Copy constructor

For self-defined classes, it may not make sense in some cases to simply copy the contents of the class. The programmer of a class can therefore tell the compiler which code to use for copying. This is done by defining the so-called copy constructor, i.e. a constructor that receives a constant reference of the same class as an argument.

Such a constructor takes an object of its own class as a parameter and initializes itself based on the passed object. A copy constructor is applied to uninitialized objects. It assigns a copy of the source object to an uninitialized target object.

Syntax

class ClassName {
    public:
        // create a default constructor
        ClassName() {
            // Constructor code
        }

        // create a copy constructor
        ClassName(ClassName &obj) {
            // Constructor code
        }
};

int main() {
    // create object using default constructor
    ClassName cn1;
    // create object using copy constructor
    ClassName cn2 = cn1;

    return 0;
}

Example: Copy Constructor

#include <iostream>
using namespace std;

class Cube {
    private:
        int length, width, height;
    public:
        Cube(int a, int b, int c) {
            length = a;
            width = b;
            height = c;
        }

        Cube(Cube &obj) {
            length = obj.length;
            width = obj.width;
            height = obj.height;
        }

        int getVolume() {
            return length * width * height;
        }
};

int main() {
    Cube c1(2, 4, 6);
    Cube c2 = c1;

    cout << c1.getVolume() << endl;
    cout << c2.getVolume() << endl;

    return 0;
}

Output

48
48