When is Copy Constructor Called in C++

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.

Copy Constructor Call When Passing Object by Value to a Function

When you pass an object of a class by value to a function, the copy constructor is called and it creates a copy of the original object to be used within the function. This process is called copy constructor through function argument.

Example

In this example, we demonstrate how the copy constructor is invoked when an object is passed by value to a function:

#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;
}

Following is the output to the above program:

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

Copy Constructor Call During Object Initialization

When you create a new object using an existing object of the same class (object initialization), then the copy constructor is called. This process is called copy constructor through object initialization.

Example

In this example, we demonstrate how the copy constructor is invoked when a new object is initialized using an existing object:

#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;
}

Following is the output to the above program:

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!