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

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.

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.

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

Shallow copy: Copies the member values from one object into another.

Deep Copy:    Copies the member values from one object into another.
                     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.)

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.

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.

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.

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 !!

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.

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

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.

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)

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.