CS148 Introduction to Programming II

1. CS148 Introduction to Programming II Ayman Abdel-Hamid Department of Computer Science Old Dominion University 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 CS148 Spring 2003

3. Dynamically Allocates data pointed to by msgStr Deallocates data pointed to by msgStr 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) 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; }; CS148 Spring 2003

4. Free Store 4 15 2001 ‘X’ ‘y’ ‘Z’ ‘\0’ msg Free Store After the call to destructor 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) • 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"); • } CS148 Spring 2003

5. Free Store 5 5 4 16 15 16 2002 2001 2002 ‘X’ ‘X’ ‘A’ ‘A’ ‘y’ ‘y’ ‘B’ ‘B’ ‘Z’ ‘Z’ ‘C’ ‘C’ ‘\0’ ‘\0’ ‘D’ ‘D’ ‘\0’ ‘\0’ 5 Free Store 16 2002 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 • void main () • { • //create an instance of class Date • Date date1 (4,15,2001,“XyZ"); • Date date2 (5,16,2002,”ABCD”); • date1 = date2; • } What happens when the destructor is invoked when main function is exited? 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? CS148 Spring 2003

7. main function 5 4 15 16 Free Store 2001 2002 ‘X’ ‘A’ ‘y’ ‘B’ ‘Z’ ‘C’ ‘\0’ ‘D’ ‘\0’ copyFrom function 5 Copy of date2 Created using a shallow copy 16 2002 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 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 CS148 Spring 2003

9. main function Free Store 4 15 2001 5 16 ‘X’ ‘A’ ‘A’ ‘y’ ‘B’ ‘B’ ‘Z’ ‘C’ ‘C’ ‘\0’ ‘D’ ‘D’ ‘\0’ ‘\0’ 2002 copyFrom function 5 Copy of date2 created using the copy-constructor 16 2002 Deep copy using a copy-constructor 2/2 • date1.copyFrom(date2); //in the presence of a copy-constructor • 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 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 CS148 Spring 2003