Copy Constructor in C++

1. COPY CONSTRUCTOR
2. A copy constructor is a special constructor in the C++ programming language for creating a new object as a copy of an existing object.

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.
• Don't write a copy constructor if shallow copies are ok
• If the object has no pointers to dynamically allocated memory, a shallow copy is probably sufficient. Therefore the default copy constructor, default assignment operator, and default destructor are ok and you don't need to write your own.

  If you need a copy constructor, you also need a destructor and operator=

  If you need a copy constructor, it's because you need something like a deep copy, or some other management of resources. Thus is is almost certain that you will need a destructor and override the assignment operator.

  Copy constructor syntax

  The copy constructor takes a reference to a const parameter. It is const to guarantee that the copy constructor doesn't change it, and it is a reference because a value parameter would require making a copy, which would invoke the copy constructor, which would make a copy of its parameter, which would invoke the copy constructor, which ...
  Here is an example of a copy constructor for the Point class, which doesn't really need one because the default copy constructor's action of copying fields would work fine, but it shows how it works.

  //=== file Point.h =============================================
  class Point {
      public:
          . . .
          Point(const Point& p);   // copy constructor
          . . .
  //=== file Point.cpp ==========================================
  . . .
  Point::Point(const Point& p) {
      x = p.x;
      y = p.y;
  }
      . . .
  //=== file my_program.cpp ====================================
  . . .
  Point p;            // calls default constructor
  Point s = p;        // calls copy constructor.
  p = s;              // assignment, not copy constructor.
  

  Difference between copy constructor and assignment

  A copy constructor is used to initialize a newly declared variable from an existing variable. This makes a deep copy like assignment, but it is somewhat simpler:
  1. There is no need to test to see if it is being initialized from itself.
  2. There is no need to clean up (eg, delete) an existing value (there is none).
  3. A reference to itself is not returned.