What is the difference between shallow and deep copy in C++?

Answer 1

A shallow copy of an object copies all of the member field values. This works well if the fields are values, but may not be what you want for fields that point to dynamically allocated memory. The pointer will be copied. but the memory it points to will not be copied -- the field in both the original object and the copy will then point to the same dynamically allocated memory, which is not usually what you want. The default copy constructor and assignment operator make shallow copies.

A deep copy copies all fields, and makes copies of dynamically allocated memory pointed to by the fields. To make a deep copy, you must write a copy constructor and overload the assignment operator, otherwise the copy will point to the original, with disasterous consequences.

Answer 2

A shallow copy means that C++ copies each member of the class individually using the assignment operator. When classes are simple (eg. do not contain any dynamically allocated memory), this works very well.
A deep copy duplicates the object or variable being pointed to so that the destination (the object being assigned to) receives it’s own local copy. This way, the destination can do whatever it wants to it’s local copy and the object that was copied from will not be affected. Doing deep copies requires that we write our own copy constructors and overloaded assignment operators.

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