Pointer & Strings: Part Three

Continuing with the previous post, today we will further study pointers and strings.

Pointer Expressions and Pointer Arithmetic:

Pointer arithmetic is of the following types:

–Increment/decrement pointer  (++ or --)

–Add/subtract an integer to/from a pointer( + or += , - or -=)

–Pointers may be subtracted from each other

but remember that pointer arithmetic is meaningless unless performed on pointer to array

consider an example where there is a 5 element int array on a machine using 4 byte integer space.

–vPtr points to first element v[ 0 ], which is at location 3000

Vptr = v or vptr = &v[0]

–vPtr += 2; sets vPtr to 3008

vPtr points to v[ 2 ]

 when subtracting pointers (must both point to the same array, if not then LOGIC ERROR)

–Returns number of elements between two addresses

  vPtr2 = &v[ 2 ];
vPtr = &v[ 0 ];
vPtr2 - vPtr = 2

As far as pointer assignment is concerned:

–Pointer can be assigned to another pointer if both are of same type

–If not of the same type, cast operator must be used

–Exception: Void Pointer: pointer to void (type void *)

Void Pointer: Pointer to void (type void *):

int nValue = 5;

void *pVoid = &nValue;    //no need for casting

// can not dereference pVoid because it is a void pointer

int *pInt = static_cast<int*>(pVoid); // cast from void* to int*

cout << *pInt << endl; // can dereference pInt

Void Pointer:

the code given below shows an example of void pointers:

void f (void *,void*,int);

main()
{
int x=9,y=5;
char r='w',t='z';

                f(&r,&t,sizeof(r));
f(&x,&y,sizeof(x));

                cout<<x<<endl;
cout<<r<<endl;

}

void f(void* x, void* y, int sizeofptr)
{

int *nptr1, *nptr2;
char *nptr3, *nptr4;

if (sizeofptr == sizeof(int))
{ nptr1=(int* )x;
nptr2=(int* )y;
*nptr1=*nptr1**nptr2;}
else
{nptr3=(char* )x;
nptr4=(char* )y;
*nptr3=toupper(*nptr3);}

}

Relationship between Arrays and Pointers:

Arrays and pointers are closely related:

–        Array name is like a constant pointer

–        Pointers can do array subscripting operations:

                                    int b[5];

                                    int *bptr;

You can point ‘bptr’ to array ‘b’ using either;

                                    bptr = b;    //remember name of array is constant pointer to address of the first                                                           location of array  

                        Or

                                    bptr = &b[0];

Array elements can easily be accessed with pointers

–        Element b[ n ] can be accessed by  *( bPtr + n )

•          Called pointer/offset notation

•          ‘n’ is the offset

•          DO NOT forget the brackets, since * bPtr + n  will simply add n to *bPtr

–        Accessing array addresses with pointers:

•          &b[ 3 ] same as bPtr + 3

–        Array name can be treated as pointer;

•          b[ 3 ] same as *( b + 3 )

•          This is possible since name of the array is a pointer itself.

–        Pointers can be subscripted (pointer/subscript notation)

•          bPtr[ 3 ] same as b[ 3 ]

Array of Pointers:

An array can contain pointers

–This is commonly used to store array of strings

char *suit[ 4 ] = {"Hearts", "Diamonds",
                  "Clubs", "Spades" };

–Each element of suit points to char * (a string)

–Array does not store strings, only pointers to strings

–suit array has fixed size, but strings can be of any size