Pointer

lvalues

• In C++, any expression that refers to an internal memory location is called an lvalue

• Appear on left side of assignment statement• e.g.

x = 1.0;intarray[2] = 17;

• Constants and expressions are not lvalues

Memory and Data Representation• Every lvalue is stored somewhere in memory

and therefore has an address• Once allocated, the address of an lvalue never

changes, even though its contents may change• Depending on type of data, different lvalues

require different amounts of memory• The address of an lvalue is itself data that can be

manipulated and stored in memory

Pointer Declaration

• A pointer is a variable storing the address of another variable

• Declaration:type *ptr;

• Example:int *p1, *p2;

• p1 and p2 are referred to as pointers-to-int, and may contain the addresses of integers

Pointer Operations

• C++ defines two operators that manipulate pointer values:

& address-of* value-pointed-to (dereference)

• Operand of & must be lvalue, and & returns the address of the lvalue

• Operand of * must be a pointer, and * returns the value pointed to by the pointer

Getting the Address of a Variable

• Each variable in program is stored at a unique address

• Use address operator & to get address of a variable:int num = -99;cout << &num; // prints address// in hexadecimal

Pointer Variables

• Definition:int *intptr;

• Read as:“intptr can hold the address of an int”

• Spacing in definition does not matter:int * intptr; // same as aboveint* intptr; // same as above

Pointer Variables

• Assigning an address to a pointer variable:int *intptr;intptr = &num;

• Memory layout:num intptr25 0x4a00

address of num: 0x4a00

The Indirection Operator

• The indirection operator (*) dereferences a pointer.

• It allows you to access the item that the pointer points to.

int x = 25;int *intptr = &x;cout << *intptr << endl;

This prints 25.

Special pointer NULL

• NULL is the value of a pointer that does not currently point to any data (usually value 0)

• Can not dereference a NULL pointer• Exists for applications

infile.open(”file.txt”);if (infile == NULL)

printf(“File not open”);

Pointers as Function Parameters• A pointer can be a parameter• Works like reference variable to allow change to

argument from within function• Requires:

1) asterisk * on parameter in prototype and headingvoid getNum(int *ptr); // ptr is pointer to an int 2) asterisk * in body to dereference the pointercin >> *ptr;

3) address as argument to the functiongetNum(&num); // pass address of num to getNum

Example

void swap(int *x, int *y){ int temp;

temp = *x;*x = *y;*y = temp;

}

int num1 = 2, num2 = -3;swap(&num1, &num2);

(Program Continues)

Another Example

• To modify value of argument, use pointervoid SetToZero(int *ip){

*ip = 0;}

which makes “ip” a pointer to the location of the value of the argument of the call

SetToZero(&x);

Advantage of use of & operator

• When calling a function usingfunc(x);

you know the value of the variable x will not change, whereas calling a function usingfunc(&x);

is allowed to change the value of x• Easier to predict effects of function call

SwapInteger function• When sorting arrays, we wanted to use

SwapIntegers(array[low], array[high]);

• Using pointers allows the following:void SwapIntegers(int *p1, int *p2){

int temp;temp = *p1;*p1 = *p2;*p2 = temp;

}

and SwapIntegers(&array[low], &array[high]);

Returning Multiple Results• Convert time (minutes) to hours and minutes

void ConvertTime(int time, int *hours, int *mins)

{*hours = time / 60;*mins = time % 60;

}

• Call with ConvertTime(t, &h, &m);

Or Use Two functionsint Hours(int time){return (time / 60);

}

int Minutes(int time){return (time % 60);

}