1. CS148 Introduction to Programming II
Ayman Abdel-Hamid
Department of Computer Science
Old Dominion University
Lecture 16: 3/24/2003
Lecture 16: 3/24/2003
CS148 Spring 2003

2. Outline Classes and dynamic data Use of destructors Shallow copy
Deep copy using a deep copy function
Deep copy using a copy-constructor
Lecture 16: 3/24/2003
CS148 Spring 2003

3. Destructors and Dynamic Data 1/2
When a class instance allocates dynamic data on the free store (using new) , a destructor is used to properly deallocate the previously allocated data (using delete)
Dynamically Allocates data pointed to by msgStr
class Date {
public:
Date (int Mo, int Day, int Yr, const char* msgStr);
void print();
~Date(); //destructor invoked when a class object is destroyed
private:
int mo;
int day;
int yr;
char* msg; };
Deallocates data pointed to by msgStr
Lecture 16: 3/24/2003
CS148 Spring 2003

4. Destructors and Dynamic Data 2/2
When a class instance allocates dynamic data on the free store (using new in a constructor) , a destructor is used to properly deallocate the previously allocated data (using delete) 4
msg
Free Store
‘X’
‘y’
‘Z’
‘\0’ 15
2001
Date::Date(int Mo, int Day, int Yr, const char* msgStr) { …
//allocate space for msg
msg = new char[strlen(msgStr) + 1];
//copy the contents of msgStr into msg
strcpy (msg,msgStr); }
Date::~Date()
delete [] msg;
void main ()
//create an instance of class Date
Date date1 (4,15,2001,“XyZ");
Free Store
After the call to destructor
Lecture 16: 3/24/2003
CS148 Spring 2003

5. Shallow copy
When using the built in assignment operator with class objects a shallow copy is performed. A shallow copy copies one class object to another without copying any pointed-to data
Free Store
‘X’
‘y’
‘Z’
‘\0’ 4 15
2001 5 16
2002
‘A’
‘B’
‘C’
‘D’
void main () {
//create an instance of class Date
Date date1 (4,15,2001,“XyZ");
Date date2 (5,16,2002,”ABCD”);
date1 = date2; }
Free Store
‘X’
‘y’
‘Z’
‘\0’ 5 16
2002
‘A’
‘B’
‘C’
‘D’
What happens when the destructor is invoked when main function is exited?
Lecture 16: 3/24/2003
CS148 Spring 2003

6. Deep copy using a deep copy function 1/2
A deep copy is an operation that not only copies one class object to another but also makes copies of any pointed-to data
Instead of built-in assignment operator use a deep copy function
//deep copy function
void Date::copyFrom (Date otherDate) {
mo = otherDate.mo;
day = otherDate.day;
yr = otherDate.yr;
delete []msg;
msg = new char[strlen(otherDate.msg)+1];
strcpy(msg,otherDate.msg); }
void main ()
//create an instance of class Date
Date date1 (4,15,2001,“XyZ");
Date date2 (5,16,2002,”ABCD”);
date1.copyFrom(date2);
What happens when date2 is passed by value to copyFrom?
What happens when copyFrom is exited?
What happens when the destructor is called upon main function exit?
Lecture 16: 3/24/2003
CS148 Spring 2003

7. Deep copy using a deep copy function 2/2
date1.copyFrom(date2);
By default, date2 is copied to the copyFrom function using a shallow copy
When copyFrom is exiting, date2 copy is destroyed prompting a call to the destructor
The destructor deallocates the original pointed-to char array
Free Store
‘X’
‘y’
‘Z’
‘\0’ 4 15
2001 5 16
2002
‘A’
‘B’
‘C’
‘D’
main function 5 16
2002
copyFrom function
Copy of date2
Created using a shallow copy
Lecture 16: 3/24/2003
CS148 Spring 2003

8. Deep copy using a copy-constructor
Include a copy-constructor Date (const Date & otherDate);
The copy constructor will be invoked when
Passing a copy of an argument to a parameter (pass by value)
Initialization in a variable declaration Date date3 = date2;
Returning an object as the value of a function return someObject;
//copy constructor
Date::Date (const Date& otherDate) {
mo = otherDate.mo;
day = otherDate.day;
yr = otherDate.yr;
msg = new char[strlen(otherDate.msg)+1];
strcpy(msg,otherDate.msg); }
void main ()
//create an instance of class Date
Date date1 (4,15,2001,“XyZ");
Date date2 (5,16,2002,”ABCD”);
date1.copyFrom(date2);
The date2 copy will be created by invoking the copy-constructor on date2
Lecture 16: 3/24/2003
CS148 Spring 2003

9. Deep copy using a copy-constructor 2/2
date1.copyFrom(date2); //in the presence of a copy-constructor
Free Store
‘X’
‘y’
‘Z’
‘\0’
‘A’
‘B’
‘C’
‘D’
main function 4 15
2001 5 16
2002
When copyFrom is exiting, date2 copy is destroyed prompting a call to the destructor
The destructor deallocates the dynamic char array created using the copy-constructor leaving the one pointed to by date2.msg intact
‘A’
‘B’
‘C’
‘D’
‘\0’ 5 16
2002
copyFrom function
Copy of date2 created using the copy-constructor
Lecture 16: 3/24/2003
CS148 Spring 2003

10. Summary
When dealing with classes and dynamic data provide the following functions
A destructor to deallocate dynamic data
A deep copy function that copies pointed-to data in addition to class object members
A copy-constructor to guarantee deep copying of class objects
If deep copying is not provided, the default is shallow copying which does not copy pointed-to data
Lecture 16: 3/24/2003
CS148 Spring 2003