How to implement a copy constructors in C++?

Here we will see how the copy constructors are implemented in C++. Before discussing that we should know what is the copy constructor.

The copy constructor is a constructor which creates an object by initializing it with an object of the same class, which has been created previously. The copy constructor is used to −

• Initialize one object from another of the same type.
• Copy an object to pass it as an argument to a function.
• Copy an object to return it from a function.

If a copy constructor is not defined in a class, the compiler itself defines one. If the class has pointer variables and has some dynamic memory allocations, then it is a must to have a copy constructor. The most common form of copy constructor is shown here −

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

Here, obj is a reference to an object that is being used to initialize another object.

Example

#include <iostream>
using namespace std;
class Line {
   public:
      int getLength( void );
      Line( int len ); // simple constructor
      Line( const Line &obj); // copy constructor
      ~Line(); // destructor
   private:
      int *ptr;
};
// Member functions definitions including constructor
Line::Line(int len) {
   cout << "Normal constructor allocating ptr" << endl;
   // allocate memory for the pointer;
   ptr = new int;
   *ptr = len;
}
Line::Line(const Line &obj) {
   cout << "Copy constructor allocating ptr." << endl;
   ptr = new int;
   *ptr = *obj.ptr; // copy the value
}
Line::~Line(void) {
   cout << "Freeing memory!" << endl;
   delete ptr;
}
int Line::getLength( void ) {
   return *ptr;
}
void display(Line obj) {
   cout << "Length of line : " << obj.getLength() <<endl;
}
// Main function for the program
int main() {
   Line line(10);
   display(line);
   return 0;
}

Output

Normal constructor allocating ptr
Copy constructor allocating ptr.
Length of line : 10
Freeing memory!
Freeing memory!

Let us see the same example but with a small change to create another object using existing object of the same type −

Example

#include <iostream>
using namespace std;
class Line {
   public:
      int getLength( void );
      Line( int len ); // simple constructor
      Line( const Line &obj); // copy constructor
      ~Line(); // destructor
   private:
      int *ptr;
};
// Member functions definitions including constructor
Line::Line(int len) {
   cout << "Normal constructor allocating ptr" << endl;
   // allocate memory for the pointer;
   ptr = new int;
   *ptr = len;
}
Line::Line(const Line &obj) {
   cout << "Copy constructor allocating ptr." << endl;
   ptr = new int;
   *ptr = *obj.ptr; // copy the value
}
Line::~Line(void) {
   cout << "Freeing memory!" << endl;
   delete ptr;
}
int Line::getLength( void ) {
   return *ptr;
}
void display(Line obj) {
   cout << "Length of line : " << obj.getLength() <<endl;
}
// Main function for the program
int main() {
   Line line1(10);
   Line line2 = line1; // This also calls copy constructor
   display(line1);
   display(line2);
   return 0;
}

Output

Normal constructor allocating ptr
Copy constructor allocating ptr.
Copy constructor allocating ptr.
Length of line : 10
Freeing memory!
Copy constructor allocating ptr.
Length of line : 10
Freeing memory!
Freeing memory!
Freeing memory!