Understanding Copy Constructors: Shallow vs Deep Copying in C++, Slides of Computer Science

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Classes

Copy Constructors

Copy Constructor

• Constructor with a Reference to THAT CLASS as an ARGUMENT
• X(X &) Format
• Initialization of a New Object by Another Object of that Class
• An Object is Passed by Value to a Function
• An Object is Returned from a Function
• Shallow and Deep Copying -- Involvement of Pointers
• Implicit Copy Constructor is Created by the Compiler to Perform Member-wise Initialization

Copy Constructor – Example 1 ….

• // date.cpp #include “date.h” Date::date() //default constructor {day = 0; month = 0; year = 0; string_date = 0;}

date::date(char *ip_date){ //Constructor day = 0; month = 0; year = 0; string_date = new char [size]; strcpy(string_date, ip_date); }

//Shallow Copy Constructor date(const date &ip_date) {string_date = ip_date.string_date;}

//Destructor date::~date() {delete[] (string_date);}

Copy Constructor – Example 1 ….

• // client.cpp

#include “date.h”

main() {

date today("June 21, 1995");

//Use of shallow copy constructor

date extra_day = today;

• “Shallow copy” simply means that all the

data member values are copied.

Copy Constructor – Example 2 ….

• // date.cpp #include “date.h” Date::date() //default constructor {day = 0; month = 0; year = 0; string_date = 0;}

date::date(char *ip_date){ //Constructor day = 0; month = 0; year = 0; string_date = new char [size]; strcpy(string_date, ip_date); } //Deep Copy Constructor date(const date &ip_date) { string_date = new char[size]; strcpy(string_date, ip_date.string_date); } //Destructor date::~date() {delete[] (string_date);}

Copy Constructor – Example 2 ….

• // client.cpp

#include “date.h” main() { date today("June 21, 1995"); //Deep copy constructor date extra_day = today; }

• “Deep copy” means that objects pointed to by

data members are also copied. Deep copying may be required whenever you have data members that are pointers.

Copy Constructor – Example 3 ….

• // student.cpp #include “student.h” /* Member functions of the “Student" class */ Student ::Student(char *studentName, char *studentId, char *studentEmail){ name = new char[size]; strcpy(name, studentName); id = new char[size]; strcpy(id, studentId); email = new char[size]; strcpy(email, studentEmail);} Student::~Student(){ delete [] name; delete [] id; delete [] email;} //Shallow copy constructor Student::Student (Student &s){ name = s.name; id = s.id; email = s.email; }

Copy Constructor – Example 3 ….

• // client.cpp

#include “student.h”

/* “csStudent" is an instance of “Student" */

main(){

Student csStudent1("Susie Creamchese“,

"screamch@iupui.edu");

Student csStudent2 = csStudent1;

What happens when csStudent2 changes its name?

• // student.cppCopy Constructor – Example 4 ….

#include “student.h” /* Member functions of the “Student" class */ Student::Student(char *studentName, char *studentId, char *studentEmail){ name = new char[size]; strcpy(name, studentName); id = new char[size]; strcpy(id, studentId); email = new char[size]; strcpy(email, studentEmail);} Student::~Student(){ delete [] name; delete [] id; delete [] email;} // Deep copy constructor Student::Student(Student &s){ name = new char[size]; strcpy(name, s.name); id = new char[size]; strcpy(id, s.id); email = new char[size]; strcpy(email, s.email);}

Copy Constructor – Example 4 ….

• // client.cpp

#include “student.h”

main(){

Student csStudent1 (“Susie Creamchese”,

“screamch@iupui.edu”);

Student csStudent2 = csStudent1;

What happens when csStudent2 changes its name?