OC Details-1. Deep copy and shallow copy detailed

Overview

• Copy: Copy an object with the same content as the source object

• The following two protocols are required to implement a copy

  • Nscopying
  • Nsmutablecopying

• Copy returns the type of object

  • Variable, mutablecopy the object returned by the message
  • Immutable, the object returned by the copy message

• Types of copies

  • Shallow copy, just copy a pointer to the source object, no object created, no memory allocated
  • Deep copy, copy source object, create new object, allocate memory

• Attention

  • The system has different memory handling for the container class object and the non-container class object, that is, when an object that is not strongly referenced by another object is removed from the container, the object is destroyed

Copy and Retain

• Copy

  • is to create a new object, a copy of the content
  • Copy represents the same contents as two objects, and the reference count for the new object is 1
  • Irrelevant to the reference count of the old object, there is no change to the object
  • Copy reduces the object-to-context

• Retain

  • A pointer is created, a copy of the pointer
  • Same object address, same content
  • The reference count of the object +1

Copy behavior of different objects

• Non-container object ( 如NSString ))

  • For non-mutable objects

    • Rules

      • Copy, shallow copy (pointer copy)
      • Mutablecopy, deep Copy (object copy), return object variable (produce new Mutable object)

    • Example

      - (void) imutableinstancecopy{nsstring *string = @ "Welcome to Xcode"; //copy, Shallow copy nsstring *stringcopy = [string copy]; //mutablecopy, the returned object is variable nsmutablestring *stringmutablecopy = [string mutablecopy]; [Stringmutablecopy appendString:@ "!"]; //string is the same as Stringcopy's memory address NSLog (@ "string:%p", string); NSLog (@ "strongcopy:%p", stringcopy); //string and stringmutablecopy have different memory addresses, allocating a new memory NSLog (@ "stringmcopy:%p", Stringmutablecopy);}   

  • For mutable objects

    • Rules

      • Copy, deep copy (object copy), return object not mutable
      • Mutablecopy, deep Copy (object copy)

    • Example

      - (void) mutableinstancecopy{nsmutablestring *mutablestring = [Nsmutablestring stringwithstring:@ "Welcome to Xcode"];Deep copy, return object not variableNSString *stringcopy = [mutablestringCopy];nsmutablestring *mutablestringcopy = [mutablestringcopy]; //run, this sentence will be an error message, "Attempt to mutate immutable object with appendString:" [mutablestringcopy Appendstring:@ "~ ~ ~"]; //deep copy, return object variable nsmutablestring *stringmutablecopy = [ Mutablestring Mutablecopy]; [Stringmutablecopy appendstring:@ "!"]; //three and mutablestring memory address are different nslog ( @ "mutablestring:%p", mutablestring); nslog (@ "string:%p", stringcopy); nslog (@ "mutablestringcopy:%p", mutablestringcopy); nslog (@ "stringmutblecopy:%p", Stringmutablecopy);}    

• Container Object ( NSArray )

  • Follow the copy principle for non-container objects

  • Attention

    • The element inside the container is the pointer assignment (shallow copy)

    • Example

      - (void) containerinstanceshallowcopy{Nsarray *array = [Nsarray arraywithobjects:[Nsmutablestring stringwithstring:@ "Welcome"],@ "to",@ "Xcode",NIL];Shallow copyNsarray *arraycopy = [arrayCopy];Deep copyNsmutablearray *arraymutablecopy = [array mutablecopy];NSLog (@ "Array:%p", array);NSLog (@ "arrayCopy:%p", arrayCopy); NSLog (@ "arraymutablecopy:%p", arraymutablecopy); the objects in the container are shallow copies, that is, they have only one copy in memory nsmutablestring *teststring = [array Objectatindex:0]; [teststring appendString:@ "You"]; //Three array contents change NSLog at the same time (@ "array[0]:%@", array[0]); NSLog (@ "arraycopy[0]:%@", arraycopy[0]); NSLog (@ "arraymutablecopy[0]:%@", arraymutablecopy[0]);}        

  • Achieve deep replication in real sense

    - (void) containerinstancedeepcopy{Nsarray *array = [Nsarray arraywithobjects:[Nsmutablestring stringwithstring:@ "Welcome"],@ "to",@ "Xcode",NIL];Object in array is pointer copyNsarray *deepcopyarray = [[Nsarray alloc] Initwitharray:array];True above the deep copy, the object in the array is the object copyNsarray *truedeepcopyarray = [Nskeyedunarchiver unarchiveobjectwithdata:[Nskeyedarchiver Archiveddatawithrootobject:array]];NSLog (@ "Array:%p", array);NSLog (@ "Deepcopyarray:%p", Deepcopyarray); NSLog (@ "Truedeepcopyarray:%p", Truedeepcopyarray); //Change the first element of an array [[[Array Objectatindex:0] appendString:@ "You"]; //affects only the first element of the Deepcopyarray array NSLog (@ "array[0]:%@", array[0]); NSLog (@ "arraycopy[0]:%@", deepcopyarray[0]); //Does not affect the first element of the Truedeepcopyarray array, is the true deep copy NSLog (@ "arraymutablecopy[0]:%@", truedeepcopyarray[ 0]);}                 

• Custom Objects

  • In defining objects to implement a copy, you need to comply with the nscoping and nsmutablecoping protocols and implement the following methods

    • -(ID) Copywithzone: (Nszone *) zone, variable copy
    • -(ID) Mutablecopywithzone: (Nszone *) zone, non-variable copy

  • Example (copy of custom object person)

    • Compliance protocol, setting member properties

      @interface Person : NSObject <NSCopying, NSMutableCopying>/**姓名*/@property (nonatomic, copy) NSMutableString *name;/**地址*/@property (nonatomic, copy) NSString *address;/**年龄*/@property (nonatomic, assign) NSInteger age;@end

    • Overriding initialization method

       -(instancetype) init{if (self = [super  Init]) {self.name = [[nsmutablestring Alloc] initwithstring:@< Span class= "hljs-string" > "Xiaoyaowang"]; self.address = @ "empty mountain Xinyu after"; self.age = 3;} return self;}        

    • Implementation-(ID) Copywithzone: (Nszone *) zone

       -(id) Copywithzone: (    Nszone *) zone{person *p = [[[self class] allocwithzone:zone] init]; P.name = [self.name copy]; P.address = [self.address Span class= "Hljs-keyword" >copy]; P.age = self.age; return p;}              

    • Implementation-(ID) Mutablecopywithzone: (Nszone *) zone

      - (id)mutableCopyWithZone:(NSZone *)zone{    Person *p = [[[self class] allocWithZone:zone] init];    //注意，此处是mutableCopy方法    p.name = [self.name mutableCopy]; p.address = [self.address copy]; p.age = self.age; return p;}

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OC Details-1. Deep copy and shallow copy detailed