry1:0x60800003b480 - ( value1, value2 ) 2017-07-20 18:33:53.708 beck.wang[1502:194476] arry2:0x60800003b480 - ( value1, value2 ) 2017-07-20 18:33:53.708 beck.wang[1502:194476] arry3:0x60800004cd20 - ( value1, value2 )
分析:arry1、arry2 地址一样,arr3 地址不一样,NSArray的copy是浅拷贝,且copy返回的对象是不可变对象;mutableCopy是深拷贝。
2.2 可变对象NSMutableArray
- (void) NSMutableArrayTest { NSMutableArray *marry1 = [[NSMutableArray alloc] initWithObjects:@"value1", @"value2",nil]; NSMutableArray *marry2 = [marry1 copy]; //copy返回的是不可变对象,marry2不能被修改,因此会崩溃 //[marry2 addObject:@"value3"]; NSMutableArray *marry3 = [marry1 mutableCopy]; NSLog(@"marry1:%p - %@ \r\n",marry1,marry1); NSLog(@"marry2:%p - %@ \r\n",marry2,marry2); NSLog(@"marry3:%p - %@ \r\n",marry3,marry3); }
打印结果:
2017-07-20 18:55:43.243 beck.wang[1577:204641] marry1:0x600000048d60 - ( value1, value2 ) 2017-07-20 18:55:43.244 beck.wang[1577:204641] marry2:0x600000026000 - ( value1, value2 ) 2017-07-20 18:55:43.244 beck.wang[1577:204641] marry3:0x6000000494b0 - ( value1, value2 )
分析:marry1、marry2、marr3 地址都不一样,NSMutableArray对象copy与mutableCopy都是深拷贝,且copy返回的对象是不可变对象。
特别注意的是:对于集合类的可变对象来说,深拷贝并非严格意义上的深复制,只能算是单层深复制,即虽然新开辟了内存地址,但是存放在内存上的值(也就是数组里的元素仍然之乡员数组元素值,并没有另外复制一份),这就叫做单层深复制。
举例说明:
- (void)singleNSMutableArrayTest { NSMutableArray *marry1 = [[NSMutableArray alloc] init]; NSMutableString *mstr1 = [[NSMutableString alloc]initWithString:@"value1"]; NSMutableString *mstr2 = [[NSMutableString alloc]initWithString:@"value2"]; [marry1 addObject:mstr1]; [marry1 addObject:mstr2]; NSMutableArray *marry2 = [marry1 copy]; NSMutableArray *marry3 = [marry1 mutableCopy]; NSLog(@"marry1:%p - %@ \r\n",marry1,marry1); NSLog(@"marry2:%p - %@ \r\n",marry2,marry2); NSLog(@"marry3:%p - %@ \r\n",marry3,marry3); NSLog(@"数组元素地址:value1:%p - value2:%p \r\n",marry1[0],marry1[1]); NSLog(@"数组元素地址:value1:%p - value2:%p \r\n",marry2[0],marry2[1]); NSLog(@"数组元素地址:value1:%p - value2:%p \r\n",marry3[0],marry3[1]); NSLog(@"\r\n------------------修改原值后------------------------\r\n"); [mstr1 appendFormat:@"aaa"]; NSLog(@"marry1:%p - %@ \r\n",marry1,marry1); NSLog(@"marry2:%p - %@ \r\n",marry2,marry2); NSLog(@"marry3:%p - %@ \r\n",marry3,marry3); NSLog(@"数组元素地址:value1:%p - value2:%p \r\n",marry1[0],marry1[1]); NSLog(@"数组元素地址:value1:%p - value2:%p \r\n",marry2[0],marry2[1]); NSLog(@"数组元素地址:value1:%p - value2:%p \r\n",marry3[0],marry3[1]); }
打印结果:
2017-07-20 19:48:24.539 beck.wang[1750:230132] marry1:0x60800004ae00 - ( value1, value2 ) 2017-07-20 19:48:24.539 beck.wang[1750:230132] marry2:0x608000023f00 - ( value1, value2 ) 2017-07-20 19:48:24.539 beck.wang[1750:230132] marry3:0x60800004abc0 - ( value1, value2 ) 2017-07-20 19:48:24.540 beck.wang[1750:230132] 数组元素地址:value1:0x60800006df40 - value2:0x60800006cb40 2017-07-20 19:48:24.540 beck.wang[1750:230132] 数组元素地址:value1:0x60800006df40 - value2:0x60800006cb40 2017-07-20 19:48:24.540 beck.wang[1750:230132] 数组元素地址:value1:0x60800006df40 - value2:0x60800006cb40 2017-07-20 19:48:24.540 beck.wang[1750:230132] ------------------修改原值后------------------------ 2017-07-20 19:48:24.540 beck.wang[1750:230132] marry1:0x60800004ae00 - ( value1aaa, value2 ) 2017-07-20 19:48:24.540 beck.wang[1750:230132] marry2:0x608000023f00 - ( value1aaa, value2 ) 2017-07-20 19:48:24.540 beck.wang[1750:230132] marry3:0x60800004abc0 - ( value1aaa, value2 ) 2017-07-20 19:48:24.541 beck.wang[1750:230132] 数组元素地址:value1:0x60800006df40 - value2:0x60800006cb40 2017-07-20 19:48:24.541 beck.wang[1750:230132] 数组元素地址:value1:0x60800006df40 - value2:0x60800006cb40 2017-07-20 19:48:24.541 beck.wang[1750:230132] 数组元素地址:value1:0x60800006df40 - value2:0x60800006cb40
分析:在修改原值之前,marry1、marry2、marr3 地址都不一样,很明显copy和mutableCopy都是深拷贝,但是从修改原值后的打印结果来看,这里的深拷贝只是单层深拷贝:新开辟了内存地址,但是数组中的值还是指向原数组的,这样才能在修改原值后,marry2 marr3中的值都修改了。另外,从打印的数组元素地址可以很明显的看出来,修改前后marry1、marry、marr3的数组元素地址都是一模一样的,更加佐证了这一点。
2.3 思维扩展:集合对象的完全深拷贝
2.2中提到了集合类的对象来说,深拷贝只是单层深拷贝,那有没有办法实现每一层都深拷贝呢?回答是肯定的,目前我们可以这么做:
(1)归档解档大法
- (void) deplyFullCopy { NSMutableArray *marry1 = [[NSMutableArray alloc] init]; NSMutableString *mstr1 = [[NSMutableString alloc]initWithString:@"value1"]; NSMutableString *mstr2 = [[NSMutableString alloc]initWithString:@"value2"]; [marry1 addObject:mstr1]; [marry1 addObject:mstr2]; NSData *data = [NSKeyedArchiver archivedDataWithRootObject:marry1]; NSArray *marray2 = [NSKeyedUnarchiver unarchiveTopLevelObjectWithData:data error:nil]; NSLog(@"marry1:%p - %@ \r\n",marry1,marry1); NSLog(@"marry2:%p - %@ \r\n",marray2,marray2); NSLog(@"数组元素地址:value1:%p - value2:%p \r\n",marry1[0],marry1[1]); NSLog(@"数组元素地址:value1:%p - value2:%p \r\n",marray2[0],marray2[1]); }
打印结果:
2017-07-20 20:04:38.726 beck.wang[1833:242158] marry1:0x600000048a00 - ( value1, value2 ) 2017-07-20 20:04:38.726 beck.wang[1833:242158] marry2:0x600000049780 - ( value1, value2 ) 2017-07-20 20:04:38.726 beck.wang[1833:242158] 数组元素地址:value1:0x600000066300 - value2:0x600000067000 2017-07-20 20:04:38.726 beck.wang[1833:242158] 数组元素地址:value1:0x600000066740 - value2:0x600000066f40
分析:我们可以看到,开辟了新的内存地址的同时,数组元素的指针地址也不同了,实现了完全的深拷贝。
(2)- (instancetype)initWithArray:(NSArray<ObjectType> *)array copyItems:(BOOL)flag;
- (void) deplyFullCopy2 { NSMutableArray *marry1 = [[NSMutableArray alloc] init]; NSMutableString *mstr1 = [[NSMutableString alloc]initWithString:@"value1"]; NSMutableString *mstr2 = [[NSMutableString alloc]initWithString:@"value2"]; [marry1 addObject:mstr1]; [marry1 addObject:mstr2]; NSArray *marray2 = [[NSArray alloc] initWithArray:marry1 copyItems:YES]; NSLog(@"marry1:%p - %@ \r\n",marry1,marry1); NSLog(@"marry2:%p - %@ \r\n",marray2,marray2); NSLog(@"数组元素地址:value1:%p - value2:%p \r\n",marry1[0],marry1[1]); NSLog(@"数组元素地址:value1:%p - value2:%p \r\n",marray2[0],marray2[1]); }
打印结果:
2017-07-20 20:08:04.201 beck.wang[1868:246161] marry1:0x610000050320 - ( value1, value2 ) 2017-07-20 20:08:04.202 beck.wang[1868:246161] marry2:0x6100002214c0 - ( value1, value2 ) 2017-07-20 20:08:04.202 beck.wang[1868:246161] 数组元素地址:value1:0x610000265600 - value2:0x610000266400 2017-07-20 20:08:04.202 beck.wang[1868:246161] 数组元素地址:value1:0xa003165756c61766 - value2:0xa003265756c61766
分析:同上。
No1:可变对象的copy和mutableCopy方法都是深拷贝(区别完全深拷贝与单层深拷贝) 。
No2:不可变对象的copy方法是浅拷贝,mutableCopy方法是深拷贝。
No3:copy方法返回的对象都是不可变对象。
万语千言汇成一张图
深浅拷贝讲完了,后续我会抽时间图文并茂的分析内存定义语句的区别, 鉴于水平有限,如果有不当之处还望批评指正,谢谢!