Deep Copy vs. Shallow Copy: When Do I Need a True Copy of My Data?

Deep Copy vs. Shallow Copy

Question:

What are the key differences between deep copy and shallow copy?

Answer:

Shallow Copy:

• Copies the object's values but retains references to shared objects.

• Example:

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

  In this shallow copy, pi references the same int object in both the original and copied objects.

Deep Copy:

• Creates a complete copy of the original object, including all embedded objects.

• Example:

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

  In this deep copy, pi points to a new int object with the same value as the original.

Copy Constructor Type:

The default copy constructor depends on the behavior of each member's copy constructor:

• For scalar types, the default assignment operator is used, resulting in a shallow copy.
• However, it is not strictly correct to say that the default copy constructor always performs a shallow copy. It could implement a deep copy, or even a combination of deep and shallow copying, depending on the member types' copy behaviors.

Example:

In the following example, the default copy constructor creates a deep copy for the std::vector member due to its implementation:

<code class="c++">class Y {
private:
    std::vector<int> v;
public:
    Y() {}
    Y(const Y& copy) : v(copy.v) {}
};</code>

In this case, the std::vector's copy constructor creates a deep copy of its contents.

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