Deep copy vs Shallow Copy

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Possible Duplicate    What is the difference between a deep copy and a shallow copy        What is the difference between deep and shallow copy  What type of a copy does a copy constructor do

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The quintessential example of this is an array of pointers to structs or objects (that are mutable).

A shallow copy copies the array and maintains references to the original objects.

A deep copy will copy (clone) the objects too so they bear no relation to the original. Implicit in this is that the object themselves are deep copied. This is where it gets hard because there's no real way to know if something was deep copied or not.

The copy constructor is used to initilize 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 Constructors and destructors.

The default copy constructor is shallow. You can make your own copy constructors deep or shallow, as appropriate. See C++ Notes: OOP: Copy Constructors.

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Shallow copy   Some members of the copy may reference the same objects as the original   class X   private      int i      int  pi  public      X             pi new int              X const X amp  copy        lt -- copy ctor           i copy i   pi copy pi               Here  the pi member of the original and copied X object will both point to the same int     Deep copy   All members of the original are cloned  recursively  if necessary   There are no shared objects   class X   private      int i      int  pi  public      X             pi new int              X const X amp  copy        lt -- copy ctor           i copy i   pi new int  copy pi        lt -- note this line in particular               Here  the pi member of the original and copied X object will point to different int objects  but both of these have the same value     The default copy constructor  which is automatically provided if you don t provide one yourself  creates only shallow copies      Correction  Several comments below have correctly pointed out that it is wrong to say that the default copy constructor always performs a shallow copy  or a deep copy  for that matter   Whether a type s copy constructor creates a shallow copy  or deep copy  or something in-between the two  depends on the combination of each member s copy behaviour  a member s type s copy constructor can be made to do whatever it wants  after all    Here s what section 12 8  paragraph 8 of the 1998 C   standard says about the above code examples      The implicitly defined copy   constructor for class X performs a   memberwise copy of its subobjects          Each subobject is copied in the   manner appropriate to its type           I f the subobject is of scalar type    the builtin assignment operator is   used

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Deep copy literally performs a deep copy  It means  that if your class has some fields that are references  their values will be copied  not references themselves  If  for example you have two instances of a class  A  amp  B with fields of reference type  and perform a deep copy  changing a value of that field in A won t affect a value in B  And vise-versa  Things are different with shallow copy  because only references are copied  therefore  changing this field in a copied object would affect the original object       What type of a copy does a copy constructor does    It is implementation - dependent  This means that there are no strict rules about that  you can implement it like a deep copy or shallow copy  however as far as i know it is a common practice to implement a deep copy in a copy constructor  A default copy constructor performs a shallow copy though

[c++] Deep copy vs Shallow Copy

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