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Copy Constructor and when it’s called?

Constructor which initializes its object member with another object of the same class.

Declaration :-

NO_RETURN_TYPE  CLASSNAME (const CLASSNAME &  CLASSNAMEObject)

clsABC  is the class, I am going to use in every example listed below

Copy constructors are called in following cases:

• when you construct an object based on another object of the same class

  clsABC objAbc ;
  
  clsABC objabc1 (objAbc); // Copy constructor is called
  
  clsABC objabc2 = objAbc; // Copy constructor is called
  
  clsABC objabc3;
  
  objabc3 = objAbc; // Operator Overloading is required here.
  

• when a function returns an object of that class by value

  clsABC   functionCopyContructorCalled() {}
  
  clsABC& functionCopyContructorNotCalled() { return HEAPVALUEObject;}
  

  In second example, since we returning by address, we should ensure the object whom memory allocated on heap should return, otherwise if you try to return object which is allocated in stack in function, you may receive the object, whose destructor is already being called when function ends.

• when the object of that class is passed by value as an argument to a function

  void   functionCopyContructorCalled(clsABC objABC) {}
  
  void   functionCopyContructorNotCalled(clsABC& objABC) {}�
  // since object is passed by address
  

Static Member variable (Why we need global declaration)

Let start with small code

class CStaticMember{
private:
	static int s_iCallrecords;
public:
	static int returnCallrecords() { return  ++s_iCallrecords; }
};

int _tmain(int argc, _TCHAR* argv[])
{
	int i = CStaticMember::returnCallrecords();
	return 0;
}

If you see the code I have declared one static member variable (s_iCallrecords)and one static member method (returnCallrecords), however when I compile the program it’s giving me following error only for static member variable

“error LNK2001: unresolved external symbol "private: static int CStaticMember::s_iCallrecords" (?s_iCallrecords@CStaticMember@@0HA)”

Not for the static member method, you must be thinking why?, it’s because we are only declaring the member variable in CStaticMember class i.e. we are not providing it storage/memory and in case of static method, we are declaring as well as defining the member function in class itself, that’s why it’s don’t give you any link error.

Also, since static member has file scope, not the class scope, i.e. it will not get created or dissolved with each object of the class, which in case of normal variables.

So solve this problem: you have to provide global declaration of the variable, just like you do for your class methods, if giving it definition outside the class.

If C++ support in-place initialization of the variable (like .Net/Java Languages do), we can solve this problem without defining it outside the class.

This is our modified code looks like, i have include following line to make it working:-

int CStaticMember::s_iCallrecords = 0;

Modified code :-

class CStaticMember{
private:
	static int s_iCallrecords;
public:
	static int returnCallrecords() { return  ++s_iCallrecords; }
};

int CStaticMember::s_iCallrecords = 0;

int _tmain(int argc, _TCHAR* argv[])
{
	int i = CStaticMember::returnCallrecords();
	return 0;
}