ECE 264 Object-Oriented Software Development

1. ECE 264Object-Oriented Software Development Instructor: Dr. Honggang Wang Fall 2012 Lecture 24: Pointers and Dynamic Allocation

2. Lecture outline • Announcements / reminders • Contact your project groups—design due 11/19 • Meeting with project group next Wednesday in my office (during the lab session) regarding your project questions and design • 2:00-2:15 pm meeting with group 1 • 2:15-2:30 pm meeting with group 2 • 2:30-2:45 pm meeting with group 3 • 2:45-3:00 pm meeting with group 4 • Exam 2: 11/12 • Lab 7 due Tuesday, 11/06 • Today • Review: pointers • Basic usage • Pointer arithmetic • Common problems • Continue with pointers • Dynamic memory allocation • Destructors ECE 264: Lecture 24

3. Refresher on pointers • Allocators (malloc, new) return pointer to allocated space • Pointer: address of another object • We implicitly use these when we pass function arguments by reference in C++ • Can get address of existing object using & • Can get value of existing pointer using * • Pointer declaration: <base type>* <pointer name> • Base type determines how reference is interpreted • Be careful when declaring multiple pointers • Be sure to initialize pointer before use ECE 264: Lecture 24

4. Refresher on pointers, pt. 2 • Can assign pointers to one another; e.g. int x, *xp, *ip; xp = &x; ip = xp; • Array/pointer duality • Array name is pointer to first element • Can dereference array name (*arr) • Can treat pointers like arrays (ptr[i]) • Can perform pointer arithmetic on both • If ptr and arr point to first element of same array • ptr + i = arr + i = address of element arr[i] • ptr++  moves pointer to next array element • arr++  illegal: can’t change array name ECE 264: Lecture 24

5. Dynamic memory allocation • Up until now, allocated memory statically • Assumed we knew data size at compile time • What if data size is input-dependent and unknown until run time? • In C, dynamic memory allocation handled through mallocand free • In C++, we use newanddelete • Allocator (new) returns pointer to allocated space ECE 264: Lecture 24

6. Allocation with new • In C, malloc()allocates space on heap • malloc(sizeof(int))allocates space for 1 integer • malloc(20*sizeof(int))allocates an array of 20 integers • Both calls return pointer to first byte of element • In C++, we use new • new intallocates space for 1 integer • new int[20] allocates an array of 20 integers • As with malloc(),newreturns pointer to first byte • Directly initialize with value in parentheses • e.g. new int(3) ECE 264: Lecture 24

7. Example int *iPtr; iPtr = new int; // 4 bytes are allocated // iPtr points to 1st byte double *dPtr; dPtr = new double[20]; // 160 bytes allocated // dPtr points to 1st byte heap iPtr dPtr 4 bytes allocated for single int variable 160 bytes allocated for 20 double variables ECE 264: Lecture 24

8. Dynamic allocation example int main() { int *iPtr, *jPtr, i; iPtr = new int; jPtr = new int(3); double *dPtr; dPtr = new double[6]; *iPtr = 7; cout << *iPtr << ',' << *jPtr << endl; for(i=0; i<6; i++) dPtr[i] = 5; for(i=0; i<6; i++) cout << (*dPtr)++ << ' '; cout << endl; for(i=0; i<6; i++) cout << dPtr[i] << ' '; return 0; } OUTPUT 7, 3 5 6 7 8 9 10 11 5 5 5 5 5 Why? ECE 264: Lecture 24

9. Dynamically allocated objects • May want to dynamically allocate objects • Ex. array of Points where size is unknown at compile time Point *pointArray; int numPoints; cin << numPoints; pointArray = new Point[numPoints]; • Ex. linked list data structure—to add an element to the list, must allocate new element class linkedList { private: int elem; linkedList *next; public: void addElement(int i) ... } void linkedList::addElement(int i) { next = new linkedList(i); } ECE 264: Lecture 24

10. Referencing objects through pointers • Recall: use dot operator (.) to reference members of object, e.g: Point p1; p1.setX(2); • With pointers, use -> linkedList *list1 = new linkedList; list1->addElement(2); ECE 264: Lecture 24

11. Deallocation with delete • Space allocated using new should be freed • In C, we used free() • In C++, we use delete • You should only use delete to free memory allocated by new • Any other use will result in an error ECE 264: Lecture 24

12. 100 delete example int *ptr; ptr = new int (100); ptr delete ptr; //free the memory ptr • delete frees space on heap ... • ... but ptr still points to same address! • Solution: assign freed pointers to NULL: • ptr = NULL; ? ECE 264: Lecture 24

13. Example: delete with arrays double *dptr; const int SIZE = 10; dptr = new double[SIZE];//80 bytes for(int i=0; i<SIZE; ++i) cin >> dptr[i]; fun1(dptr, SIZE); // pass array to fun1 delete [] dptr; //free all 10 elements dptr = NULL; ECE 264: Lecture 24

14. Final notes • Next time • Dynamic allocation code examples • Acknowledgements: this lecture borrows heavily from lecture slides provided with the following texts: • Deitel & Deitel, C++ How to Program, 8th ed. • Etter & Ingber, Engineering Problem Solving with C++, 2nd ed. ECE 264: Lecture 24