What is the difference between a deep copy and a shallow copy

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In short  it depends on what points to what  In a shallow copy  object B points to object A s location in memory  In deep copy  all things in object A s memory location get copied to object B s memory location   This wiki article has a great diagram   http   en wikipedia org wiki Object copy

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To add just a little more for confusion between shallow copy and simply assign a new variable name to list    Say we have   x          1 2 3        4 5 6           This statement creates 3 lists  2 inner lists and one outer list  A reference to the outer list is then made available under the name x  If we do  y   x   no data gets copied  We still have the same 3 lists in memory somewhere  All this did is make the outer list available under the name y  in addition to its previous name x  If we do  y   list x    or  y   x      This creates a new list with the same contents as x  List x contained a reference to the 2 inner lists  so the new list will also contain a reference to those same 2 inner lists  Only one list is copied   the outer list  Now there are 4 lists in memory  the two inner lists  the outer list  and the copy of the outer list  The original outer list is available under the name x  and the new outer list is made available under the name y   The inner lists have not been copied  You can access and edit the inner lists from either x or y at this point   If you have a two dimensional  or higher  list  or any kind of nested data structure  and you want to make a full copy of everything  then you want to use the deepcopy   function in the copy module  Your solution also works for 2-D lists  as iterates over the items in the outer list and makes a copy of each of them  then builds a new outer list for all the inner copies    source  https   www reddit com r learnpython comments 1afldr why is copying a list so damn difficult in python

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Copying ararys    Array is a class  which means it is reference type so array1   array2 results in two variables that reference the same array   But look at this example     static void Main                 int   arr1   new int     1  2  3  4  5             int   arr2   new int     6  7  8  9  0             Console WriteLine arr1 2          arr2 2            arr2   arr1          Console WriteLine arr1 2          arr2 2             arr2    int   arr1 Clone            arr1 2    12          Console WriteLine arr1 2          arr2 2            shallow clone means that only the memory represented by the cloned array is copied     If  the  array  contains  value  type  objects   the  values are copied    if the array contains reference type  only the references are copied - so as a result there are two arrays whose members reference the same objects    To create a deep copy   where reference type are duplicated  you must loop through the array and clone each element manually

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In short  it depends on what points to what  In a shallow copy  object B points to object A s location in memory  In deep copy  all things in object A s memory location get copied to object B s memory location   This wiki article has a great diagram   http   en wikipedia org wiki Object copy

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Shallow Cloning  Definition   A shallow copy of an object copies the    main    object  but doesn   t copy the inner objects    When a custom object  eg  Employee  has just primitive  String type variables then you use Shallow Cloning    Employee e   new Employee 2   john cena    Employee e2 e clone      You return super clone    in the overridden clone   method and your job is over   Deep Cloning  Definition   Unlike the shallow copy  a deep copy is a fully independent copy of an object   Means when an Employee object holds another custom object     Employee e   new Employee 2   john cena   new Address 12   West Newbury    Massachusetts      Then you have to write the code to clone the  Address  object as well in the overridden clone   method  Otherwise the Address object won t clone and it causes a bug when you change value of Address in cloned Employee object  which reflects the original one too

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Shallow copy  nbsp Copies the member values from one object into another   Deep Copy  nbsp  nbsp  nbsp  nbsp Copies the member values from one object into another   nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp Any pointer objects are duplicated and Deep Copied   Example   class String        int   size       char  data      String  s1  Ace         s1 size   3 s1 data 0x0000F000  String  s2   shallowCopy s1       s2 size  3 s2 data   0X0000F000 String  s3   deepCopy s1       s3 size  3 s3 data   0x0000F00F                           With Ace copied to this location

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Shallow copies duplicate as little as possible   A shallow copy of a collection is a copy of the collection structure  not the elements   With a shallow copy  two collections now share the individual elements   Deep copies duplicate everything   A deep copy of a collection is two collections with all of the elements in the original collection duplicated

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Shallow Cloning  Definition   A shallow copy of an object copies the    main    object  but doesn   t copy the inner objects    When a custom object  eg  Employee  has just primitive  String type variables then you use Shallow Cloning    Employee e   new Employee 2   john cena    Employee e2 e clone      You return super clone    in the overridden clone   method and your job is over   Deep Cloning  Definition   Unlike the shallow copy  a deep copy is a fully independent copy of an object   Means when an Employee object holds another custom object     Employee e   new Employee 2   john cena   new Address 12   West Newbury    Massachusetts      Then you have to write the code to clone the  Address  object as well in the overridden clone   method  Otherwise the Address object won t clone and it causes a bug when you change value of Address in cloned Employee object  which reflects the original one too

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Taken from  blog   http   sickprogrammersarea blogspot in 2014 03 technical-interview-questions-on-c 6 html  Deep copy involves using the contents of one object to create another instance of the same class  In a deep copy  the two objects may contain ht same information but the target object will have its own buffers and resources  the destruction of either object will not affect the remaining object  The overloaded assignment operator would create a deep copy of objects   Shallow copy involves copying the contents of one object into another instance of the same class thus creating a mirror image  Owing to straight copying of references and pointers  the two objects will share the same externally contained contents of the other object to be unpredictable   Explanation   Using a copy constructor we simply copy the data values member by member  This method of copying is called shallow copy  If the object is a simple class  comprised of built in types and no pointers this would be acceptable  This function would use the values and the objects and its behavior would not be altered with a shallow copy  only the addresses of pointers that are members are copied and not the value the address is pointing to  The data values of the object would then be inadvertently altered by the function  When the function goes out of scope  the copy of the object with all its data is popped off the stack   If the object has any pointers a deep copy needs to be executed  With the deep copy of an object  memory is allocated for the object in free store and the elements pointed to are copied  A deep copy is used for objects that are returned from a function

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Shallow copies duplicate as little as possible   A shallow copy of a collection is a copy of the collection structure  not the elements   With a shallow copy  two collections now share the individual elements   Deep copies duplicate everything   A deep copy of a collection is two collections with all of the elements in the original collection duplicated

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Shallow copy  nbsp Copies the member values from one object into another   Deep Copy  nbsp  nbsp  nbsp  nbsp Copies the member values from one object into another   nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp Any pointer objects are duplicated and Deep Copied   Example   class String        int   size       char  data      String  s1  Ace         s1 size   3 s1 data 0x0000F000  String  s2   shallowCopy s1       s2 size  3 s2 data   0X0000F000 String  s3   deepCopy s1       s3 size  3 s3 data   0x0000F00F                           With Ace copied to this location

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In short  it depends on what points to what  In a shallow copy  object B points to object A s location in memory  In deep copy  all things in object A s memory location get copied to object B s memory location   This wiki article has a great diagram   http   en wikipedia org wiki Object copy

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struct sample       char   ptr    void shallowcpy sample  amp  dest  sample  amp  src        dest ptr src ptr    void deepcpy sample  amp  dest  sample  amp  src        dest ptr malloc strlen src ptr  1       memcpy dest ptr src ptr

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Shallow copies duplicate as little as possible   A shallow copy of a collection is a copy of the collection structure  not the elements   With a shallow copy  two collections now share the individual elements   Deep copies duplicate everything   A deep copy of a collection is two collections with all of the elements in the original collection duplicated

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In object oriented programming  a type includes a collection of member fields  These fields may be stored either by value or by reference  i e   a pointer to a value    In a shallow copy  a new instance of the type is created and the values are copied into the new instance  The reference pointers are also copied just like the values  Therefore  the references are pointing to the original objects  Any changes to the members that are stored by reference appear in both the original and the copy  since no copy was made of the referenced object   In a deep copy  the fields that are stored by value are copied as before  but the pointers to objects stored by reference are not copied  Instead  a deep copy is made of the referenced object  and a pointer to the new object is stored  Any changes that are made to those referenced objects will not affect other copies of the object

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Shallow copy  nbsp Copies the member values from one object into another   Deep Copy  nbsp  nbsp  nbsp  nbsp Copies the member values from one object into another   nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp Any pointer objects are duplicated and Deep Copied   Example   class String        int   size       char  data      String  s1  Ace         s1 size   3 s1 data 0x0000F000  String  s2   shallowCopy s1       s2 size  3 s2 data   0X0000F000 String  s3   deepCopy s1       s3 size  3 s3 data   0x0000F00F                           With Ace copied to this location

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var source     firstName  Jane   lastname  Jones     var shallow   ShallowCopyOf source   var deep   DeepCopyOf source   source lastName    Smith   WriteLine source lastName      prints Smith WriteLine shallow lastName      prints Smith WriteLine deep lastName      prints Jones

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Just for the sake of easy understanding you could follow this article  https   www cs utexas edu  scottm cs307 handouts deepCopying htm    Shallow Copy        Deep Copy

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Shallow copy will not create new reference but deep copy will create the new reference   Here is the program to explain the deep and shallow copy   public class DeepAndShollowCopy       int id      String name      List lt String gt  testlist   new ArrayList lt  gt                    To performing Shallow Copy         Note  Here we are not creating any references         public DeepAndShollowCopy int id  String name  List lt String gt testlist                    System out println  Shallow Copy for Object initialization           this id   id          this name   name          this testlist   testlist                              To performing Deep Copy         Note  Here we are creating one references  Al arraylist object         public DeepAndShollowCopy int id  String name  List lt String gt  testlist            System out println  Deep Copy for Object initialization            this id   id          this name   name          String item          List lt String gt  Al   new ArrayList lt  gt             Iterator lt String gt  itr   testlist iterator            while  itr hasNext                  item   itr next                Al add item                     this testlist   Al              public static void main String   args            List lt String gt  list   new ArrayList lt  gt             list add  Java            list add  Oracle            list add  C              DeepAndShollowCopy copy new DeepAndShollowCopy 10  Testing   list           System out println copy toString                Override     public String toString             return  DeepAndShollowCopy  id     id      name     name      testlist     testlist

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Breadth vs Depth  think in terms of a tree of references with your object as the root node  Shallow     The variables A and B refer to different areas of memory  when B is assigned to A the two variables refer to the same area of memory  Later modifications to the contents of either are instantly reflected in the contents of other  as they share contents  Deep     The variables A and B refer to different areas of memory  when B is assigned to A the values in the memory area which A points to are copied into the memory area to which B points  Later modifications to the contents of either remain unique to A or B  the contents are not shared

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Deep Copy  A deep copy copies all fields  and makes copies of dynamically allocated memory pointed to by the fields  A deep copy occurs when an object is copied along with the objects to which it refers    Shallow Copy  Shallow copy is a bit-wise copy of an object  A new object is created that has an exact copy of the values in the original object  If any of the fields of the object are references to other objects  just the reference addresses are copied i e   only the memory address is copied

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Breadth vs Depth  think in terms of a tree of references with your object as the root node  Shallow     The variables A and B refer to different areas of memory  when B is assigned to A the two variables refer to the same area of memory  Later modifications to the contents of either are instantly reflected in the contents of other  as they share contents  Deep     The variables A and B refer to different areas of memory  when B is assigned to A the values in the memory area which A points to are copied into the memory area to which B points  Later modifications to the contents of either remain unique to A or B  the contents are not shared

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Especially For iOS Developers    If B is a shallow copy of A  then for primitive data it s like B    A assign   and for objects it s like B    A retain    B and A point to the same memory location  If B is a deep copy of A  then it is like B    A copy    B and A point to different memory locations  B memory address is same as A s  B has same contents as A s

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I came to understand from the following lines   Shallow copy copies an object value type int  float  bool  fields in to target object and object s reference types string  class etc  are copied as references in target object  In this target reference types will be pointing to the memory location of source object   Deep copy copies an object s value and reference types into a complete new copy of the target objects  This means both the value types and reference types will be allocated a new memory locations

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char   Source    Hello  world     char   ShallowCopy   Source       char   DeepCopy   new char strlen Source  1   strcpy DeepCopy Source              ShallowCopy  points to the same location in memory as  Source  does   DeepCopy  points to a different location in memory  but the contents are the same

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In object oriented programming  a type includes a collection of member fields  These fields may be stored either by value or by reference  i e   a pointer to a value    In a shallow copy  a new instance of the type is created and the values are copied into the new instance  The reference pointers are also copied just like the values  Therefore  the references are pointing to the original objects  Any changes to the members that are stored by reference appear in both the original and the copy  since no copy was made of the referenced object   In a deep copy  the fields that are stored by value are copied as before  but the pointers to objects stored by reference are not copied  Instead  a deep copy is made of the referenced object  and a pointer to the new object is stored  Any changes that are made to those referenced objects will not affect other copies of the object

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A shallow copy constructs a new compound object and insert its references into it to the original object   Unlike shallow copy  deepcopy constructs new compound object and also inserts copies of the original objects of original compound object   Lets take an example   import copy x   1  2   y copy copy x  z  copy deepcopy x  print y is z    Above code prints FALSE   Let see how   Original compound object x  1  2    called as compound because it has object inside object  Inception      as you can see in the image  there is a list inside list   Then we create a shallow copy of it using y   copy copy x   What python does here is  it will create a new compound object but objects inside them are pointing to the orignal objects     In the image it has created a new copy for outer list  but the inner list remains same as the original one   Now we create deepcopy of it using z   copy deepcopy x   what python does here is  it will create new object for outer list as well as inner list  as shown in the image below  red highlighted      At the end code prints False  as y and z are not same objects   HTH

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Shallow copy will not create new reference but deep copy will create the new reference   Here is the program to explain the deep and shallow copy   public class DeepAndShollowCopy       int id      String name      List lt String gt  testlist   new ArrayList lt  gt                    To performing Shallow Copy         Note  Here we are not creating any references         public DeepAndShollowCopy int id  String name  List lt String gt testlist                    System out println  Shallow Copy for Object initialization           this id   id          this name   name          this testlist   testlist                              To performing Deep Copy         Note  Here we are creating one references  Al arraylist object         public DeepAndShollowCopy int id  String name  List lt String gt  testlist            System out println  Deep Copy for Object initialization            this id   id          this name   name          String item          List lt String gt  Al   new ArrayList lt  gt             Iterator lt String gt  itr   testlist iterator            while  itr hasNext                  item   itr next                Al add item                     this testlist   Al              public static void main String   args            List lt String gt  list   new ArrayList lt  gt             list add  Java            list add  Oracle            list add  C              DeepAndShollowCopy copy new DeepAndShollowCopy 10  Testing   list           System out println copy toString                Override     public String toString             return  DeepAndShollowCopy  id     id      name     name      testlist     testlist

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Shallow copying is creating a new object and then copying the non-static fields of the current object to the new object  If a field is a value type --  a bit-by-bit copy of the field is performed  for a reference type --  the reference is copied but the referred object is not  therefore the original object and its clone refer to the same object   Deep copy is creating a new object and then copying the nonstatic fields of the current object to the new object  If a field is a value type --  a bit-by-bit copy of the field is performed  If a field is a reference type --  a new copy of the referred object is performed  The classes to be cloned must be flagged as  Serializable

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To add more to other answers     a Shallow Copy of an object performs copy by value for value types based properties  and copy by reference for reference types based properties  a Deep Copy of an object performs copy by value for value types based properties  as well as copy by value for reference types based properties deep in the hierarchy  of reference types

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Imagine there are two arrays called arr1 and arr2    arr1   arr2      shallow copy arr1   arr2 clone      deep copy

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Adding to all the above definitions  one more and most commonly used deep copy  is in the copy constructor  or overloading assignment oprator  of the class   Shallow copy --  is when you are not providing copy constructor  Here  only the object gets copied but not all the members of the class are copied   Deep copy --  is when you have decided to implement copy constructor or overload assignment in your class and allows copying all the members of the class   MyClass amp  MyClass const MyClass amp  obj     copy constructor for MyClass                write your code  to copy all the members and return the new object   MyClass amp  operator  const MyClass amp  obj     overloading assignment operator                 write your code  to copy all the members and return the new object

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In Simple Terms  a Shallow Copy is similar to Call By Reference and a Deep Copy is similar to Call By Value  In Call By Reference  Both formal and actual parameters of a function refers to same memory location and the value   In Call By Value  Both formal and actual parameters of a functions refers to different memory location but having the same value

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I would like to give example rather than the formal definition   var originalObject          a   1       b   2       c   3       This code shows a shallow copy   var copyObject1   originalObject   console log copyObject1 a              it will print 1  console log originalObject a            it will also print 1  copyObject1 a   4   console log copyObject1 a               now it will print 4  console log originalObject a            now it will also print 4  var copyObject2   Object assign     originalObject    console log copyObject2 a             it will print 1  console log originalObject a           it will also print 1  copyObject2 a   4   console log copyObject2 a             now it will print 4  console log originalObject a           now it will print 1   This code shows a deep copy   var copyObject2   Object assign     originalObject    console log copyObject2 a             it will print 1  console log originalObject a           it will also print 1  copyObject2 a   4   console log copyObject2 a             now it will print 4  console log originalObject a              now it will print 1

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In short  it depends on what points to what  In a shallow copy  object B points to object A s location in memory  In deep copy  all things in object A s memory location get copied to object B s memory location   This wiki article has a great diagram   http   en wikipedia org wiki Object copy

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Copying ararys    Array is a class  which means it is reference type so array1   array2 results in two variables that reference the same array   But look at this example     static void Main                 int   arr1   new int     1  2  3  4  5             int   arr2   new int     6  7  8  9  0             Console WriteLine arr1 2          arr2 2            arr2   arr1          Console WriteLine arr1 2          arr2 2             arr2    int   arr1 Clone            arr1 2    12          Console WriteLine arr1 2          arr2 2            shallow clone means that only the memory represented by the cloned array is copied     If  the  array  contains  value  type  objects   the  values are copied    if the array contains reference type  only the references are copied - so as a result there are two arrays whose members reference the same objects    To create a deep copy   where reference type are duplicated  you must loop through the array and clone each element manually

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The copy constructor is used to initialize the new object with the previously created object of the same class  By default compiler wrote a shallow copy  Shallow copy works fine when dynamic memory allocation is not involved because when dynamic memory allocation is involved then both objects will points towards the same memory location in a heap  Therefore to remove this problem we wrote deep copy so both objects have their own copy of attributes in a memory  In order to read the details with complete examples and explanations you could see the article C   constructors

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I haven t seen a short  easy to understand answer here--so I ll give it a try   With a shallow copy  any object pointed to by the source is also pointed to by the destination  so that no referenced objects are copied    With a deep copy  any object pointed to by the source is copied and the copy is pointed to by the destination  so there will now be 2 of each referenced object    This recurses down the object tree

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ShallowCopy  points to the same location in memory as  Source  does   DeepCopy  points to a different location in memory  but the contents are the same

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char   Source    Hello  world     char   ShallowCopy   Source       char   DeepCopy   new char strlen Source  1   strcpy DeepCopy Source              ShallowCopy  points to the same location in memory as  Source  does   DeepCopy  points to a different location in memory  but the contents are the same

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In Simple Terms  a Shallow Copy is similar to Call By Reference and a Deep Copy is similar to Call By Value  In Call By Reference  Both formal and actual parameters of a function refers to same memory location and the value   In Call By Value  Both formal and actual parameters of a functions refers to different memory location but having the same value

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Just for the sake of easy understanding you could follow this article  https   www cs utexas edu  scottm cs307 handouts deepCopying htm    Shallow Copy        Deep Copy

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Try to consider following image    For example Object MemberwiseClone creates a shallow copy link  and using ICloneable interface you can get deep copy as described here

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I haven t seen a short  easy to understand answer here--so I ll give it a try   With a shallow copy  any object pointed to by the source is also pointed to by the destination  so that no referenced objects are copied    With a deep copy  any object pointed to by the source is copied and the copy is pointed to by the destination  so there will now be 2 of each referenced object    This recurses down the object tree

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I haven t seen a short  easy to understand answer here--so I ll give it a try   With a shallow copy  any object pointed to by the source is also pointed to by the destination  so that no referenced objects are copied    With a deep copy  any object pointed to by the source is copied and the copy is pointed to by the destination  so there will now be 2 of each referenced object    This recurses down the object tree

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Try to consider following image    For example Object MemberwiseClone creates a shallow copy link  and using ICloneable interface you can get deep copy as described here

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Shallow copy  nbsp Copies the member values from one object into another   Deep Copy  nbsp  nbsp  nbsp  nbsp Copies the member values from one object into another   nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp Any pointer objects are duplicated and Deep Copied   Example   class String        int   size       char  data      String  s1  Ace         s1 size   3 s1 data 0x0000F000  String  s2   shallowCopy s1       s2 size  3 s2 data   0X0000F000 String  s3   deepCopy s1       s3 size  3 s3 data   0x0000F00F                           With Ace copied to this location

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In object oriented programming  a type includes a collection of member fields  These fields may be stored either by value or by reference  i e   a pointer to a value    In a shallow copy  a new instance of the type is created and the values are copied into the new instance  The reference pointers are also copied just like the values  Therefore  the references are pointing to the original objects  Any changes to the members that are stored by reference appear in both the original and the copy  since no copy was made of the referenced object   In a deep copy  the fields that are stored by value are copied as before  but the pointers to objects stored by reference are not copied  Instead  a deep copy is made of the referenced object  and a pointer to the new object is stored  Any changes that are made to those referenced objects will not affect other copies of the object

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Adding to all the above definitions  one more and most commonly used deep copy  is in the copy constructor  or overloading assignment oprator  of the class   Shallow copy --  is when you are not providing copy constructor  Here  only the object gets copied but not all the members of the class are copied   Deep copy --  is when you have decided to implement copy constructor or overload assignment in your class and allows copying all the members of the class   MyClass amp  MyClass const MyClass amp  obj     copy constructor for MyClass                write your code  to copy all the members and return the new object   MyClass amp  operator  const MyClass amp  obj     overloading assignment operator                 write your code  to copy all the members and return the new object

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What is Shallow Copy   Shallow copy is a bit-wise copy of an object  A new object is created that has an exact copy of the values in the original object  If any of the fields of the object are references to other objects  only the reference addresses are copied i e   only the memory address is copied    In this figure  the MainObject1 has fields field1 of type int  and ContainObject1 of type ContainObject  When you do a shallow copy of MainObject1  MainObject2 is created with field2 containing the copied value of field1 and still pointing to ContainObject1 itself  Note that since field1 is of primitive type  its value is copied to field2 but since ContainedObject1 is an object  MainObject2 still points to ContainObject1  So any changes made to ContainObject1 in MainObject1 will be reflected in MainObject2   Now if this is shallow copy  lets see what s deep copy    What is Deep Copy   A deep copy copies all fields  and makes copies of dynamically allocated memory pointed to by the fields  A deep copy occurs when an object is copied along with the objects to which it refers     In this figure  the MainObject1 have fields field1 of type int  and ContainObject1 of type ContainObject  When you do a deep copy of MainObject1  MainObject2 is created with field2 containing the copied value of field1 and ContainObject2 containing the copied value of ContainObject1  Note any changes made to ContainObject1 in MainObject1 will not reflect in MainObject2    good article

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struct sample       char   ptr    void shallowcpy sample  amp  dest  sample  amp  src        dest ptr src ptr    void deepcpy sample  amp  dest  sample  amp  src        dest ptr malloc strlen src ptr  1       memcpy dest ptr src ptr

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char   Source    Hello  world     char   ShallowCopy   Source       char   DeepCopy   new char strlen Source  1   strcpy DeepCopy Source              ShallowCopy  points to the same location in memory as  Source  does   DeepCopy  points to a different location in memory  but the contents are the same

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In object oriented programming  a type includes a collection of member fields  These fields may be stored either by value or by reference  i e   a pointer to a value    In a shallow copy  a new instance of the type is created and the values are copied into the new instance  The reference pointers are also copied just like the values  Therefore  the references are pointing to the original objects  Any changes to the members that are stored by reference appear in both the original and the copy  since no copy was made of the referenced object   In a deep copy  the fields that are stored by value are copied as before  but the pointers to objects stored by reference are not copied  Instead  a deep copy is made of the referenced object  and a pointer to the new object is stored  Any changes that are made to those referenced objects will not affect other copies of the object

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Breadth vs Depth  think in terms of a tree of references with your object as the root node  Shallow     The variables A and B refer to different areas of memory  when B is assigned to A the two variables refer to the same area of memory  Later modifications to the contents of either are instantly reflected in the contents of other  as they share contents  Deep     The variables A and B refer to different areas of memory  when B is assigned to A the values in the memory area which A points to are copied into the memory area to which B points  Later modifications to the contents of either remain unique to A or B  the contents are not shared

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I came to understand from the following lines   Shallow copy copies an object value type int  float  bool  fields in to target object and object s reference types string  class etc  are copied as references in target object  In this target reference types will be pointing to the memory location of source object   Deep copy copies an object s value and reference types into a complete new copy of the target objects  This means both the value types and reference types will be allocated a new memory locations

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I haven t seen a short  easy to understand answer here--so I ll give it a try   With a shallow copy  any object pointed to by the source is also pointed to by the destination  so that no referenced objects are copied    With a deep copy  any object pointed to by the source is copied and the copy is pointed to by the destination  so there will now be 2 of each referenced object    This recurses down the object tree

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Shallow copying is creating a new object and then copying the non-static fields of the current object to the new object  If a field is a value type --  a bit-by-bit copy of the field is performed  for a reference type --  the reference is copied but the referred object is not  therefore the original object and its clone refer to the same object   Deep copy is creating a new object and then copying the nonstatic fields of the current object to the new object  If a field is a value type --  a bit-by-bit copy of the field is performed  If a field is a reference type --  a new copy of the referred object is performed  The classes to be cloned must be flagged as  Serializable

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What is Shallow Copy   Shallow copy is a bit-wise copy of an object  A new object is created that has an exact copy of the values in the original object  If any of the fields of the object are references to other objects  only the reference addresses are copied i e   only the memory address is copied    In this figure  the MainObject1 has fields field1 of type int  and ContainObject1 of type ContainObject  When you do a shallow copy of MainObject1  MainObject2 is created with field2 containing the copied value of field1 and still pointing to ContainObject1 itself  Note that since field1 is of primitive type  its value is copied to field2 but since ContainedObject1 is an object  MainObject2 still points to ContainObject1  So any changes made to ContainObject1 in MainObject1 will be reflected in MainObject2   Now if this is shallow copy  lets see what s deep copy    What is Deep Copy   A deep copy copies all fields  and makes copies of dynamically allocated memory pointed to by the fields  A deep copy occurs when an object is copied along with the objects to which it refers     In this figure  the MainObject1 have fields field1 of type int  and ContainObject1 of type ContainObject  When you do a deep copy of MainObject1  MainObject2 is created with field2 containing the copied value of field1 and ContainObject2 containing the copied value of ContainObject1  Note any changes made to ContainObject1 in MainObject1 will not reflect in MainObject2    good article

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Shallow Copy- Reference variable inside original and shallow-copied objects have reference to common object  Deep Copy- Reference variable inside original and deep-copied objects have reference to different object   clone always does shallow copy   public class Language implements Cloneable           String name      public Language String name           this name name                 public String getName             return name                  Override     protected Object clone   throws CloneNotSupportedException           return super clone             main class is following- public static void main String args    throws ClassNotFoundException  CloneNotSupportedException         ArrayList lt Language gt  list new ArrayList lt Language gt           list add new Language  quot C quot           list add new Language  quot JAVA quot            ArrayList lt Language gt  shallow  ArrayList lt Language gt   list clone            We used here clone since this always shallow copied         System out println list  shallow                for int i 0 i lt list size   i          System out println list get i   shallow get i     true              ArrayList lt Language gt  deep new ArrayList lt Language gt           for Language language list             deep add  Language  language clone                   System out println list  deep         for int i 0 i lt list size   i              System out println list get i   deep get i     false            OutPut of above will be-  false true  true false false false  Any change made in origional object will reflect in shallow object not in deep object    list get 0  name  quot ViSuaLBaSiC quot     System out println shallow get 0  getName    quot    quot  deep get 0  getName      OutPut- ViSuaLBaSiC    C

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var source     firstName  Jane   lastname  Jones     var shallow   ShallowCopyOf source   var deep   DeepCopyOf source   source lastName    Smith   WriteLine source lastName      prints Smith WriteLine shallow lastName      prints Smith WriteLine deep lastName      prints Jones

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Shallow copies duplicate as little as possible   A shallow copy of a collection is a copy of the collection structure  not the elements   With a shallow copy  two collections now share the individual elements   Deep copies duplicate everything   A deep copy of a collection is two collections with all of the elements in the original collection duplicated

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Imagine two objects  A and B of same type  t with respect to C    and you are thinking about shallow deep copying A to B   Shallow Copy  Simply makes a copy of the reference to A into B  Think about it as a copy of A s Address  So  the addresses of A and B will be the same i e  they will be pointing to the same memory location i e  data contents   Deep copy  Simply makes a copy of all the members of A  allocates memory in a different location for B and then assigns the copied members to B to achieve deep copy  In this way  if A becomes non-existant B is still valid in the memory  The correct term to use would be cloning  where you know that they both are totally the same  but yet different  i e  stored as two different entities in the memory space   You can also provide your clone wrapper where you can decide via inclusion exclusion list which properties to select during deep copy  This is quite a common practice when you create APIs   You can choose to do a Shallow Copy ONLY IF you understand the stakes involved  When you have enormous number of pointers to deal with in C   or C  doing a shallow copy of an object is REALLY a bad idea    EXAMPLE OF DEEP COPY  An example is  when you are trying to do image processing and object recognition you need to mask  Irrelevant and Repetitive Motion  out of your processing areas  If you are using image pointers  then you might have the specification to save those mask images  NOW    if you do a shallow copy of the image  when the pointer references are KILLED from the stack  you lost the reference and its copy i e  there will be a runtime error of access violation at some point  In this case  what you need is a deep copy of your image by CLONING it  In this way you can retrieve the masks in case you need them in the future   EXAMPLE OF SHALLOW COPY I am not extremely knowledgeable compared to the users in StackOverflow so feel free to delete this part and put a good example if you can clarify  But I really think it is not a good idea to do shallow copy if you know that your program is gonna run for an infinite period of time i e  continuous  push-pop  operation over the stack with function calls  If you are demonstrating something to an amateur or novice person  e g  C C   tutorial stuff  then it is probably okay  But if you are running an application such as surveillance and detection system  or Sonar Tracking System  you are not supposed to keep shallow copying your objects around because it will kill your program sooner or later

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Imagine there are two arrays called arr1 and arr2    arr1   arr2      shallow copy arr1   arr2 clone      deep copy

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Deep Copy  A deep copy copies all fields  and makes copies of dynamically allocated memory pointed to by the fields  A deep copy occurs when an object is copied along with the objects to which it refers    Shallow Copy  Shallow copy is a bit-wise copy of an object  A new object is created that has an exact copy of the values in the original object  If any of the fields of the object are references to other objects  just the reference addresses are copied i e   only the memory address is copied

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Breadth vs Depth  think in terms of a tree of references with your object as the root node  Shallow     The variables A and B refer to different areas of memory  when B is assigned to A the two variables refer to the same area of memory  Later modifications to the contents of either are instantly reflected in the contents of other  as they share contents  Deep     The variables A and B refer to different areas of memory  when B is assigned to A the values in the memory area which A points to are copied into the memory area to which B points  Later modifications to the contents of either remain unique to A or B  the contents are not shared

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var source     firstName  Jane   lastname  Jones     var shallow   ShallowCopyOf source   var deep   DeepCopyOf source   source lastName    Smith   WriteLine source lastName      prints Smith WriteLine shallow lastName      prints Smith WriteLine deep lastName      prints Jones

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The copy constructor is used to initialize the new object with the previously created object of the same class  By default compiler wrote a shallow copy  Shallow copy works fine when dynamic memory allocation is not involved because when dynamic memory allocation is involved then both objects will points towards the same memory location in a heap  Therefore to remove this problem we wrote deep copy so both objects have their own copy of attributes in a memory  In order to read the details with complete examples and explanations you could see the article C   constructors

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To add just a little more for confusion between shallow copy and simply assign a new variable name to list    Say we have   x          1 2 3        4 5 6           This statement creates 3 lists  2 inner lists and one outer list  A reference to the outer list is then made available under the name x  If we do  y   x   no data gets copied  We still have the same 3 lists in memory somewhere  All this did is make the outer list available under the name y  in addition to its previous name x  If we do  y   list x    or  y   x      This creates a new list with the same contents as x  List x contained a reference to the 2 inner lists  so the new list will also contain a reference to those same 2 inner lists  Only one list is copied   the outer list  Now there are 4 lists in memory  the two inner lists  the outer list  and the copy of the outer list  The original outer list is available under the name x  and the new outer list is made available under the name y   The inner lists have not been copied  You can access and edit the inner lists from either x or y at this point   If you have a two dimensional  or higher  list  or any kind of nested data structure  and you want to make a full copy of everything  then you want to use the deepcopy   function in the copy module  Your solution also works for 2-D lists  as iterates over the items in the outer list and makes a copy of each of them  then builds a new outer list for all the inner copies    source  https   www reddit com r learnpython comments 1afldr why is copying a list so damn difficult in python

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I would like to give example rather than the formal definition   var originalObject          a   1       b   2       c   3       This code shows a shallow copy   var copyObject1   originalObject   console log copyObject1 a              it will print 1  console log originalObject a            it will also print 1  copyObject1 a   4   console log copyObject1 a               now it will print 4  console log originalObject a            now it will also print 4  var copyObject2   Object assign     originalObject    console log copyObject2 a             it will print 1  console log originalObject a           it will also print 1  copyObject2 a   4   console log copyObject2 a             now it will print 4  console log originalObject a           now it will print 1   This code shows a deep copy   var copyObject2   Object assign     originalObject    console log copyObject2 a             it will print 1  console log originalObject a           it will also print 1  copyObject2 a   4   console log copyObject2 a             now it will print 4  console log originalObject a              now it will print 1

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A shallow copy constructs a new compound object and insert its references into it to the original object   Unlike shallow copy  deepcopy constructs new compound object and also inserts copies of the original objects of original compound object   Lets take an example   import copy x   1  2   y copy copy x  z  copy deepcopy x  print y is z    Above code prints FALSE   Let see how   Original compound object x  1  2    called as compound because it has object inside object  Inception      as you can see in the image  there is a list inside list   Then we create a shallow copy of it using y   copy copy x   What python does here is  it will create a new compound object but objects inside them are pointing to the orignal objects     In the image it has created a new copy for outer list  but the inner list remains same as the original one   Now we create deepcopy of it using z   copy deepcopy x   what python does here is  it will create new object for outer list as well as inner list  as shown in the image below  red highlighted      At the end code prints False  as y and z are not same objects   HTH

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To add more to other answers     a Shallow Copy of an object performs copy by value for value types based properties  and copy by reference for reference types based properties  a Deep Copy of an object performs copy by value for value types based properties  as well as copy by value for reference types based properties deep in the hierarchy  of reference types

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Shallow Copy- Reference variable inside original and shallow-copied objects have reference to common object  Deep Copy- Reference variable inside original and deep-copied objects have reference to different object   clone always does shallow copy   public class Language implements Cloneable           String name      public Language String name           this name name                 public String getName             return name                  Override     protected Object clone   throws CloneNotSupportedException           return super clone             main class is following- public static void main String args    throws ClassNotFoundException  CloneNotSupportedException         ArrayList lt Language gt  list new ArrayList lt Language gt           list add new Language  quot C quot           list add new Language  quot JAVA quot            ArrayList lt Language gt  shallow  ArrayList lt Language gt   list clone            We used here clone since this always shallow copied         System out println list  shallow                for int i 0 i lt list size   i          System out println list get i   shallow get i     true              ArrayList lt Language gt  deep new ArrayList lt Language gt           for Language language list             deep add  Language  language clone                   System out println list  deep         for int i 0 i lt list size   i              System out println list get i   deep get i     false            OutPut of above will be-  false true  true false false false  Any change made in origional object will reflect in shallow object not in deep object    list get 0  name  quot ViSuaLBaSiC quot     System out println shallow get 0  getName    quot    quot  deep get 0  getName      OutPut- ViSuaLBaSiC    C

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ShallowCopy  points to the same location in memory as  Source  does   DeepCopy  points to a different location in memory  but the contents are the same

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char   Source    Hello  world     char   ShallowCopy   Source       char   DeepCopy   new char strlen Source  1   strcpy DeepCopy Source              ShallowCopy  points to the same location in memory as  Source  does   DeepCopy  points to a different location in memory  but the contents are the same

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Especially For iOS Developers    If B is a shallow copy of A  then for primitive data it s like B    A assign   and for objects it s like B    A retain    B and A point to the same memory location  If B is a deep copy of A  then it is like B    A copy    B and A point to different memory locations  B memory address is same as A s  B has same contents as A s

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Imagine two objects  A and B of same type  t with respect to C    and you are thinking about shallow deep copying A to B   Shallow Copy  Simply makes a copy of the reference to A into B  Think about it as a copy of A s Address  So  the addresses of A and B will be the same i e  they will be pointing to the same memory location i e  data contents   Deep copy  Simply makes a copy of all the members of A  allocates memory in a different location for B and then assigns the copied members to B to achieve deep copy  In this way  if A becomes non-existant B is still valid in the memory  The correct term to use would be cloning  where you know that they both are totally the same  but yet different  i e  stored as two different entities in the memory space   You can also provide your clone wrapper where you can decide via inclusion exclusion list which properties to select during deep copy  This is quite a common practice when you create APIs   You can choose to do a Shallow Copy ONLY IF you understand the stakes involved  When you have enormous number of pointers to deal with in C   or C  doing a shallow copy of an object is REALLY a bad idea    EXAMPLE OF DEEP COPY  An example is  when you are trying to do image processing and object recognition you need to mask  Irrelevant and Repetitive Motion  out of your processing areas  If you are using image pointers  then you might have the specification to save those mask images  NOW    if you do a shallow copy of the image  when the pointer references are KILLED from the stack  you lost the reference and its copy i e  there will be a runtime error of access violation at some point  In this case  what you need is a deep copy of your image by CLONING it  In this way you can retrieve the masks in case you need them in the future   EXAMPLE OF SHALLOW COPY I am not extremely knowledgeable compared to the users in StackOverflow so feel free to delete this part and put a good example if you can clarify  But I really think it is not a good idea to do shallow copy if you know that your program is gonna run for an infinite period of time i e  continuous  push-pop  operation over the stack with function calls  If you are demonstrating something to an amateur or novice person  e g  C C   tutorial stuff  then it is probably okay  But if you are running an application such as surveillance and detection system  or Sonar Tracking System  you are not supposed to keep shallow copying your objects around because it will kill your program sooner or later

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Taken from  blog   http   sickprogrammersarea blogspot in 2014 03 technical-interview-questions-on-c 6 html  Deep copy involves using the contents of one object to create another instance of the same class  In a deep copy  the two objects may contain ht same information but the target object will have its own buffers and resources  the destruction of either object will not affect the remaining object  The overloaded assignment operator would create a deep copy of objects   Shallow copy involves copying the contents of one object into another instance of the same class thus creating a mirror image  Owing to straight copying of references and pointers  the two objects will share the same externally contained contents of the other object to be unpredictable   Explanation   Using a copy constructor we simply copy the data values member by member  This method of copying is called shallow copy  If the object is a simple class  comprised of built in types and no pointers this would be acceptable  This function would use the values and the objects and its behavior would not be altered with a shallow copy  only the addresses of pointers that are members are copied and not the value the address is pointing to  The data values of the object would then be inadvertently altered by the function  When the function goes out of scope  the copy of the object with all its data is popped off the stack   If the object has any pointers a deep copy needs to be executed  With the deep copy of an object  memory is allocated for the object in free store and the elements pointed to are copied  A deep copy is used for objects that are returned from a function

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var source     firstName  Jane   lastname  Jones     var shallow   ShallowCopyOf source   var deep   DeepCopyOf source   source lastName    Smith   WriteLine source lastName      prints Smith WriteLine shallow lastName      prints Smith WriteLine deep lastName      prints Jones

[language-agnostic] What is the difference between a deep copy and a shallow copy?

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