C++ LANGUAGE COPY CONSTRUCTOR

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C++ LANGUAGE COPY CONSTRUCTOR

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. 
#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; 
} 

When the above code is compiled and executed, it produces the following result: 

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: 
#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; 
} 
When the above code is compiled and executed, it produces the following result: 
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!