Pointer Howto For Beginners

Inhaltsverzeichnis

1 the basics
2 malloc, sizeof and free
3 pointer & structs

the basics

* ... is used to access the content of a pointer
& ... is used to retrieve the memory-address of a variable or a pointer

this will tell address of a variable:

void main() {
   int i=7;
   cout << &i;

   /* Ausgabe zB 0x00FFAB75... */
   return 0;
}

a pointer usually is declared and initialized that way:

[datatype_to_point] *[pointer_name] = NULL;

lets declare a pointer called ptr_int for pointing to an integer:

int *ptr_int = NULL;

a pointer always wants a hardware-address to be set to:

ptr_int = &i;

now we can access the content. here's a whole example:

void main() {
   int i = 7;
   int *ptr_int = NULL;

   ptr_int = &i;
   
   cout << ptr_int << endl;     /* output: eg 0x00FFABE5... */
   cout << *ptr_int << endl;    /* output: 7 */

   *ptr_int = 666;              /* here we set the value with the pointer */
 
   cout << i;                   /* output: 666 */
   return 0;
}

malloc, sizeof and free

sizeof (datatype) .......... tells us how many bytes a variable-type requires (sizeof(int) will return 4)
malloc (size_in_bytes) ... searches for a free space in the heap, reserves it, and returns the hardware-address
free (pointername) ........ frees the memory again

we combine it like this:

void main() {
   double *ptr_double = NULL;
   
   ptr_double = (double*) malloc( sizeof( double ) );

   *ptr_double = 789.0;

   cout << ptr_double << endl;   /* output: 0x00FFABCD... */
   cout << *ptr_double << endl;  /* output: 789 */

   free(ptr_double);             /* free memory */

   return 0;
}

pointer & structs

lets take an example:

struct robopos {
	int x;
	int y;
	int z;
};

int main() {
	robopos *ptr_robopos = NULL;

	ptr_robopos = (struct robopos*) malloc(sizeof(struct robopos));

	ptr_robopos->x = 7;
	ptr_robopos->y = 8;
	ptr_robopos->z = ptr_robopos->x + ptr_robopos->y;

	cout << ptr_robopos->z << endl;

	free(ptr_robopos);
		
	cout << endl;
	return 0;
}