Copy Constructor in C++

What is Copy Constructor?

Copy constructor creates a new object of the same class using an existing object. It creates a copy/replica of the existing object.

Syntax of Copy Constructor

( const  &givenObject)

Here givenObject is the object of same class which is already created. Copy constructor takes its own class object (which already exists) as parameter while constructing another object. 

What Copy Constructor does?

It copies value of all member variables from existing object to new object. It copies values member-to-member.

Example of Copy Constructor

#include <string.h>
#include <iostream>
using namespace std;

class Point
{

public:
	Point()//default constructor
	{
		cout << "defalut constructor is called to construct the object" << endl;
	}; 
	
	~Point(){};

	Point(const int &xValue, const int &yValue)  //parameterized constructor
	{
		x = xValue;
		y = yValue;
		cout << "parameterized constructor is called to construct the object" << endl;
	}

	Point(const Point& pointObj) //copy constructor
	{
		x = pointObj.x;
		y = pointObj.y;
		cout << "copy constructor is called to construct the object" << endl;
	}

	void Print()
	{
		cout << "X-cordinate of point is : " << x << endl;
		cout << "Y-cordinate of point is : " << y << endl;
	}

private:
	int x;
	int y;
};

	
int main()
{
	Point p1;  //default constructor is called to constrct object 'p1'

	Point p2(10, 20);   //parameterized constructor is called to construct object 'p2'

	Point cp(p2); //copy constructor is called to construct object 'cp'

	cout << "print p1" << endl;
	
	p1.Print();

	cout << "print p2" << endl;
	
	p2.Print();

	cout << "print xp" << endl;
	
	cp.Print();

	return 0;
}
Try It

When is copy constructor called?

  1. When an object is constructed based on another object of the same class.
  2. When an object of the class is passed (to a function) by value as an argument.
  3. When an object of the class is returned by value.
  4. When compiler generates a temporary object.

When you need a copy constructor?

If the class has pointer variables and has some dynamic memory allocations, then it is a must to have a copy constructor.

What if you don’t define a copy constructor for a class?

If you don't define copy constructor, the C++ compiler creates a default copy constructor for each class which does a member-wise copy (shallow copy) between objects.

Then why we need a copy constructor?

Since copy constructor does a member-wise copy and if the class has some pointer member variable where memory allocation/deallocation is done specifically while constructing a new object a new memory block need to be allocated corresponding pointer member variable.

Example: In class string we must need to define a constructor as it has a pointer *name and we are allocating memory dynamically to it using new.

#include 
#include 

using namespace std;

class MyString
{

private:

	char *name;
	int size;

public:

	~MyString() { delete[] name; } // destructor

	MyString(const char *str = NULL) // constructor
	{
		size = strlen(str);
		name = new char[size + 1];
		//strcpy(dst, src);
		strcpy(name, str);
	}

	MyString(const MyString& old_str) // copy constructor
	{
		size = old_str.size;
		name = new char[size + 1];
		strcpy(name, old_str.name);
	}

	void Print()
	{
		cout << "name:" << name << endl;
	}

};



int main()
{	
	//Example 1:
	MyString str1("Hi");

	str1.Print();
	
	MyString str2(str1); //copy constructor called

	str2.Print(); //This will crash if copy constructor not handled appropriately
	
	//Example:2
	MyString *str3 = new MyString("Hello");

	if (str3)
	{
		str3->Print();
		MyString str4(*str3);
		delete str3;
		str4.Print();
	}
	return 0;
}
Try It

Comment copy constructor and then try to example 1 and example 2, one by one. str2.Print() and str4.Print() – will crash as there is no dynamic allocation done while creating str2 and str4.

Difference between Default Constructor and Copy Constructor

 

Default Constructor

Copy Constructor

Syntax

()

( const &givenObject)

 

Usage

Create a new object

Creates a new object from an existing object and does member-wise copy

Parameter

Doesn’t take any argument

It takes its own class object as a parameter

What if user doesn’t provide

Compiler provides it

Compiler provides it, but default copy constructor doesn’t do dynamic allocation

Difference between Parameterized Constructor and Copy Constructor

Copy constructor’s syntax is fixed it is always defined as – It always excepts an existing object of same class as "const reference"

Syntax

Copy Constructor

( const &givenObject)

Parameterized constructor can be defined to accepts any type of arguments

Classname(int a);
Classname(char *a);
Classname(int x, OtherClass abc);

When is copy constructor called?

  1. When an object of the class is returned by value.
  2. When an object of the class is passed (to a function) by value as an argument.
  3. When an object is constructed based on another object of the same class.
  4. When compiler generates a temporary object.

Why Copy Constructor’s parameter is const reference?

MyString(const MyString obj) – Here if we pass Class object "obj' to copy constructor as value(not reference), compiler will start creating a copy of "obj" and to create a copy of the object, copy constructor would be called again and it would create an infinite loop and program will go in deadlock state.

MyString(MyString &obj) – the parameter "obj" should not get modified while copying.

So here the interesting part is compiler doesn’t allow to create you a constructor without const reference.


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