Thread: sizeof() function on a pointer to an array of a struct

Whew now that that confusing Thread Title is done and over with.. I think I explained it correct...

Well... here is my problem..

Code:

RGBQUAD *mColorTable;

mColorTable = new RGBQUAD[256];
sizeof(mColorTable);

thats obviously not my exact code, but you get the general idea of what I'm trying to do..

Additional info: RGBQUAD struct contains 4 member variables.

Therefore the size of mColorTable should be 4 * 256 = 1024.
However, since mColorTable is pointing to the first index in the array the code is returning 4 * 1 = 4.

Obviously i can rid of pointers all together and just use

Code:

RGBQUAD mColorTable[256];
sizeof(mColorTable);

but I do not know the size of the array until runtime, which is why I am using pointers.

thnx for any help...

P.S. for those wondering I am trying to create a class for loading BITMAPS of various sizes and color depths... this portion is for storing the color table of my BITMAP.. currently I am only able to store the first entry of the color table, which is black meaning all I get when I try and draw my BITMAP is a black screen

A pointer is a reference to the object it is assigned. It holds the address of the object it is "pointing" to. A pointer will always be 4 bytes in size (whether this a pointer to a char, int, float or an object of any size in bytes) and therefore you will not be able to get the size of an object directly through a pointer.

>> they're 8 bytes on a 64-bit machine


That may depend on your compiler. Pointers are the same size as integers.

>> Pointers are the same size as integers.
Wrong. There is no coorelation between sizeof(void*) and sizeof(int).
[edit]
or any other integer. Just because sizeof() returns the same size for a pointer as it does for an integer, doesn't mean one can make that statement.
Yes, pointers are the same size as int's and long's on most all 32-bit compilers.
[/edit]

However, it is compiler dependent. You can read about MS's 64-bit compilation model (LLP64) here.
All of the *nix 64-bit compiler implementations use the LP64 model, where the "long" data type becomes 8 bytes.

To get your learn-on even further, start here.

gg

Okay, the size of a pointer comment may confuse newbies. I was only using int as a comparison. Not to cause any confusion.