Copy constructor(Deep copy, Shallow copy) in C++

Copy constructor

A copy constructor is a constructor that copies all the information of an object by passing the object as an argument to a function. The compiler defines a default copy constructor if copy constructor is not defined. 

Usually, a copy constructor is used for copying objects. For example, in a game, you use it to create multiple monsters.

The basic form of a copy constructor 

classname (const classname &obj) {
   //body
}

Example Code

class Bank {
private:
    int num;
    int cnum;
public:
 
    Bank() { //constructor
        num = 0;
        cnum = 0;
    }
 
    ~Bank() {
        cout << "destructor" << endl; //destructor
    }
 
    Bank(const Bank& bank) { //copy constructor
        cout << "copy constructor called" << endl;
        this->num = bank.num;
        this->cnum = bank.cnum;
    }
};
 
int main()
{
    Bank bank;
    int a = 20;
    int b = 30;
    cout << a << b << endl;
    Bank bank2 = bank;
 
    return 0;
}

Result

Shallow copy and Deep copy

It is a shallow copy that only the reference is copied. Unlike a shallow copy, deep copy creates a new copy of a referred object.

Shallow copy example Code

class Bank {
private:
    int *num;
    int arrNum;
public:
    Bank(const Bank &pBank) {
        num = pBank.num; // Shallow copy occurred 
    }
    Bank(void) {
        cout << "Enter a number: ";
        cin >> arrNum;
        cout << "the number of elements is: " << arrNum << endl;
 
        num = new int[arrNum];
 
        for (int i = 0; i < arrNum; i++)
        {
            cout << "Enter the numbers: " << i + 1 << ":";
            cin >> num[i];
        }
 
        for (int j = 0; j < arrNum; j++)
        {
            cout << num[j] << " , " << endl;
        }
    }
 
    ~Bank(void) {
        cout << endl;
        cout << "Destructor" << endl;
        delete[] num; //Error occurred when it executes. 
        // bank2 can not executed because bank1 has already deleted
    }
};
 
int main(void)
{
    Bank bank1;
    Bank bank2 = bank1;
 
    return 0;
}

Deep copy example Code

class Bank {
private:
    int *num;
    int arrNum;
public:
    Bank(const Bank &pBank) {
        num = new int[arrNum]; //Deep copy occurred 
    }
    Bank(void) {
        cout << "Enter a number: ";
        cin >> arrNum;
        cout << "the number of elements is: " << arrNum << endl;
 
        num = new int[arrNum];
 
        for (int i = 0; i < arrNum; i++)
        {
            cout << "Enter the numbers: " << i + 1 << ":";
            cin >> num[i];
        }
 
        for (int j = 0; j < arrNum; j++)
        {
            cout << num[j] << " , " << endl;
        }
    }
 
    ~Bank(void) {
        cout << endl;
        cout << "Destructor" << endl;
        delete[] num; 
    }
};
 
int main(void)
{
    Bank bank1;
    Bank bank2 = bank1;
 
    return 0;
}

Result