[PDF] Programming in C and C++ - Lecture 3: Pointers and Structures





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This specific file (iostream) includes the declarations of the basic standard input-output library in C++ and it is included because its functionality is going to be used later in the program using namespace std; All the elements of the standard C++ library are declared within what is called a namespace the namespace with the name std

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Programming in C and C++

Lecture 3: Pointers and StructuresNeel Krishnaswami and Alan Mycroft

Pointers

Computer memory is often abstracted as a sequence of bytes, grouped into words Each byte has a unique address or index into this sequence The size of a word (and byte!) determines the size of addressable memory in the machine A pointer in C is a variable which contains the memory address of another variable (this can, itself, be a pointer) Pointers are declared or dened using an asterisk( * ); for example:char* pc;orint** ppi; The asterisk binds to the variable name, not the type specier; for examplechar* pc,c; A pointer does not necessarily take the same amount of storage space as the type it points to 1

ExampleExample

0x2c 0x30 0x34 0x38 0x4c 0x50 0x60

05421c52 00

00 00 62
char cchar *pc int iint *pi int **ppi 00 00 00

38 4c000000

41
41

Little

Big

3/ 252

Manipulating pointers

The value pointed to by a pointer can be retrieved or dereferenced by using the unary * operator; for example: int p int x p; The memory address of a variable is returned with the unary ampersand ( & ) operator; for exampleint* p= & x;

Dereferenced pointer values can be used in normal

expressions; for example:*pi+= 5 ;or(*pi)++ 3

Example

1#include

2

3intmain (void) {

4intx =1,y=2;

5int* pi;

6int** ppi;

7

8pi= & x; ppi= & pi;

9printf("%p, %p, %d=%d=%d\n",ppi,pi,x,*pi,**ppi);

10pi= & y;

11printf("%p, %p, %d=%d=%d\n",ppi,pi,y,*pi,**ppi);

12

13return0 ;

14} 4

Pointers and arrays

A C array uses consecutive memory addresses without padding to store data An array name (used in an expression without an index) represents the memory address of the rst element of the array; for example: char c[ 10 char pc c; // This is the same char pc c[ 0 ];// as this Pointers can be used to index into any element of an array; for example: int i[ 10 int pi i[ 5 5

Pointer arithmetic

Pointer arithmetic can be used to adjust where a pointer points; for example, ifpcpoints to the rst element of an array, after executingpc+=3;thenpcpoints to the fourth element A pointer can even be dereferenced using array notation; for examplepc[2]represents the value of the array element which is two elements beyond the array element currently pointed to bypc

In summary, for an arrayc,*(c+i)== c[i] and

c i c[i] A pointer is a variable, but an array name is not; therefore pc c andpc++are valid, butc= pc andc++are not 6

Pointer Arithmetic Example

1#include

2

3intmain (void) {

4charstr[] = "A string." ;

5char* pc= str;

6

7printf("%c %c %c\n",str[0],*pc,pc[3]);

8pc+= 2 ;

9printf("%c %c %c\n",*pc, pc[2], pc[5]);

10

11return0 ;

12} 7

Pointers as function arguments

Recall that all arguments to a function are copied, i.e. passed-by-value; modication of the local value does not aect the original In the second lecture we dened functions which took an array as an argument; for examplevoidreverse( chars[]) Why, then, does reverse aect the values of the array after the function returns (i.e. the array values havent been copied)? becausesis re-written tochar* sand the caller implicitly passes a pointer to the start of the array Pointers of any type can be passed as parameters and return types of functions Pointers allow a function to alter parameters passed to it 8

Example

Compareswp1(a,b)withswp2(&a,&b):1voidswp1 (intx, inty) 2{

3inttemp = x;

4x= y;

5y= temp;

6}1voidswp2 (int* px,int* py)

2{

3inttemp = * px;

4*px= * py;

5*py= temp;

6}9

Arrays of pointers

C allows the creation of arrays of pointers; for example int a[ 5 Arrays of pointers are particularly useful with strings An example is C support of command line arguments: int main int argc, char argv[]) { ... } In this caseargvis an array of character pointers, andargc tells the programmer the length of the array 10

Diagram of Argument List Layout

NULLargv:

firstarg\0progname\0 secondarg\0argv[0] argv[3] argv[2]argv[1] argc: 311

Multi-dimensional arrays

Multi-dimensional arrays can be declared in C; for example: int i[ 5 10 Values of the array can be accessed using square brackets; for example:i[3][2] When passing a two dimensional array to a function, the rst dimension is not needed; for example, the following are equivalent: void f int i[ 5 10 void f int i[][ 10 void f int i)[ 10 In arrays with higher dimensionality, all but the rst dimension must be specied 12

Pointers to functions

C allows the programmer to use pointers to functions This allows functions to be passed as arguments to functions For example, we may wish to parameterise a sort algorithm on dierent comparison operators (e.g. lexicographically or numerically) If the sort routine accepts a pointer to a function, the sort routine can call this function when deciding how to order values 13

Function Pointer Example

1voidsort (inta[], const int len,

2int( *compare)(int,int )) {

3for(inti = 0 ; i< len -1; i++)

4for(intj = 0 ; j< len -1-i; j++)

5if(( *compare)(a[j],a[j+1])) {

6inttmp = a[j];

7a[j]= a[j +1], a[j+1]= tmp;

8} 9} 10

11intinc (inta, int b) { return a > b ? 1 : 0 ; }

Source of some confusion: either or both of the*s in*compare may be omitted due to language (over-)generosity. 14

Using a Higher-Order Function in C

1#include

2#include"example8.h"

3

4intmain (void) {

5inta[] = { 1,4,3,2,5};

6unsignedint len = 5 ;

7sort(a,len,inc);//or sort(a,len,&inc);

8

9int* pa= a; //C99

10printf("[");

11while(len --) { printf("%d%s",* pa++, len?" ":""); }

12printf("]\n");

13

14return0 ;

15}15

Thevoid *pointer

C has a \typeless" or \generic" pointer:void* p

This can be a pointer to any object (but not legally to a function) This can be useful when dealing with dynamic memory

Enables polymorphic code; for example:

sort( void p, const unsigned int len, int comp)( voidquotesdbs_dbs8.pdfusesText_14
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