Casting unsigned byte array to array of Structs

Question

Using C++ (GCC specifically, should have put this sooner), I'm storing raw texture data in an array of unsigned bytes, in a RGBA format, with 32 bits per pixel (8 bits per color value with Alpha, so on and so forth...). The thing is, I want to write a function that returns the raw data as a array of Colors, where a Color is a struct defined as the following:

struct Color
{
    uint8 r;
    uint8 g;
    uint8 b;
    uint8 a;
};

Plus functions and whatnot, but those are the only variables in the struct. My thinking is that since each color is 4 bytes long, what I can somehow cast the raw byte array to a Color array that is 1/4 of the original size (in "length" of array, not in absolute size). I think that reinterpret_cast is what I am looking for, but I cannot find anything after a google search that confirms 100% that you can convert it into an array of structs instead of just one struct.

So I guess I am just asking someone to either confirm that this is indeed possible with reinterpret_cast, or if there is a different cast or way to do this. Thanks.

EDIT: My wording is a little weird, so as an arbitrary example I'd like to somehow cast a array of 16 unsigned bytes into an array of 4 Colors.

EDIT: Also I know it's kind of a little late, but I cant seem to find how to cast a small portion of the array at a specific place to a single struct using a reinterpret_cast, if that is possible, without copying to a smaller array and casting like that. So any help with this problem would also be greatly appreciated.

Besides uint8 doesn't specify a standard type, you might get a victim of padding with what you're trying to do. You should be very sure what you have to apply a reinterpret_cast<>, otherwise your fridge might explode or any other unexpected (== undefined) behavior may occur!! — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Jun 18, 2014 at 2:07
You can arrive at the "reinterpret_cast" option by simply excluding all other types of casts. Const_cast, static_cast and dynamic_cast won't work in your case for sure. — Oleg
– Oleg, Commented Jun 18, 2014 at 2:07
Sorry should have been more specific. I have a global include file that defines uint8 as uint8_t, which is defined in GCC (and perhaps others, but I only use GCC) as unsigned char and I'm using GCC. — sm81095
– sm81095, Commented Jun 18, 2014 at 2:32

Remy Lebeau · Accepted Answer · 2014-06-18 02:03:11Z

6

as an arbitrary example I'd like to somehow cast a array of 16 unsigned bytes into an array of 4 Colors.

Like this:

#pragma pack(push, 1)
struct Color
{
    uint8 r;
    uint8 g;
    uint8 b;
    uint8 a;
};
#pragma pack(pop)

uint8 bytearray[16];
...
Color *colorarray = reinterpret_cast<Color*>(bytearray);

Then you can do things like this:

for (int idx = 0; idx < 4; ++idx)
{
    Color &c = colorarray[idx];
    // use c.r, c.g, c.b, c.a as needed...
}

answered Jun 18, 2014 at 2:03

Remy Lebeau

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3 Comments

Drew Dormann Over a year ago

This is how I would do it. Mind that a C++ compiler doesn't have to support #pragma pack, and a particular environment might not even need it. It depends on how standard you want your solution to be.

Cheers and hth. - Alf Over a year ago

the pragmas are non-standard. bit fields are standard but don't get you all the way: no way to ensure lack of padding at end. however, as a practical thing one can just static assert the size.

sm81095 Over a year ago

@Praetorian I'm not accusing anyone here of thinking that. I was merely making an observation about other people I've met in my travels across the various help boards.

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