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A deep copy is often used when working with objects that have dynamically allocated memory, such as objects created on the heap using the new operator. In this case, a simple copy of the object using the default copy constructor or assignment operator would result in both objects pointing to the same memory location, which can cause problems if one object is modified.
Deep Copy Explained
Deep copy is a concept in object-oriented programming that becomes crucial when dealing with objects which have pointers or references to other memory spaces like dynamically allocated memory or other objects.
Imagine you have a "House" object. This house object contains a number of "Room" objects. Now, if you want to create an exact copy of this house, you'd also want to create copies of each of these rooms.
If you do a shallow copy (the default copy behavior in C++), you end up with a new house, but the rooms in this new house are exactly the same rooms as the original house. If you make a change in any of the rooms, it will reflect in both the houses because they both refer to the same set of rooms.
Now, think of a deep copy. When you make a deep copy of the house, not only do you create a new house, but you also create new, separate rooms for this house. Now, if you change anything in the rooms of the new house, it will not affect the rooms in the original house. They are independent of each other.
In terms of programming and specifically in C++, when you're making a deep copy, you need to define a custom copy constructor. In this copy constructor, you would allocate new memory for the pointer in the new object (the new house in our analogy) and then copy the actual values (rooms) from the source object to the newly allocated memory.
By doing so, any changes you make to the objects (rooms) in the copied object (new house) do not affect the objects (rooms) in the original object (old house), preventing any unintentional changes or deletions. This is why it's known as a deep copy - you're copying everything, not just the top-level structure.
Here is an example of a deep copy:
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