Programming Fundamental. Instructor Name: Lecture-18

Transcription

1 Programming Fundamental Instructor Name: Lecture-18

2 Today s Lecture Pointers 2

3 Introduction Pointers Pointers are a special type of variables in which a memory address is stored. They contain a memory address, not the value of the variable. We have already seen that we can directly assign a value to a variable and as well as we have used addresses to assign value in case of call by reference. So, two ways to assign values direct reference and indirect reference. In direct reference a value is directly assigned to variable In, indirect reference value is referred through the use of memory address Pointers, like any other variables, must be declared before they can be used 3

4 Pointers Introduction Memory address: integer pointer 4

5 Pointers & Address Operator Each variable in a program is stored at a unique address in memory Use the address operator & to get the address of a variable: int num = -23; cout << &num; // prints address // in hexadecimal The address of a memory location is a pointer 5

6 Why use Pointers? Pointers are useful for: Working with memory locations that regular variables don t give you access to Working with strings and arrays Creating new variables in memory while the program is running Creating arbitrarily-sized lists of values in memory 6

7 Pointers Declaration Pointers work by pointing to a particular data type. We can have pointer to an integer, pointer to a double, pointer to a character and so on. The syntax of declaring a pointer is: Pointer s data type *name ; Here name is the name of the pointer and data type is the type of the data to which the pointer (name) points. There is no space between asterisk (*) and the name. Each variable being declared as a pointer must be preceded by *. int *myptr; The easiest way to read pointers variables is from right to left. For above declaration, we say myptr is pointer to an integer. 7

8 Pointers Initializing Pointers Can initialize to NULL or 0 (zero) int *ptr = NULL; A pointer with the value 0 or NULL points to nothing and is known as a null pointer. Can initialize to addresses of other variables int num, *numptr = &num; int val[isize], *valptr = val; Initial value must have correct type float cost; int *ptr = &cost; // won't work 8

9 Pointers What address a pointer holds Suppose, we declare a pointer variable ptr and a variable x and assign a value 10 to it. We write this as under. int *ptr ; int x ; x = 25 ; Here x is a name of a memory location where a value 10 is stored. We want to store the address of this memory location (which is labeled as x) into the pointer ptr. x ptr = &x ; ptr 0x7e00 25 Address of x: 0x7e00 9

10 Pointers Example1 OUTPUT 10

11 Pointers Example2 OUTPUT 11

12 Pointers Example3 The code void doubleit(int x, int * p) { } *p = 2 * x; int main() { } int a = 16; doubleit(9, &a); return 0; a gets 18 Box diagram main a 16 doubleit x 9 p p (8200) x (8196) a (8192) Memory Layout doubleit main 12

13 Pointer to Pointer Pointers If a pointer pointing to address of another pointer then it is called pointer to pointer. What is the output?

14 Pointers Dereferencing a Pointer An interesting property of pointers is that they can be used to access the variable they point to directly. This is done by preceding the pointer name with the dereference operator (*). The operator itself can be read as "value pointed to by". A dereference pointer can be used on the left hand side of an assignment statement *yptr=9; A dereference pointer may be used to receive an input value as cin>>*yptr; 14

15 Pointers Dereferencing a Pointer Consider the following price of code int x=25,baz,*foo; foo=&x; baz=*foo; This could be read as: "baz equal to value pointed to by foo", and the statement would actually assign the value 25 to baz, since foo is 1776, and the value pointed to by 1776 would be 25. The asterisk (*) used when declaring a pointer only means that it is a pointer and should not be confused with the dereference operator (*). They are simply two different things represented with the same sign. 15

16 Pointers & Arrays Relationship Between Pointer & Arrays Array names can be used as pointer and vice versa The name of the array is a constant pointer which contains the memory address of the first element of the array. To access an array, any pointer to the first element can be used instead of the name of the array. Example Output 10 int numbers[] = {10, 20, 30, 40, 50}; cout << "The first element of the array is "; cout << *numbers << endl; 16

17 Pointers & Arrays Relationship Between Pointer & Arrays Array name is starting address of array int vals[] = {4, 7, 11}; starting address of vals: 0x4a00 cout << vals; // displays 0x4a00 cout << vals[0]; // displays 4 Array name can be used as a pointer constant cout << *vals; // displays 4 Pointer can be used as an array name int *valptr = vals; cout << valptr[1]; // displays 7 17

18 Pointers & Arrays Pointers in Expression Given: int vals[]={4,7,11}; int *valptr = vals; What is valptr + 1? It means (address in valptr) + (1 * size of an int) cout << *(valptr+1); // displays 7 cout << *(valptr+2); // displays 11 Must use ( ) in expression 18

19 Pointers & Arrays Pointer Arithmetic Some arithmetic operators can be used with pointers: Increment and decrement operators ++, -- Integers can be added to or subtracted from pointers using the operators +, -, +=, and -= One pointer can be subtracted from another by using the subtraction operator Assume the variable definitions int vals[]={4,7,11}; int *valptr = vals; Examples of use of ++ and valptr++; // points at 7 valptr--; // now points at 4 19

20 Pointers & Arrays Pointer Arithmetic Example #include <iostream> using namespace std; main() { int x =10; int *yptr; yptr = &x; cout << The address yptr points to = << yptr << endl ; cout << The contents yptr points to = << *yptr << endl; (*yptr) ++; cout << After increment, the contents are << *yptr << endl; cout << The value of x is = << x << endl; } 20

21 Pointers & Arrays Pointer Arithmetic Assume the variable definitions: int vals[]={4,7,11}; int *valptr = vals; Example of the use of + to add an int to a pointer: cout << *(valptr + 2) This statement will print 11 Example of use of +=: valptr = vals; // points at 4 valptr += 2; // points at 11 21

22 Pointer Arithmetic Pointers & Arrays We have two pointers yptr1 and yptr2 to integer and written as yptr1 + yptr2 ; The compiler will show an error in this statement. But we can subtract pointers main () { int y[10], *yptr1, *yptr2; yptr1 = &y[0]; yptr2 = &y[3]; cout << " The difference of " <<yptr2<<" - "<<yptr1 <<" = "<<yptr2 - yptr1; } 22

23 Comparing Pointers Pointers & Arrays Relational operators can be used to compare addresses in pointers Comparing addresses in pointers is not the same as comparing contents pointed at by pointers: if (ptr1 == ptr2) // compares addresses if (*ptr1 == *ptr2) // compares contents If one address comes before another address in memory, the first address is considered less than the second. array[0] array[1] array[2] array[3] array[4] (Addresses) 0x5A00 0x5A04 0x5A08 0x5A0C 0x5A0F 23

24 Pointers & Functions Pointers as Function Parameter There are three ways in C++ to pass arguments to a function Pass-by-value pass-by-reference with reference arguments pass-by-reference with pointer arguments Reference arguments also enable programs to pass large data objects to a function and avoid the overhead of passing the objects by value (which, of course, requires making a copy of the object). 24

25 Pointers & Functions Pointers as Function Parameter A pointer can be used as function parameter Works like a reference parameter to allow change to argument from within function A pointer parameter must be explicitly dereferenced to access the contents at that address Requires: 1) asterisk * on parameter in prototype and heading void getnum(int *ptr); 2) asterisk * in body to dereference the pointer cin >> *ptr; 3) address as argument to the function getnum(&num); 25

26 Pointers & Functions Pointers as Function Parameter void swap(int *x, int *y) { int temp; temp = *x; *x = *y; *y = temp; } int num1 = 2, num2 = -3; swap(&num1, &num2); 26

27 Pointers with const Keyword Pointers to Constant and Constant Pointers Pointer to a constant: cannot change the value that is pointed at Constant pointer: address in pointer cannot change once pointer is initialized 27

28 Pointers with const Keyword Pointers to Constant Must use const keyword in pointer definition: const double taxrates[] = {0.65, 0.8, 0.75}; const double *rateptr; Use const keyword for pointers in function headers to protect data from modification from within function What does the definition means? rates is a pointer to a constant double. This means that the value of pointer rates can be changed but the value stored at that location cannot be changed 28

29 Pointers with const Keyword Constant Pointers Defined with const keyword adjacent to variable name: int classsize = 24; int * const classptr = &classsize; Must be initialized when defined Can be used without initialization as a function parameter Initialized by argument when function is called Function can receive different arguments on different calls While the address in the pointer cannot change, the data at that address may be changed 29

30 Pointers with const Keyword Constant Pointers What does the definition means? 30

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