Chapter 8 Pointers

1. Chapter 8Pointers C++, How to Program Deitel & Deitel CSRU1600 Yanjun Li

2. Pointer Variable Declarations • Pointer variables contain memory addresses as values • Normally, variable contains specific value (direct reference) • Pointers contain address of variable that has specific value (indirect reference) • Example : int *countPtr; //a pointer to int int count; //int variable //pointer variables to double double *xPtr, *yPtr; CSRU1600 Yanjun Li

3. Variable and Pointer Variable • Indirection • Referencing value through pointer CSRU1600 Yanjun Li

4. Pointer Variable Initialization (1) • Initialized to 0, NULL • 0 or NULL points to nothing (null pointer) • 0 is the only integer value that can be assigned directly to a pointer variable without casting the integer to a pointer type first. • Example: int * countPtr; countPtr = 0; CSRU1600 Yanjun Li

5. Pointer Variable Initialization (2) • Initialized to an address • Using the address operator (&) • Example int y = 5; //int variable y int *yPtr; //declare a pointer yPtr to int yPtr = &y; //and assign the address of y to yPtr. CSRU1600 Yanjun Li

6. Pointer Operators • Indirection/Dereferenceing operator(*) • Returns synonym for the object its operand points to • *yPtrreturnsy (becauseyPtrpoints toy) *yptr = 9; -> y = 9; • An attempt to dereference a variable that is not a pointer is a compilation error. • * and & are inverses of each other • *&y -> y • &*yPtr -> yPtr CSRU1600 Yanjun Li

7. Pointer Types int x; x = 12; int* ptr; ptr = &x; std::cout << *ptr; *ptr is the value in the place to which ptr points 2000 12 x 3000 2000 ptr CSRU1600 Yanjun Li

8. Pointer Types int x; x = 12; int* ptr; ptr = &x; *ptr = 5; // changes the value // at adddress ptr to 5 2000 12 5 x 3000 2000 ptr CSRU1600 Yanjun Li

9. Pointer Types 4000 A Z ch 5000 6000 4000 4000 q p char ch; ch = ‘A’; char* q; q = &ch; *q = ‘Z’; char* p; p = q; // the right side has value 4000 // now p and q both point to ch CSRU1600 Yanjun Li

10. Pointers and Arrays (1) • Arrays and pointers are closely related • Array name is like constant pointer • The value saved in an array name is the address of the first element of the array. • We can use pointer as the array name. CSRU1600 Yanjun Li

11. Pointers and Arrays (2) • Accessing array elements with pointers • Assume declarations: int b[ 5 ];int *bPtr; bPtr = b; • Element b[ n ] can be accessed by *( bPtr + n ) • Called pointer/offset notation • Addresses • &b[ 3 ] is same as bPtr + 3 • Array name can be treated as pointer • b[ 3 ] is same as*( b + 3 ) • Pointers can be subscripted (pointer/subscript notation) • bPtr[ 3 ] is same as b[ 3 ] CSRU1600 Yanjun Li

12. Pointer Arithmetic (1) • Pointer arithmetic • Increment/decrement pointer (++ or --) • Add/subtract an integer to/from a pointer (+ or +=,- or -=) • Pointers may be subtracted from each other • Pointer arithmetic is meaningless unless performed on a pointer to an array CSRU1600 Yanjun Li

13. Pointer Assignment • Pointer can be assigned to another pointer if both are of same type • Example int area =1; double *pArea = &area; //wrong CSRU1600 Yanjun Li

14. Passing Arguments to Functions by Reference with Pointers • Three ways to pass arguments to a function • Pass-by-value • Pass-by-reference with reference arguments • Pass-by-reference with pointer arguments • A function can return only zero or one value explicitly. • Arguments passed to a function using reference arguments • Function can modify original values of arguments • More than one value “returned” implicitly. CSRU1600 Yanjun Li

15. Pass-by-reference With Pointer Arguments • Simulates pass-by-reference • Use pointers and indirection operator • Pass address of argument using & operator • * operator used as alias/nickname for variable inside of function • Example void squareByReference(int *); //function prototype //function declaration void squareByReference(int *nPtr) { *nPtr = *nPtr * *nPtr; } int x; squareByReference(&x); //call this function CSRU1600 Yanjun Li

16. sizeofoperator (1) • Returns size of operand in bytes • For arrays, sizeof returns ( size of 1 element ) * ( number of elements ) • If sizeof( int ) returns 4thenint myArray[ 10 ]; cout << sizeof( myArray ); will print 40 • Can be used with • Variable names • Type names • Constant values CSRU1600 Yanjun Li

17. sizeofoperator (2) • Using the sizeof operator in a function to find the size in bytes of an array parameter results in the size in bytes of a pointer, not the size in bytes of the array. • Is performed at compiler-time • For double realArray[ 22 ]; • Use sizeof realArray / sizeof( double ) to calculate the number of elements in realArray • Parentheses are only required if the operand is a type name CSRU1600 Yanjun Li

18. Reference • Reproduced from the Cyber Classroom for C++, How to Program, 5/e by Deitel & Deitel. • Reproduced by permission of Pearson Education, Inc. CSRU1600 Yanjun Li