Chapter 08

1. Chapter 08 Pointers and Pointer-Based Strings (Part I)

2. OBJECTIVES In this part you will learn: • What pointers are. • The similarities and differences between pointers and references and when to use each. • To use pointers to pass arguments to functions by reference.

3. 8.1 Introduction • Pointers • Powerful, but difficult to master • Can be used to perform pass-by-reference • Can be used to create and manipulate dynamic data structures • Close relationship with arrays and strings • Example: pointer-based strings char *

4. 8.2 Pointer Variable Declarations and Initialization • 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) address value a (34) 45 b (28) a (34) 34 45 Pointer variable

5. 8.2 Pointer Variable Declarations and Initialization • Pointer declarations • * indicates variable is a pointer • Example • int *myPtr; • Declares pointer to int, of type int * • Multiple pointers require multiple asterisks int *myPtr1, *myPtr2; int * myPtr; myPtr (28) a (34) 34 45

6. eXERCISE • Examples char *str; double *dbValue; str 123 123 ‘a’ dbValue 54 54 0.001

7. Pointer initialization • Initialized to 0, NULL, or an address • 0 or NULL points to nothing (null pointer) • Example1: int *myPtr = 0; • Example 2: int *myPtr = NULL;

8. 8.3 Pointer Operators • Address operator (&) • Returns memory address of its operand • Example • int y = 5;int *yPtr;yPtr = &y;assigns the address of variable y to pointer variable yPtr • Variable yPtr“points to”y • yPtr indirectly references variable y’s value

9. 8.3 Pointer Operators • Example int y = 5; int *yPtr = &y; cout << y << endl; cout << yPtr << endl; y (14) yPtr 14 5

10. Error-Prevention Tip 8.1 • Initialize pointers to prevent pointing to unknown or uninitialized areas of memory.

11. Question: How to get the value of y from its address?

12. 8.3 Pointer Operators • * operator • Also called indirection operator or dereferencing operator • *yPtr returns y (because yPtr points to y) • Dereferenced pointer is an lvalue *yptr = 9; (equivalent to y = 9;)

13. Example • int y = 5; • int *yPtr = &y; • cout << y << endl; • cout << yPtr << endl; • *yPtr = 9; • cout << y << endl; • cout << yPtr << endl; • y = 12; • cout << *yPtr << endl; • cout << y << endl; y (14) yPtr 14 5 y (14) yPtr y (14) yPtr

14. 8.3 Pointer Operators • * and & are inverses of each other • Will “cancel one another out” when applied consecutively in either order • Example: • int y = 5; • int *yPtr = &y; • cout << y << endl; • cout << *(yPtr) << endl; • cout << *(&y) << endl;

15. Common Programming Error 8.2 • Dereferencing a pointer that has not been properly initialized or that has not been assigned to point to a specific location in memory could cause a fatal execution-time error. • int y = 5; • int *yPtr; • cout << y << endl; • cout << *(yPtr) << endl; Error, because yPtr is not properly initialized.

16. Common Programming Error • An attempt to dereference a variable that is not a pointer is a compilation error. • Dereferencing a null pointer is normally a fatal execution-time error. • int y = 5; • cout << *y << endl; Error, because y is not a pointer variable. • int y = 5; • int *yPtr = NULL; • cout << y << endl; • cout << *(yPtr) << endl; Error, because NULL denotes noting, which is not a valid address.

18. 8.4 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 one value • Arguments passed to a function using reference arguments • Function can modify original values of arguments • More than one value “returned”

19. Pass-by-Value • Example #include <iostream> void swap(int x, int y); int main() { int a, b; cin >> a >> b; swap(a, b); cout << a << “ “ << b; return 0; } void swap(int x, in y) { int temp; cout << x << “ “ << y; temp = x; x = y; y = temp; cout << x << “ “ << y; } a b 4 3 temp 4 x y 4 3 3 4

20. 8.4 Passing Arguments to Functions by Reference with Pointers • Pass-by-reference with pointer arguments • Simulates pass-by-reference • Use pointers and indirection operator. • Pass address of argument using & operator • Arrays not passed with & because array name is already a pointer. • * operator used as alias/nickname for variable inside of function.

21. Passing Arguments to Functions by Reference with Pointers • Example #include <iostream> void swap(int *x, int *y); int main() { int a, b; cin >> a >> b; swap(&a, &b); cout << a << “ “ << b; return 0; } void swap(int *x, in *y) { int temp; cout << *x << “ “ << *y; temp = *x; *x = *y; *y = temp; cout << *x << “ “ << *y; } a (89) b (100) 3 4 4 3 temp 3 x y 89 100

22. Pass-by-Reference • Example #include <iostream> void swap(int &x, int &y); int main() { int a, b; cin >> a >> b; swap(a, b); cout << a << “ “ << b; return 0; } void swap(int &x, in &y) { int temp; cout << x << “ “ << y; temp = x; x = y; y = temp; cout << x << “ “ << y; }

23. Software Engineering Observation 8.1 • Use pass-by-value to pass arguments to a function unless the caller explicitly requires that the called function directly modify the value of the argument variable in the caller. • This is another example of the principle of least privilege.