1. Pointers: Part I

2. Why pointers? - low-level, but efficient manipulation of memory - dynamic objects  Objects whose memory is allocated during program execution. ( Dynamic objects can survive after the function ends in which they were allocated).  Dynamic objects allow flexible-sized arrays and lists

3. Computer Memory and Variables  A variable is a symbolic name assigned to some memory storage  The size of this storage depends on the type of the variable: one byte char, 4 bytes int  The name of variable is in ‘symbol table’ by compiler: each symbol is associated with its type, location, scope …  So, the compiler manages its location  good for programmers! 100 …… Memory address:1024 … 1020 Symbol table a: int, 1024, … int a=100;

4. Pointers  A pointer is a variable used for storing the address of a memory cell.  We can use the pointer to reference this memory cell, so to ‘manipulate’ it! 100 ……1024… Memory address:1024 10032 … 1020 Integer a Pointer p n Object whose value represents the location (memory address) of another object int a; int* p;

5. Define a pointer type variable Examples of (uninitialized) pointers int* ip; char* s; int *ip; // ip is a pointer to an int char *s; // s is a pointer to a char TypeName *variablename; int *p; Each type of object (variable) has a pointer type, therefore a pointer variable of that type. TypeName* variablename; int* p; equivalent

6. Getting an address: address operator & int a=100; “&a”  “the address of a” 100………… Memory address:1024 int a = 100; cout << a;  100 Cout << &a;  1024 … 1020 a

7. Store the address in a pointer variable  The value of pointer p is the address of variable a 10088…1024… Memory address:102410032 … 1020 ap int a = 100; int* p = &a; cout << a << " " << &a <<endl; cout << p << " " << &p <<endl; Result is: 100 1024 1024 10032 p is pointing to a  A pointer is also a variable, so it has its own memory address

8. Dereferencing Operator *  We can access to the value stored in the variable pointed to by preceding the pointer with the “star” operator (*), 10088…1024… Memory address:102410032 … 1020 int a = 100; int* p = &a; cout << a << endl; cout << &a << endl; cout << p << " " << *p << endl; cout << &p << endl; a p *p gives 100

9. An asterisk (‘*’) has two usages  In a definition, an asterisk indicates that the object is a pointer. char* s; // s is of type pointer to char (char *s; is possible)  In expressions, an asterisk before a pointer indicates the object the pointer pointed to, called dereferencing int i = 1, j; int* ptr; // ptr is an int pointer ptr = &i; // ptr points to i j = *ptr + 1;// j is assigned 2 cout << *ptr << j << endl; // display " 12 "

10. Summary on two operators * and & * has two usages: - pointer - dereferencing & has two usages: - getting address - reference (‘call by ref’, see later)

11. Null Address  0 is a pointer constant that represents the empty or null address  Indicates that pointer is not pointing to storage of a valid object  Cannot dereference a pointer whose value is null int* ptr; ptr = 0; cout << *ptr << endl; // invalid, ptr // does not point to // a valid int

12. Example #include using namespace std; int main (){ int value1 = 5, value2 = 15; int* p1; int* p2; p1 = &value1; // p1 = address of value1 p2 = &value2; // p2 = address of value2 *p1 = 10; // value pointed to by p1=10 *p2 = *p1; // value pointed to by p2= value // pointed to by p1 p1 = p2; // p1 = p2 (pointer value copied) *p1 = 20; // value pointed to by p1 = 20 cout << "value1==" << value1 << "/ value2==" << value2; return 0; } Result: Value1 is 10 Value2 is 20

13. Another Pointer Example int a = 3; char s = ‘z’; double d = 1.03; int* pa = &a; char* ps = &s; double* pd = &d; cout << sizeof(pa) << sizeof(*pa) << sizeof(&pa) << endl; cout << sizeof(ps) << sizeof(*ps) << sizeof(&ps) << endl; cout << sizeof(pd) << sizeof(*pd) << sizeof(&pd) << endl;

14. Writing pointer type properly in C++ … int *a, *b; a, b are both integer pointers int* a, b; a is integer pointer, b is just integer! typedef int* IntPt; IntPt a, b; typedef int MyInt; MyInt k; int k; int* a; int* b; ? Recommended:

15. Traditional Pointer Usage void swap(char* ptr1, char* ptr2){ char temp = *ptr1; *ptr1 = *ptr2; *ptr2 = temp; } int main() { char a = 'y'; char b = 'n'; swap(&a, &b); cout << a << b << endl; return 0; } Use pass-by-value of pointers to ‘change’ variable values C language does not have ‘call by reference’, only ‘call by value’!

16. Reference: X& l int& b a; l b is an alternative name for a void f(int& b) {}; int main() { int a; f(a); }

17. Pass by reference (better than ‘pointers’) void swap(char& y, char& z) { char temp = y; y = z; z = temp; } int main() { char a = 'y'; char b = 'n'; swap(a, b); cout << a << b << endl; return 0; } y, z are ‘references’, only names, not like ptr1, ptr2 that are variables

18. it is also possible (not recommended)… void swap(char &y, char &z) { char temp = y; y = z; z = temp; } int main() { char a = 'y'; char b = 'n'; swap(a, b); cout << a << b << endl; return 0; } This is just an alternative way of writing ‘reference’.

19. Pointers and arrays

20. Arrays are pointers! The name of an array points only to the first element not the whole array. 1000 1012 1016 1004 1008

21. #include Using namespace std; void main (){ // Demonstrate array name is a pointer constant int a[5]; cout << "Address of a[0]: " << &a[0] << endl << "Name as pointer: " << a << endl; } /* result: Address of a[0]: 0x0065FDE4 Name as pointer: 0x0065FDE4 */ Array name is a pointer constant

22. Result is: 2 #include Using namespace std; void main(){ int a[5] = {2,4,6,8,22}; cout << *a << " " << a[0] << " " << *(&a[0]);..." } //main 2 4 8 6 22 a[4] a[0] a[2] a[1] a[3] Dereference of an array name This element is called a[0] or *a

23. To access an array, any pointer to the first element can be used instead of the name of the array. We could replace *p by *a #include Using namespace std; void main(){ int a[5] = {2,4,6,8,22}; int* p = a; int i = 0; cout << a[i] << " " << *p;... } 2 4 8 6 22 a[4] a[0] a[2] a[1] a[3] a p a Array name as pointer 2 2

24. Multiple Array Pointers Both a and p are pointers to the same array. 2 4 #include Using namespace std; void main(){ int a[5] = {2,4,6,8,22}; int* p = &a[1]; cout << a[0] << " " << p[-1]; cout << a[1] << " " << p[0];... } 2 4 8 6 22 a[4] a[0] a[2] a[1] a[3] p P[0] A[0]

25. Pointer Arithmetic Given a pointer p, p+n refers to the element that is offset from p by n positions. 2 4 8 6 22 a a + 2 a + 4 a + 3 a + 1 p p + 2 p + 3 p - 1 p + 1

26. *(a+n) is identical to a[n] Dereferencing Array Pointers a[3] or *(a + 3) 2 4 8 6 22 a a + 2 a + 4 a + 3 a + 1 a[2] or *(a + 2) a[1] or *(a + 1) a[0] or *(a + 0) a[4] or *(a + 4)

27. Summary * two usages: - pointer type definition: int a; int* p; - dereferencing: *p is an integer variable if p = &a; & two usages: - getting address: p = &a; - reference: int& b a; b is an alternative name for a First application in passing parameters (‘swap’ example) int a=10; int b=100; int* p; int* q; p=&a; q=&b; p = q; *p = *q; ? ?

28. A[n]: like a vector, an array *(a+n): like a ‘sequence’ or a ‘list’ Two ways of manipulating an array