OC语言 - 内存管理：copy | 深拷贝 | 浅拷贝 | 自定义拷贝

copy 实现原理

1 - copy 修饰属性时，其 setter/getter 方法内部实现原理同 retain 类似 

// - Person.h

#import <Foundation/Foundation.h>
@interface Person : NSObject
@property(nonatomic,copy)NSString *name;
@end

// - Person.m

#import "Person.h"
@implementation Person

-(void)dealloc{

    [_name release];
    NSLog(@"%@ 销毁",self);
    [super dealloc];
}


-(void)setName:(NSString *)name{

    if (_name != name) {
        [_name release];
        _name = [name copy];// 不同点：使用的是 copy
    }
}

-(NSString *)getName{

    return [[_name retain] autorelease];
}

@end

深拷贝 | 浅拷贝

1 - 对不可变 NSString、NSArray、NSDictionary 等进行 copy 属浅拷贝。它拷贝指针，并没有拷贝出新对象，copy 相当于是 retain

NSString *str01 = [NSString stringWithFormat:@"小白菜"];
NSLog(@"%lu   %p",[str01 retainCount],str01);//  1   0x100502850
NSString *newStr01 = [str01 copy];
NSLog(@"%lu    %p",[newStr01 retainCount],newStr01);// 2    0x100502850

2 - 对可变 NSMutableString、NSMutableArray、NSMutableDictionary 等进行 copy 是深拷贝。它会拷贝出一个新对象且新对象是不可变的

NSMutableString *str02 = [NSMutableString stringWithFormat:@"科技"];
NSLog(@"%lu %p",[str02 retainCount],str02); // 1  0x103306120
NSString *newStr02 = [str02 copy];// 返回不可变
NSLog(@"%lu %@ %p",[newStr02 retainCount],newStr02,newStr02);// 1 科技 0x103104230

3 - 对 NSString/NSMutableString、NSArray/NSMutableArray、NSDictionary/NSMutableDictionary 进行 mutableCopy 是深拷贝。它会拷贝出一个新对象且新对象是可变的

NSString *str03 = [NSString stringWithFormat:@"小二黑结婚"];
NSMutableString *newSt03 = [str03 mutableCopy];// 返回可变字符串
NSLog(@"%p   %p",str03,newSt03);// 0x100508460   0x1005067c0
    
NSMutableString *str04 = [NSMutableString stringWithFormat:@"平凡的世界"];
NSMutableString *newStr04 = [str04 mutableCopy];

自定义拷贝

1 - 对类进行拷贝：当一个类需要 copy 时要接受 NSCopying 协议，并且实现协议中的 copyWithZone 方法

// - Student.h

#import <Foundation/Foundation.h>
@interface Student : NSObject<NSCopying> // 协议

@property (nonatomic,copy)NSString *name;
@property (nonatomic,copy)NSString *sex;
@property (nonatomic,assign)NSInteger  age;

- (id)initWithName:(NSString *)name sex:(NSString *)sex age:(NSInteger )age;
- (void)sayHI;

@end

// - Student.m

#import "Student.h"
@implementation Student

-(void)dealloc{

    self.name = nil;
    self.sex = nil;
    [super dealloc];
}

- (void)sayHI{
    NSLog(@"I am a student,name ： %@,sex ： %@,age ： %ld",_name,_sex,_age);
}

- (id)initWithName:(NSString *)name sex:(NSString *)sex age:(NSInteger )age{
    self = [super init];

    if (self) {
        _name = name;
        _sex = sex;
        _age = age;
    }
    return self;
}

// 当一个对象 copy 时会自动调用到 copyWithZone: 方法
// NSZone 是用于记录原对象的内存结构信息
-(id)copyWithZone:(NSZone *)zone{

    // 如何实现协议中的方法
    // 根据原对象的内存结构信息创建一个新对象
    // 把原对象实例变量的内容复制给新对象
    Student *newStudent = [Student allocWithZone:zone];
    newStudent.name = self.name;
    newStudent.sex = self.sex;
    newStudent.age = self.age;

    return newStudent;
}

@end

// - main.m

#import <Foundation/Foundation.h>
#import "Student.h"

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

    Student *stu01 = [[Student alloc] initWithName:@"Bruce" sex:@"male" age:17];
    NSLog(@"%@===%@ %@ %ld",stu01,stu01.name,stu01.sex,stu01.age);// <Student: 0x101dafb80>===Bruce male 17
    NSLog(@"%lu",[stu01 retainCount]);// 1

    // copy 会拷贝出一份新的对象
    Student *newstudent = [stu01 copy];
    // newstudent 和 stu01 地址不一样
    NSLog(@"%@===%@ %@ %ld",newstudent,newstudent.name,newstudent.sex,newstudent.age);//  <Student: 0x104904320> Bruce male 17
    NSLog(@"stu04---%lu  newstudent---%lu ",[stu01 retainCount],[newstudent retainCount]);// stu01---1  newstudent---1

    Student *newstudentII = [newstudent copy];
    NSLog(@"newstudent---%lu  newstudentII---%lu ",[newstudent retainCount],[newstudentII retainCount]);//  newstudent---1  newstudentII---1

    return 0;
}

小结

1 - copy 修饰的属性仅限于 NSString、NSArray、NSDictionary，也是苹果推荐的使用方式

2 - 对于 NSMutableArray、NSMutableString、NSMutableDictionary 等可变因素一旦 copy 后就会生成一个不可变的对象，这在实际开发中是我们不想看到的

// - Person.h

#import <Foundation/Foundation.h>
@interface Person : NSObject
@property(copy,nonatomic)NSMutableArray *mutArray;

@end

// - main.m

#import <Foundation/Foundation.h>
#import "Person.h"
int main(int argc, const char * argv[]) {

    Person *p = [Person new];
    
    NSMutableArray *array01 = [NSMutableArray arrayWithCapacity:1];
    p.mutArray = array01;
    [p.mutArray addObject:@"5"];// crash
    
    // 因为属性 mutArray 是 copy 修饰的
    // 经过点语法赋值之后 p.mutArray 拿到的是新拷贝出的不可变数组
    // 不可变数组调用了一个并不存在的方法 addObject，故 crash

    return 0;

}