Deep vs. Shallow Copy: When Should You Choose Which?

Deep vs. Shallow Copy in Programming

Shallow copying involves creating a new object that references the same underlying data as the original. In contrast, deep copying constructs a new object that contains independent duplicates of all data associated with the original.

Consider the following C class:

<code class="c++">class X {
public:
    int i;
    int *pi;
    X() : pi(new int) {}
    X(const X& copy) : i(copy.i), pi(copy.pi) {}
};</code>

In this case, shallow copying will result in both the original and copied X objects pointing to the same int object when assigning pi:

<code class="c++">X original;
X copy(original);</code>

On the other hand, deep copying includes a step in the copy constructor where a new int object is allocated:

<code class="c++">class X {
public:
    int i;
    int *pi;
    X() : pi(new int) {}
    X(const X& copy) : i(copy.i), pi(new int(*copy.pi)) {}
};</code>

As a result, the copied X object will have its own int object with the same value as the original.

Initially, the default copy constructor was thought to always perform shallow copies. However, it has been clarified that the behavior depends on the type of each member. The standard specifies memberwise copying, meaning that scalar types (like int in this example) are copied using the built-in assignment operator.

Therefore, the choice between deep and shallow copying depends on the specific requirements of the application and the characteristics of the data being copied.

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