1. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 1 - Pointers and More Pointers Pointers and Arrays We’ve now looked at  Pointers  Arrays  Strings Let’s now see how all these tie together Also look at advanced pointer concepts

2. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 2 - Pointers and More Pointers Pointers and Arrays Let’s begin with look at the relationship between pointers and arrays Recall that an array is collection of values of a single data type

3. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 3 - Pointers and More Pointers Pointers and Arrays Consider an array of ints, a [ ], stored in memory By definition, the value of a variable of type array Is the address of the 0th element For our array, the following are equivalent &a [ 0 ] a Thus, If a [ ] is loaded at address 3000 Then &a[0] = a = 3000

4. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 4 - Pointers and More Pointers Pointers and Arrays Such an equivalence allows us to write A bit more surprising is a [ j ] is equivalent to *(a + j) Should not be….let’s see why int a[10]; // declare an array of 10 ints int *pa; // declare a pointer to an int pa = a; // assign the address of a[ ] to the pointer int a[10]; // declare an array of 10 ints int *pa; // declare a pointer to an int pa = a; // assign the address of a[ ] to the pointer

5. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 5 - Pointers and More Pointers Pointers and Arrays We know a + j is the address of the jth element beyond a We have also seen a [ j ] = a + j * sizeof (an element of a) We have several ways of addressing an array int a[ ] = {1, 2, 3, 4, 5}; int *pa = a; a[ i ] *(pa + i) *pa++ (*pa)++ // which increments the contents We have several ways of addressing an array int a[ ] = {1, 2, 3, 4, 5}; int *pa = a; a[ i ] *(pa + i) *pa++ (*pa)++ // which increments the contents

6. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 6 - Pointers and More Pointers Pointers and Arrays Multi Dimensional Arrays We’ve been looking at arrays in one dimension It’s Possible to declare multiple dimensional arrays Our arrays have had one column and multiple rows We can extend these by simply adding more columns It all reduces to managing pointers We’ll start with 2 dimensions Then briefly explore larger arrays

7. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 7 - Pointers and More Pointers Pointers and Arrays Multi Dimensional Arrays Let’s go back to the basics We know an array is collection of values Single data type is represented as Then stored in memory as

8. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 8 - Pointers and More Pointers Pointers and Arrays Multi Dimensional Arrays Array can be specified such that the elements are objects of any type including pointers or addresses If a single dimensional array contains pointers to other arrays, we can easily build up a two dimensional array We specify array by its number of Rows Columns An array’s dimension refers to the number of columns

9. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 9 - Pointers and More Pointers Pointers and Arrays Two Dimensional Arrays The simplest complex array is two dimensional Its elements are one dimensional arrays syntax: type arrayName [ rowSize ] [ columnSize ] syntax: type arrayName [ rowSize ] [ columnSize ]

10. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 10 - Pointers and More Pointers Pointers and Arrays Two Dimensional Arrays We can write int year [12][31] The declaration says that year is Array of 12 elements Which we can use to express the months Array of 31 ints Which we can use to express the days of the month

11. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 11 - Pointers and More Pointers Pointers and Arrays Two Dimensional Arrays For two dimensional array, we can access a particular element by specifying the row and column To To get the 7th month and 13th day….. ….We write year[7][13] As expected….. First index identifies the row Second index specifies column in that row

12. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 12 - Pointers and More Pointers Pointers and Arrays Two Dimensional Array Implementation Two dimensional arrays are implemented as an array of pointers to arrays b[m][n] They are stored as an array of m pointers to arrays with n elements each Elements of each constituent array are stored in adjacent memory locations

13. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 13 - Pointers and More Pointers Pointers and Arrays Two Dimensional Array Implementation

14. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 14 - Pointers and More Pointers Pointers and Arrays Two Dimensional Array Implementation Two dimensional array differ from simple array of pointers to arrays  The declaration of two dimensional array Initializes the main array to point to component arrays All component arrays are same size  Array of pointers Must be initialized by programmer  All component arrays may be different sizes Sometimes referred to as a ragged array

15. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 15 - Pointers and More Pointers Pointers and Arrays Two Dimensional Array Implementation For array a[2] [3], the elements stored as a[0][0], a[0][1], a[0][2], a[1[0], a[1[1], a[1][2] Element a[1][2] is addressed as: *(*(a + 1) + 2) 1. a is 2 by 3 array. It points to first element of a 3 element subarray 2. a + 1 points to the second 3 element subarray 3. *(a + 1) points to the first element of the second 3 element subarray 4. *(a + 1) + 2 points to the third element of the second subarray 5. *(*(a + 1) + 2) is third element in second subarray 1. a is 2 by 3 array. It points to first element of a 3 element subarray 2. a + 1 points to the second 3 element subarray 3. *(a + 1) points to the first element of the second 3 element subarray 4. *(a + 1) + 2 points to the third element of the second subarray 5. *(*(a + 1) + 2) is third element in second subarray

16. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 16 - Pointers and More Pointers Pointers and Arrays Two Dimensional Array Implementation In general a two-dimensional arrays are accessed by  Array notation or  Pointer notation  Array notation is used when dealing with the array itself and  Pointer notation is used when all you have is the address of the array The address of any element in our example array is guaranteed to be (a + 3*i + j)

17. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 17 - Pointers and More Pointers Pointers and Arrays Dynamic Allocation of Two Dimensional Arrays Two-dimensional arrays can be allocated on the heap with the new operator with following syntax: The allocation defines x as a pointer to i arrays of j elements int (* x)[j] = new int[i][j];

18. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 18 - Pointers and More Pointers Pointers and Arrays Deleting Two Dimensional Arrays When dynamically allocated two-dimensional arrays are deleted….. ….The delete operator requires only one [ ] regardless of the number of dimensions in the array delete [] x;//Correct. delete [][] x;//Error. Use only one []

19. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 19 - Pointers and More Pointers Pointers and Arrays Multidimensional Dimensional Arrays C and C++ place no restriction on the number of dimensions an array may have Multidimensional arrays are stored in row major order Elements differing only in last subscript are stored adjacently

20. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 20 - Pointers and More Pointers Pointers and Arrays Multidimensional Dimensional Arrays Pointer conversion and addressing done as in two dimensional arrays only in more dimensions In General an expression of form…. i by j by k ….is converted to the form pointer to j by k

21. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 21 - Pointers and More Pointers Pointers and Arrays Multidimensional Dimensional Arrays The mechanics of the two-dimensional arrays can be extended to cover arrays with more dimensions... For following 3 dimensional array int a[2][3][4]; The address of an element (&a[i][j][k]) Is guaranteed to be (array + 2*3*i + 3*j + k)

22. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 22 - Pointers and More Pointers Pointers and Arrays Generic and Null Pointers The pointers we’ve dealt with point to a variable of a particular type Often we would like to use same pointer to point to a variable of any data type In C++ will see this is a simple form of polymorphism ANSI C++ introduces the generic pointer

23. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 23 - Pointers and More Pointers Pointers and Arrays Generic Pointers The generic pointer is a pointer to type void void * void * pointer guaranteed to be large enough to hold pointer to any type of object except a function type Such a pointer can be converted to type void * and back again with no loss of information

24. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 24 - Pointers and More Pointers Pointers and Arrays Generic Pointers A generic pointer cannot be dereferenced with these operators * [ ] subscript On many machines all pointers are the same size However to ensure max portability because of potentially different sizes void * pointer must converted back to pointer of appropriate type before dereferencing

25. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 25 - Pointers and More Pointers Pointers and Arrays Dereferencing Generic Pointers void * myGenericPtr; int myValue = 3; myGenericPtr = &myValue; int tempValue = *myGenericPtr; // illegal void * myGenericPtr; int myValue = 3; myGenericPtr = &myValue; int tempValue = *myGenericPtr; // illegal int a = 9; int *intPtr = &a;// point to a cout << *intPtr << endl;// prints 9 void* genPtr = static_cast (&a);// preferred C++ style cast int* intPtr2 = static_cast (genPtr); // preferred C++ style cast cout << *intPtr2 << endl; // prints 9 int a = 9; int *intPtr = &a;// point to a cout << *intPtr << endl;// prints 9 void* genPtr = static_cast (&a);// preferred C++ style cast int* intPtr2 = static_cast (genPtr); // preferred C++ style cast cout << *intPtr2 << endl; // prints 9

26. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 26 - Pointers and More Pointers Pointers and Arrays Null Pointers Dereferencing a pointer with No assigned value gives undetermined results A value of 0 is Illegal

27. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 27 - Pointers and More Pointers Pointers and Arrays Null Pointers  When declared, local pointers contain whatever values were in memory at time…. ….. dereferencing treats that value as an address  Global and Static Pointers are initialized to 0….. …..dereferencing usually gives runtime error and program termination

28. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 28 - Pointers and More Pointers Pointers and Arrays Null Pointers ANSI C++ offers solution and defines a special pointer null pointer Its value points to no object or function (void *) 0 Special macro NULL defined in header file Definition depends upon compiler and operating system Often defined as NULL ((void *) 0 )

29. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 29 - Pointers and More Pointers Pointers and Arrays Function Pointers The pointers we’ve discussed so far have been pointers to data We can also define pointers to functions or function pointers syntax return type (* functionPointer) ( ) arg list may be empty syntax return type (* functionPointer) ( ) arg list may be empty

30. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 30 - Pointers and More Pointers Pointers and Arrays Function Pointers int (* intFunctPtr) (); Read as intFunctPtr is  A pointer to a function  Taking no args  Returning an int int (* intFunctPtr) (); Read as intFunctPtr is  A pointer to a function  Taking no args  Returning an int double (* doubleFunctPtr) (int, char); Read as doubleFunctPtr is  A pointer to a function  Taking two args: an int and a char  Returning a double double (* doubleFunctPtr) (int, char); Read as doubleFunctPtr is  A pointer to a function  Taking two args: an int and a char  Returning a double

31. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 31 - Pointers and More Pointers Pointers and Arrays Function Pointers How do we obtain a pointer to function…. ….There are several ways  Use address of operator, &  Use function name with no parameter list int (* intFunctPtr) (); int myFunction(); intFunctPtr = &myFunction; or intFunctPtr = myFunction; int (* intFunctPtr) (); int myFunction(); intFunctPtr = &myFunction; or intFunctPtr = myFunction;

32. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 32 - Pointers and More Pointers Pointers and Arrays Dereferencing Function Pointers We can also dereference a function pointer in several ways (* functPtr)( argument-list) functPtr(argument-list) double (*doubleFunctPtr)(int, char); double yourFunction (int, char) doubleFunctPtr = yourFunction; (*doubleFunctPtr)(3, ‘b’); int (* intFunctPtr) (); int myFunction(); intFunctPtr = myFunction; (*intFunctPtr)();

33. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 33 - Pointers and More Pointers Pointers and Arrays Function Pointers Function and Data pointers may have significantly different representations  Based upon machine architecture  Byte Ordering  Alignment requirements  Sizes  Memory models Conversion between function and data pointers in ANSI C++ Has undefined behavior Conversion to or from type void * in ANSI C++ No guarantee of safety

34. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 34 - Pointers and More Pointers Pointers and Arrays Indirect Reference In addition to address of a variable a pointer variable can contain the address of another pointer ….We have pointer to pointer To retrieve the value we must apply dereference operator twice syntax type ** pointerName

35. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 35 - Pointers and More Pointers Pointers and Arrays Indirect Reference Let’s declare an integer int myInt = 3 A pointer to an int int* myIntPrt = &myInt; …and a pointer to that pointer int** myIntPtrPtr = &myIntPtr; Let myInt be stored at memory address 4000 myIntPtrPtr be stored at address 3000 myIntPtr be stored at address 3020

36. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 36 - Pointers and More Pointers Pointers and Arrays Indirect Reference Thus the expression int myValue = ** myIntPtrPtr Assigns the value 3 to the variable myValue Since myIntPtrPtr is pointing to 1. Memory location 3020 2. Which points to memory location 4000 3. Which contains the value 0003 Since myIntPtrPtr is pointing to 1. Memory location 3020 2. Which points to memory location 4000 3. Which contains the value 0003

37. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 37 - Pointers and More Pointers Pointers and Arrays Indirect Reference Similarly int myValue = 4; **myIntPtrPtr = myValue; Places the value 4 into memory location 4000

38. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 38 - Pointers and More Pointers Pointers and Arrays Reference Types C++ introduces an additional way of referring to an object Called reference A reference serves as an alias or alternate name for an object

39. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 39 - Pointers and More Pointers Pointers and Arrays Reference Types Rules for references A reference Must be initialized when created like a const A pointer initialized any time Once initialized to an object it cannot be changed to refer to another object A pointer can be pointed to different object anytime syntax type & varName = aValue

40. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 40 - Pointers and More Pointers Pointers and Arrays Reference Types Cannot  Have NULL reference  Point to reference  Have an array of references  Take the address of a reference  Compare references  Assign to them  Do arithmetic on them

41. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 41 - Pointers and More Pointers Pointers and Arrays Initialization of Reference Types A non const - only initialized to lvalue of exact type lvalue is value that can appear on left hand side of an assignment statement unsigned char mine; int d1, d2; int &a = 1024;// illegal - not lvalue char &b = mine;// illegal - not exact type int &c = d1 + d2;// illegal - not lvalue

42. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 42 - Pointers and More Pointers Pointers and Arrays Initialization of Reference Types const - can have an rvalue that is an lvalue not of the exact type rvalue can only appear on right hand side of an assignment statement unsigned char mine; int d1, d2; const int = 1024;// legal const char &b = mine;// legal const int &c = d1 + d2;// legal

43. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 43 - Pointers and More Pointers Pointers and Arrays References vs. Pointers Do not support pointer like declarations Cannot have arrays of references Reference to arrays We can’t declare an array of references….. int& v[ ];// array of references to ints ….because v[i]  *(v + i)// pointer to a reference We can’t declare an array of references….. int& v[ ];// array of references to ints ….because v[i]  *(v + i)// pointer to a reference

44. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 44 - Pointers and More Pointers Pointers and Arrays References as Parameters References can be used as parameters in a function call Pass by value... Function never accesses the actual arguments Manipulates local copy stored on the stack Changes made to local copies not always suitable  Large objects passed  args must be modified Pass by value... Function never accesses the actual arguments Manipulates local copy stored on the stack Changes made to local copies not always suitable  Large objects passed  args must be modified Pass by reference…. Function accesses actual arguments Address of args is passed If we don’t want arg to be changed Use const specifier Pass by reference…. Function accesses actual arguments Address of args is passed If we don’t want arg to be changed Use const specifier

45. Copyright 2001 Oxford Consulting, Ltd1 January 2001 - 45 - Pointers and More Pointers Pointers and Arrays References as Parameters void incrementR (int &number); void incrementV (int number); int main(void) { int number = 10; int number1 = 20; incrementR(number); cout << number << endl; // Will print 11 incrementV (number1); cout << number1 << endl; // Will print 20 return 0; } void incrementR (int &number); void incrementV (int number); int main(void) { int number = 10; int number1 = 20; incrementR(number); cout << number << endl; // Will print 11 incrementV (number1); cout << number1 << endl; // Will print 20 return 0; } void incrementR (int &aNumber) { aNumber++; // Passed by reference } void incrementV (int aNumber) { aNumber++; // Passed by value } void incrementR (int &aNumber) { aNumber++; // Passed by reference } void incrementV (int aNumber) { aNumber++; // Passed by value }