How to implement a copy constructors in C++?

Here we will see how the copy constructors are implemented in C++. Before discussing that we should know what is the 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.

Example

#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; }

Output

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 −

Example

#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; }

Output

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!