deep copy and shallow copy

vatsa_mitr · #1 14-Aug-2006, 04:32

Hi ,
can anybody tell me the diff between deep copy and shallow copy.

Gamer_2k4 · #2 14-Aug-2006, 07:19

I think (and someone please correct me if I'm wrong) that a shallow copy is when you create another reference to the same object.

CPP / C++ / C Code:

testObject * o1 = new testObject();
testObject * o2 = o1;

In the above case, o2 is a shallow copy of o1, because they both point to the same object.

Now a deep copy (again, someone please correct me if necessary) is when you reallocate the memory for an object, generally with a copy constructor.

CPP / C++ / C Code:

class testObject
{
public:
   testObject()
   {
      myVar = 0;
   }

   testObject(testObject &o)
   {
       myVar = o.myVar;
   }

   int myVar;
}

//elsewhere in the program
testObject * o1 = new testObject();
testObject * o2 = new testObject(*o1);

Here o2 is a deep copy of o1, because the memory is reallocated for the object. Even though they contain the same information, o1 and o2 are different objects.

Gamer_2k4

ubergeek · #3 14-Aug-2006, 07:52

Nice explanation Gamer. There's another twist to this, one that's more dangerous in code. Suppose the class testObject contains a c-style string, in other words a dynamically allocated char array.

CPP / C++ / C Code:

class testObject
{
    public:
        testObject(char *str) { m_str = new char[strlen(str)+1]; strcpy(m_str, str); }
        ~testObject() { if (m_str) delete[] m_str; }
    private:
        char *m_str;
};

int main()
{
    testObject o1("hello world");
    testObject o2 = o1; //use default copy constructor
    return 0; //when o1 and o2 go out of scope, their destructors will be called automatically
}

This program crashes as it terminates because of a double delete. Since no copy constructor is provided in testObject, the compiler creates a default one, which simply copies over the values of all the member variables. This is a shallow copy. The problem is, with the shallow copy o1.m_str == o2.m_str; in other words the two pointers point to the same place in memory. Then when they go out of scope, a destructor is called and deletes that memory. Then -- whoops -- another destructor is called and deletes the same memory. Crash.

The solution to this is to provide a copy constructor for testObject. (To be safe, there's also an assignment overload operator that does the same thing. See explanation below.)

CPP / C++ / C Code:

class testObject
{
    public:
        testObject(char *str) { m_str = new char[strlen(str)+1]; strcpy(m_str, str); }
        testObject(testObject& orig) { m_str = new char[strlen(orig.m_str)+1]; strcpy(m_str, orig.m_str); }
        ~testObject() { if (m_str) delete[] m_str; }
        testObject& operator=(testObject& other) { if (m_str) { delete[] m_str; } m_str = new char[strlen(other.m_str)+1]; strcpy(m_str, other.m_str); }
    private:
        char *m_str;
};

int main()
{
    testObject o1("hello world");
    testObject o2 = o1; //use the copy constructor I wrote for testObject (a constructor with one argument, unless declared with the explicit keyword, can be called with this syntax, so this is not assignment; rather it is equivalent to testObject o2(o1);
    testObject o3("three");
    o3 = o1; //use assignment operator
    return 0; //when o1, o2 and o3 go out of scope, their destructors will be called automatically
}

Now all three destructors will get called, but all three class instances own copies of the string "hello world" in different places in memory, so they can all delete their own strings and not crash. The copy constructor and assignment operator do deep copies.

Tomb332 · #4 14-Aug-2006, 09:12

thats right
The proper definition as i know it is

Shallow copy
Just copies the member data nothing else (big problems if there is
allocated memory)(the default copy constructor uses shallow copies)

Deep copy
Copies everything and if there is allocated memory allocates
itself some memory for the data (insted of just copying the pointer
data (ends up with 2 pointers to same place as ubergeek said))

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search
Rate This Thread
Rate This Thread:

deep copy and shallow copy

Re: deep copy and shallow copy

Re: deep copy and shallow copy

Re: deep copy and shallow copy