What is a shallow copy in c plus plus?

Answer 1

Shallow copies occur whenever we copy "naked" pointers.

int* p1 = new int {42};

int* p2 = p1; // shallow copy

We call this a shallow copy because although p1 and p2 are independent pointer variables, they both refer to the same memory. If we change the value stored in that memory, we affect both pointers:

assert (*p2 == 42);

*p1 = 0;

assert (*p2 == 42); // error!

Side-effects such as this are completely at odds with how a "normal" copy behaves:

int x = 42;

int y = x;

assert (y == 42);

x = 0;

assert (y == 42); // ok

Here, changing the value of one variable has no effect upon the other because they are completely independent variables.

To mimic this behaviour with pointer variables we must perform a deep copy, copying the memory being referred to:

int* p1 = new int {42}; int* p2 = new int {*p1}; // deep copy

assert (*p2 == 42);

*p1 = 0;

assert (*p2 == 42); // ok

A shallow copy is (usually) cheaper than a deep copy, thus it is not unusual for class designers to postpone an expensive deep copy until it becomes absolutely necessary to do so (also known as a lazy copy). However, since C++11, the inclusion of move semantics and standard library smart pointers and resource handles has greatly reduced the need for shallow copying.