Introduction to deep copy, shallow copy, custom object copy of IOS

The purpose of copy syntax: When changing a copy, it does not affect the source object;

Deep copy: A copy of the content that produces a new object. The new object counter is set to 1, and the source object counter is unchanged.

Shallow copy: Pointer copy, does not produce new objects. Source Object Counter +1.

The copy has the following two methods to implement the copy:

-(ID) copy;  

object to implement copy, must implement <NSCopying> protocol
arrays, dictionaries, and strings have already implemented the <NSCopying> protocol, with the following string as an example:

1. Non-variable string call copy implementation (shallow copy)

NSString *string = [[NSString alloc] initwithformat:@ "ABCDE"];  Copy produces an immutable copy, because the source object itself is immutable, so for the sake of performance, copy will directly return the source object itself//  source Object counter will be +1  //In a shallow copy case, copy is actually equivalent to retain  NSString *STR = [string copy];  

2. Immutable string calls Mutablecopy implementation copy, (deep copy)

NSString *string = [[NSString alloc] initwithformat:@ "ABCD"];  A new object is generated with a counter of 1. The counter for the source object does not change.  nsmutablestring *str = [string mutablecopy];//There are two objects at this time//str:1 and//string:1   //STR and string not the same object   //NSLog (@ "%i", str = = string);//0   [str appendstring:@ "ABCD"];//modify STR does not affect string   NSLog (@ "string:%@", string);   NSLog (@ "str:%@", str);  

3. Variable string call copy implementation copy (deep copy)

nsmutablestring *string = [nsmutablestring stringwithformat:@ "Age is%i", ten];   A new object is generated and the str counter is 1   nsstring *str = [string copy];  

4. mutablecopy(deep copy) of variable strings

nsmutablestring *string = [nsmutablestring stringwithformat:@ "Age is%i", ten];  A new object is generated, and the STR counter is 1  nsmutablestring *str = [string mutablecopy];  [Str appendstring:@ "1234"];//modify the new object without affecting the original object  NSLog (@ "str:%@", str);  NSLog (@ "string:%@", string);  

5. Copy the custom object, with the student object as an example

A.student to implement copy, the <NSCopying> protocol must be implemented

B. Ways to implement the <NSCopying> Agreement:

-(ID) Copywithzone: (nszone *) Zone

Student.h file

@interface student:nsobject <NSCopying>    //Copy represents the Set method will release the old object, copy new object  //Modify the outside variables, and does not affect the internal member variable  //Recommendation: NSString General use copy strategy, other objects generally with retain  @property (nonatomic, copy) NSString *name;    + (ID) studentwithname: (NSString *) name;    @end  

STUDENT.M file

#import "Student.h"    @implementation Student    + (ID) studentwithname: (NSString *) name {      //Here it is best to write [self class]      Student *stu = [[[Self class] alloc] init] autorelease];      Stu.name = name;            return stu;  }    -(void) dealloc {      [_name release];            [Super Dealloc];  }    #pragma mark Description method cannot print self internally, otherwise it will cause a dead loop  -(NSString *) Description {      return [NSString stringWithFormat : @ "[name=%@]", _name];  }    #pragma mark Copying Protocol method  //The replica object created here does not require a release  -(ID) Copywithzone: (Nszone *) zone {      Student *copy = [[[Self Class] allocwithzone:zone] init];            Copy the name to the Copy object      copy.name = self.name;            return copy;  }    @end  

Copy Student

Student *STU1 = [Student studentwithname:@ "STU1"];    Student *STU2 = [stu1 copy];  Stu2.name = @ "STU2";    NSLog (@ "stu1:%@", STU1);  NSLog (@ "stu2:%@", STU2);    [STU2 release];  

Summary:

Recommendation: NSString general use copy strategy, other objects generally use retain;

Only one case is a shallow copy: When the immutable object calls the Copy method, the other case is a deep copy;

Introduction to deep copy, shallow copy, custom object copy of IOS