Objective - C code analysis of the deep and shallow copy

ocThis language belongs to the oddest of odds,Objects of basic types are divided into“可变”类型与“不可变”类型.

Such as the following types

Mutable types can be seen as“变量”,Immutable types can be thought of as“常量”.当然,Only superficially similar.

两种类型都是NSObject的子类,都实现了NSObject中的mutableCopy与copy方法.

about these two methods,Apple是这样介绍的：

在oc中 copy和mutableCopyTwo methods are used by all objects(继承自NSObject的类)继承的,These two methods are forcopy准备的.其中,mutableCopyis to create mutable types of primitive objectscopy.这两个方法分别调用copyWithZone和mutableCopyWithZoneTwo ways to do itcopy.A class must implementcopyWithZone或者mutableCopyWithZone,才能进行copy或者mutableCopy.

两种类型（可变、不可变）,两种方法（mutableCopy、copy）,There are four combinations：

• 可变类型调用copy
• 可变类型调用mutableCopy
• 不可变类型调用copy
• 不可变类型调用mutableCopy

These four combinations derive two concepts,浅拷贝与深拷贝：

• 浅copy: 指针复制,A new object will not be created.
• 深copy: 内容复制,会创建一个新的对象.

concept space,用代码来理解：

#import <Foundation/Foundation.h>

int main(int args, const char *argv[]) {
    
    @autoreleasepool {
    
        // 可变对象调用copy,mutableCopy
        NSMutableString *mutableString1 = [[NSMutableString alloc] init];
        [mutableString1 setString:@"hello1"];
        id s1 = mutableString1.copy;
        id s2 = mutableString1.mutableCopy;
        NSLog(@"可变对象：%p %@", mutableString1,mutableString1.class);
        NSLog(@"调用copy：%p %@", s1, [s1 class]);
        NSLog(@"调用mutableCopy：%p %@ \n\n", s2, [s2 class]);
        // 可变对象调用copy,mutableCopy
        NSString *immutableString1 = @"hello2";
        id s3 = immutableString1.copy;
        id s4 = immutableString1.mutableCopy;
        NSLog(@"不可变对象：%p %@", immutableString1,immutableString1.class);
        NSLog(@"调用copy：%p %@", s3, [s3 class]);
        NSLog(@"调用mutableCopy：%p %@", s4, [s4 class]);
    }
    return 0;
}

以第一部分“可变对象调用copy,mutableCopy”为例

Create a mutable object first,Then set a value for it“hello1”

NSMutableString *mutableString1 = [[NSMutableString alloc] init];
[mutableString1 setString:@"hello1"];

因为我们不知道copy与mutableCopyWhat type of object is returned,So we use twoid变量来获取

id s1 = mutableString1.copy;
id s2 = mutableString1.mutableCopy;

%pThe value of the pointer can be printed out,[s1 class]Can know which class it is,So the original variable can be printed out、copy对象和mutableCopyThe memory address and type of the object,如下.

NSLog(@"可变对象：%p %@", mutableString1,mutableString1.class);
NSLog(@"调用copy：%p %@", s1, [s1 class]);
NSLog(@"调用mutableCopy：%p %@ \n\n", s2, [s2 class]);

打印结果如下(Omit irrelevant parts）：

可变对象：0x600003058ba0  __NSCFString
调用copy：0xaf10ee324c733912  NSTaggedPointerString
调用mutableCopy：0x600003058bd0  __NSCFString 

可以看出,All three memory addresses are different,Indicates that a new object was created,因此

Mutable type callscopy与mutableCopy都是深拷贝

Take another look at the code for the immutable part

NSString *immutableString1 = @"hello2";
id s3 = immutableString1.copy;
id s4 = immutableString1.mutableCopy;
NSLog(@"不可变对象：%p %@", immutableString1,immutableString1.class);
NSLog(@"调用copy：%p %@", s3, [s3 class]);
NSLog(@"调用mutableCopy：%p %@", s4, [s4 class]);

Basically similar to the previous code,直接看输出：

不可变对象：0x100ce40e8  __NSCFConstantString
调用copy：0x100ce40e8  __NSCFConstantString
调用mutableCopy：0x600001980cc0  __NSCFString

可以看出：

1. 不可变对象调用copy：浅拷贝
2. 不可变对象调用mutableCopy：深拷贝

一句话总结：

Only immutable objects are calledcopy是浅拷贝,其他都是深拷贝

有关copy的深度长文：https://www.jianshu.com/p/5f776a4816ee