Copy Constructors

Whenever an object obj2 is assigned to an object obj3 being newly declared: 
Person obj3 = obj2;
a bit by bit copy of obj2 is created by calling Person’s constructor and setting the new object's member values accordingly. This might not always be desirable, for example when an object's constructor performs actions that are not supposed to be repeated when the object is copied (like sending an initialization message to another object, for example, or allocating some memory space, registering with another object, etc.) For such a reason copy constructors were introduced. A copy constructor allows you to define a constructor that will be used when an object is created and assigned another one. In the following example, the class Person has a two-parameter constructor that performs an action which should take place only once: it saves the name of the person in a global array called arrayOfPersons. To prevent such an action from happening also when an object is copied, the copy constructor: 
Person(const Person &o)
is defined, which simply copies name and number in the new object without saving name into the array. So no object's name will be written twice in the array. (For sake of simplicity, it is assumed that every object will receive a different name by the user, otherwise a name-checking routine should be written).

Example


#include <iostream>
#include <string>
using namespace std;

string arrayOfPersons[30];  // contains names of all people

void printArrayOfPersons()
{
     for(int i=0; i<30 && arrayOfPersons[i] != ""; i++)
           cout << arrayOfPersons[i] << endl;
}

class Person
{
     string name;
     int number;

     void save(string aName)  // saves a person's name in arrayOfPersons
     {
           for(int i=0; i<30; i++)
                if(arrayOfPersons[i] == "")
                {
                    arrayOfPersons[i] = aName;
                    break;
                }
     }

public:
     Person(string aName, int aNumber) // overloaded two-parameter constructor
     {
        name = aName;
        number = aNumber;
        save(aName);
     }

     string getName()
     {
           return name;
     }

     int getNumber()
     {
           return number;
     }

// COPY CONSTRUCTOR: o is a reference
// to the object being assigned
     Person(const Person &o) 
     {                      
        name = o.name;
        number = o.number;     // NB: name is not saved in arrayOfPersons
     }
};


int main()
{   
     Person obj1("Jack", 22222222);
     Person obj2("Lisa", 11111111);
     Person obj3 = obj2;  // here the copy constructor is called

     // verify the copy took place
     cout << obj3.getName() << " " << obj3.getNumber() << endl;
     cout << "Array of persons:" << endl;
     printArrayOfPersons(); // show that the name has no duplicate in the array
}

Output

Lisa 11111111
Array of persons:
Jack
Lisa