1. Shallow and Deep copy #

Created Friday 10 January 2020

Shallow vs deep copy?

• Shallow copy refers to the default copying scenario, which is bit by bit in C++.
• Deep copy means copying the contents of the object. Obviously, what is copied deeply is decided by the programmer.

Need for deep copy #

• For primitive types, shallow copy is okay. But if an object stores pointer to a data structure, and we just copy the pointer(without creating a copy of the data structure), then both objects will refer to the same place in memory when reading/writing to the data structure.
• This is not good, as we actually a clone.

Context #

• Both copy constructor and copy assignment operator do shallow copy.
• Hence we need to overload both the copy constructor and copy assignement operator if we wish to perform deep copy.

Example - A dynamic array. Assume extra code is present wherever necessary.

class DynamicArray
{
	int size, capacity;
	int * arr;
	...

	DynamicArray(const& A) // deep copy
	{
		size = A.size, capacity = A.capacity;
		arr = new int[capacity]; // allocate new memory
		for(int i=0; i<size; i++)
			arr[i] = A.arr[i];
	}

	void operator=(const& B) // A = B
	{
		size = B.size, capacity = B.capacity;
		delete[] arr; // delete current memory
		arr = new int[capacity];
		for(int i=0; i<size; i++)
			arr[i] = A.arr[i];
	}
}