1. EECE.3220 Data Structures Instructor: Dr. Michael Geiger Spring 2019
Lecture 18:
Dynamic allocation

2. Announcements/reminders
Program 2 due 3/21
Exam 2: Monday, 4/1, 3-5 PM, Ball 214
Use same poll as before to request alt. exam (I’ll repost link)
Today’s lecture
Dynamic allocation
4/26/2019
Data Structures: Lecture 18

3. Data Structures: Lecture 18
Refresher on pointers
Allocators (malloc, new) return pointer to allocated space
Pointer: address of another object
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
4/26/2019
Data Structures: Lecture 18

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])
4/26/2019
Data Structures: Lecture 18

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 malloc and free
In C++, we use new and delete
Allocator (new) returns pointer to allocated space
4/26/2019
Data Structures: Lecture 18

6. Data Structures: Lecture 18
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 int allocates space for 1 integer
new int[20] allocates an array of 20 integers
As with malloc(), new returns pointer to first byte
Directly initialize with value in parentheses
e.g. new int(3)
4/26/2019
Data Structures: Lecture 18

7. Data Structures: Lecture 18
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 ? …
4 bytes allocated for single int variable
dPtr
160 bytes allocated for 20 double variables
4/26/2019
Data Structures: Lecture 18

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; cout << (*dPtr)++ << ' '; cout << endl; cout << dPtr[i] << ' '; return 0; }
OUTPUT
7, 3
Why?
4/26/2019
Data Structures: Lecture 18

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:
linkedList();
linkedList(int val);
void addElement(int i);
... }
void linkedList::addElement(int i) {
next = new linkedList(i);
4/26/2019
Data Structures: Lecture 18

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);
4/26/2019
Data Structures: Lecture 18

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
4/26/2019
Data Structures: Lecture 18

12. Data Structures: Lecture 18
delete example
int *ptr;
ptr = new int (100);
ptr
100
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;
4/26/2019
Data Structures: Lecture 18

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;
4/26/2019
Data Structures: Lecture 18

14. Data Structures: Lecture 18
Final notes
Next time
Stacks
Reminders:
Program 2 due 3/21
Exam 2: Monday, 4/1, 3-5 PM, Ball 214
Use same poll as before to request alt. exam (I’ll repost link)
4/26/2019
Data Structures: Lecture 18