Thread: structure pointer

I have the following structure RGB:

struct RGB{__int8 B, G, R;} *pLcd;

Using a pointer (*pPanel) I call the function GetBitmapBits().

pLcd = (RGB *)(*pPanel)->GetBitmapBits();

What does (RGB *) do?

(RGB *) casts the result of (*pPanel)->GetBitmapBits() to (RGB *)

so, pLcd stores a RGB Pointer.

pLcd is a variable of struct RGB.
infront of whatever value you will put
(rgb *) it would convert that value to rgb type.
like int num;
(rgb *)num;
num was earlier an integer now it is pointer of rgb type.

i am just a student so dont mind if i am wrong
i myself need the help of your esteemed self.

* <- that is _not_ a "pointer"

That is an asterisk. It tells the compiler you want it to work with the _address_ of the specified variable of the specified type, not the _contents_ of the variable.

when you say

Code:

typedef struct
   {
   unsigned char  red;
   unsigned char  blue;
   unsigned char  green;
   }RGB;

you are creating a variable _type_. you can create any number of variables of type 'RGB'.

When you pass a variable of this type to another function, or procedure, it's faster and more effecient to pass it's _address_ (that is, where it resides in RAM), so that the function can manipulate the actual data indirectly.

And here is the specific difference:

Code:

void  foo1(RGB);                                   /* prototypes */
void  foo2(RGB*);
int    main(void);

int main(void)
   {
   RGB myRGBStruct;

   myRGBStruct.red = 0xFF;
   myRGBStruct.blue = 0xCC;
   myRGBStruct.green = 0x00;

   foo1(myRGBStruct);                         /* pass a copy of myRGBStruct to the func */
   foo2(&myRGBStruct)                        /* pass the addressof the original myRGBStruct to the func */

   return(0);
   }

void foo1(RGB myRGB)
   {
   myRGB.blue = 0xEE;
   }                                                       /* <- myRGB is _gone_ off th stack when we return */
                                                            /* from this procedure, so 'myRGBStruct' in main */
                                                            /* was not affected. */

void foo2(RGB *myRGB)
   {
   myRGB->red = 0xAA;                        /* myRGBStruct.red in main(), just got changed */
   }