C++ Pointers

1. C++ Pointers Gordon College

2. 27 Regular variables • Regular variables declared • Memory allocated for value of specified type • Variable name associated with that memory location • Memory initialized with values provided (if any)

3. Pointers • Pointer Variables • contains a memory address • pointer variable should be typed int *Ptr • However could be a void pointer void *Vptr • two operators: • Dereference * • Address of &

4. Basic Pointer Operations • Dereferencing and indirection • Pointer variable stores address of a location • Accessing contents of that location requires dereferencing operator * cout << *iPtr; Dereferencing pointer int *iPtr = &i; Pointer declaration PointerFun Video

5. Basic Pointer Operations • Assignment • Pointer variables can be assigned the values of other pointer variables bound to same type int *iPtr = &i; int *jPtr = &j; jPtr = iPtr;

6. Basic Pointer Operations • Consider: *jPtr = 44; • Changes value that both pointers reference • Not good programming practice, hard to debug • Known as aliasing problem

7. Basic Pointer Operations • Comparison • Relational operators used to compare two pointers • Must be bound to same type • Most common == and != • The null address may be compared with any pointer variable int* t; double *d; if (t == d) … ERROR Fix: if (t == (int*) d)

8. Initial value for pointers • Variables are not automatically given an initial value by the system - they start with whatever garbage is left in memory when they are allocated. #include <cstdlib> . . . Node* head = NULL; // Initialized pointer to NULL. WARNING: Invalid addresses in pointer can cause problems - known as “wild pointers” when not initialized

9. Pointers vs. References • Consider this code: int i; int i; int *p = &i; int &r = i; *p = 5; r = 5; Both p and r contain addresses which point to the variable i, however these “addressing” variables are used differently in code. Why bother with pointers?

10. Pointers vs. References • Pointer arithmetic i.e. p++ p-- ptr = sptr + 1; char* GetFirstT(char* p) { for ( ; *p ; ++p) { if ( *p == 't' ) return p; } return 0; } signedmain() { char the_alphabet[] = "abcdefghijklmnopqrstuvwxyz"; char* p_t = GetFirstT(the_alphabet); cout << p_t - the_alphabet << endl; } OUTPUT: 19

11. Dynamic Memory Allocation • The new operation • Exampleint * intPtr; intPtr = new int; • An anonymous variable • Cannot be accessed directly • Warning: watch out for memory leak

12. Memory Management Problems • Memory leak - program fails to release memory when no longer needed { int *t = newint(4); } Solution: must use the delete command before existing block • Dangling Pointer - an object is deleted or deallocated, without modifying the value of the pointer. char *cp = NULL; { char c; cp = &c; } Solution: set pointer to NULL before exiting block

13. Pointer Arguments • Pointers can be passed as arguments to functions • This is logically equivalent to reference parameters • In fact, this is how early C++ compilers accomplished reference parameters

14. Pointer to a Pointer int cow(7); int* p_cow = &cow; int** p_p_cow(&p_cow); int*** p_p_p_cow = &p_p_cow; Name of Variable Type of Variable Address in Memory Value Stored Cow Int 108 7 p_cow int* 110 108 p_p_cow int** 112 110 p_p_p_cow int*** 114 112 int cow(7); int* p_cow = &cow; int** p_p_cow(&p_cow); int*** p_p_p_cow = &p_p_cow; ***p_p_p_cow = 8;

15. Pointer to objects • Declaration: classType *name = &object • Accessing the object: (*name).method(); name->method();

16. Pointers to objects (from lab2) Pet * kennel[10];// The cages in the kennel char dashes[] = "----------------------------------------"; kennel[0] = new Rodent("Mickey", "W. Disney", "01/01/2002", "01/04/2002","mouse"); kennel[1] = new Bird("Tweety", "Granny", "9.1.2001", "9/5/2001","canary"); kennel[2] = new Reptile("Ignatius", "B. Starr", "10/07/2001", "10/10/2001","iguana"); kennel[3] = new Cat("Garfield", "J. Arbuckle", "01/01/2002", "01/31/2002",20, true); kennel[4] = new Dog("Snoopy", "C. Brown", "01/01/2002", "01/02/2002",15, false, "beagle"); kennel[5] = new Cat("Sylvester", "Granny", "09/01/2001", "09/05/2001",15, false); kennel[6] = new Dog("Butch", "Granny", "09/01/2001", "09/05/2001",45, true, "bulldog"); kennel[9] = kennel[8] = kennel[7] = NULL; for (int i = 0; i < 10 && kennel[i] != NULL; i++) { cout << endl << dashes << dashes << endl << endl; cout << "Bill for: " << kennel[i] -> getName() << endl << endl; kennel[i] -> printBill(); cout << endl << dashes << dashes << endl; }