Pointers & Dynamic Arrays Shinta P. Data Structures in C++ Pointers & Dynamic Arrays Shinta P.

Static and Dynamic Memory Static Allocation allocated by the compiler at compile time once allocated, does not change Dynamic Allocation allocated by program at run time ‘new’ allocates desired amount from heap amount depends on class/type ‘delete’ deallocates an object and returns to storage manager for reallocation

Where or When Static Dynamic data stricture sizes are fixed little chance for storage needs to grow prototypes Dynamic amount of data changes from run to run data relationships change frequently

Pointers a built-in primitive type; 32 bit used to hold the storage address of a variable to define a pointer variable use the * operator in the definition eg. int *airplane_ptr ; (airplane_ptr is a variable that will point to an integer)

Pointers (cont.) To assign a value to the pointer variable use the address operator & eg. int F15; int Airplane_ptr; Airplane_ptr = &F15; Airplane_ptr F15

Pointers (cont.) Note that F15 has not been assigned a value yet to do this we must use the dereferencing operator * ex. *Airplane_ptr = 5 (dereferencing * reads: location pointed to by var) 5 Airplane_ptr F15

Alamat=123f , 4byte Alamat=421f , 4byte Int nilai=50; int *pnilai; cout<<nilai; // cetak 50 cout<<&nilai; // 123f pnilai=&nilai; // pointer pnilai menunjuk alamat nilai cout<<pnilai; // 123f cout<<*pnilai; // 50 nilai =50 *pnilai=123f

char *head; char data1=‘A’; char data2=‘B’; head=&data1; head= &data2; Without pointer char head; char data1=‘A’; char data2=‘B’; head= data1; head= data2; head= ‘C’; cout<<data1; cout<<data2; char *head; char data1=‘A’; char data2=‘B’; head=&data1; head= &data2; *head= ‘C’; cout<<data1; cout<<data2; With pointer

char *head; char data1=‘A’; char data2=‘B’; head=&data1; head= &data2; Without pointer Data1 Data2 A Display: AB Head B char head; char data1=‘A’; char data2=‘B’; head= data1; head= data2; head= ‘C’; cout<<data1; cout<<data2; char *head; char data1=‘A’; char data2=‘B’; head=&data1; head= &data2; *head= ‘C’; cout<<data1; cout<<data2; With pointer

char *head; char data1=‘A’; char data2=‘B’; head=&data1; head= &data2; Without pointer Data1 Data2 A Display: AB Head C char head; char data1=‘A’; char data2=‘B’; head= data1; head= data2; head= ‘C’; cout<<data1; cout<<data2; char *head; char data1=‘A’; char data2=‘B’; head=&data1; head= &data2; *head= ‘C’; cout<<data1; cout<<data2; Display: AC With pointer

Pointers (cont.) ...so whats the big deal?? Which is exactly equivalent to: F15 = 5; ...so whats the big deal??

The Big Deal.... We’ve been looking at the trivial case Pointers to primitives aren’t very useful things get more interesting with arrays we can make : an array that grows as the application needs more room for data an array that shrinks as the application needs less room for data and much better with dynamic objects p1

More Pointers int i = 50; int j = 75; int *p1 ; int * p2 ; p1 = &i ; p2 = & j; cout << *p1; p1 = p2 ; *p2 =0; cout <<*p1; i j 50 75 p2 p1

More Pointers int i = 50; int j = 75; int *p1 ; int * p2 ; p1 = &i ; p2 = & j; cout << *p1; p1 = p2 ; *p2 =0; cout <<*p1; i j 50 Display: 500 p2 p1

int i = 50; int j = 75; int *p1 ; int * p2 ; p1 = &i ; p2 = & j; cout << *p1; p2 = p1 ; *p2 =0; cout <<*p1; i j 75 Display: 500 p2 p1

Pointers to arrays The name of an array is a pointer to the 0th element of the array (the beginning of the array) int array[5] ; // array is equivalent to & array[0] *array = 5; is like array[0] = 5; int j = *(array+4) is like int j = array[1] cout << *array; is like cout << array[0];

Pointers to arrays Pass by reference - in C++ arrays are always pass by reference (there is no pass by value for an array) this is a big improvement over C in C to pass an array to a function it had to be passed by passing a pointer to the beginning of the array then doing pointer arithmetic to manipulate the contentsd of the array

new returns the address of a piece of dynamically allocated storage ex. int *i; //create a pointer i = new int // get a new integer *i = 75 // assign it a value 75 i

Dynamic Arrays arrays can be allocated at run time double * p; int count ; cout << “how many elements? “ << “\n”; cin >> count; p = new double[count];

Dynamic Arrays You can effectively change the size of an array at run-time if it was originally allocated dynamically. … from previous example double * temp; temp = new double[20]; /* copy the contents of the old array into the new one */ for (int I=0 ; I < 10 ; I++) temp[I] = p[I]; /* dispose of the original array */ delete p; p = temp; /* now the array has twice as many elements */

Value semantics The value semantics of a class determine how values are copied from one object to another. In C++ the value semantics consist of two operations: the assignment operator the copy constructor The copy constructor is a constructor that creates and initializes an object to the value of another (existing) object the copy constructor has one parameter whose type is the same as the class name Ex. Date Today; Today.month=5 ; Today.year=2000; Today.day = 21; Date Tomorrow(Today)

Copy Constructor Date :: Date(const & Date t) { month = t.month; day = t.month; year = t.year; }