Smaller than a byte

[Waldermort]

I have aa array of struct in my app, each struct is the size of a byte. My problem is that this array is very large, up to 256mb if the machine can cope with it. I know each struct is small, but I can half that size. I need 1 bit for a flag and 3 bits for a state. How can I declare the stuct to effectivly have two struct members each of which is 4 bits in size?

struct block
{
BYTE Flag : 1;
BYTE State : 3;
};

This is what I currently have, but it's size is still 1 byte even though only 4 bits are being used.

Waldermort

[David Crow]

How are you packing the structure?

---

"A good athlete is the result of a good and worthy opponent." - David Crow

"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne

[Waldermort]

standard #pragma pack ( push, 1 )

Waldermort

[Waldermort]

This seems to work, but accessing the members is a pain

struct block
{
union
{
struct Upper
{
BYTE State : 1;
BYTE Repaint : 3;
};
struct Lower
{
BYTE State : 1;
BYTE Repaint : 3;
};
} U;
}

Waldermort

[led mike]

WalderMort wrote:

This is what I currently have, but it's size is still 1 byte even though only 4 bits are being used.

I believe the minimum size of your structure is the size of the base type, in this case BYTE.

[Mark Salsbery]

Doesn't Upper and Lower use the same bits? Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

[Waldermort]

Yeah, I just realised, I'm still trying to get it...

Waldermort

[Mark Salsbery]

AFAIK, byte granularity is all you're going to get unless you pack two pairs in each struct:

struct block
{
BYTE State : 1;
BYTE Repaint : 3;
BYTE State2 : 1;
BYTE Repaint2 : 3;
};

Do you have to have a struct type?  Can you wrap the array in a class and provide accessors to packed State/Repaint bits instead?

Mark Salsbery Microsoft MVP - Visual C++ :java:

[Chris Losinger]

have you looked at bitfields ?

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[Waldermort]

Funny you say that, I was just playing around with the bitfields and found my solution. Though I'm not sure of how portable it may be.

struct block
{
union
{
struct Upper
{
BYTE Pack : 4;
BYTE State : 3;
BYTE Repaint : 1;
};
struct Lower
{
BYTE State : 3;
BYTE Repaint : 1;
};

    BYTE Total;
} U;

};

Waldermort

[Sameerkumar Namdeo]

Original struct struct aablock { BYTE Flag : 1; BYTE State : 3; }; Original array of size 512 MAXSIZE = 512 aablock aa[MAXSIZE]; Modified Struct struct bbblock { BYTE FlagOdd : 1; BYTE StateOdd : 3; BYTE FlagEven : 1; BYTE StateEven : 3; }; try to suffle the elements if size of this struct > sizeof(BYTE) if you are having an array of size say 512 then you may declare array with half of that size. Each element will store 2 elements of original struct bbblock bb[MAXSIZE/2]; j = -1; for(i = 0; i < MAXSIZE/2; ++i) { j++ // j = 0, 2, 4, 6... bb[i].FlagEven = aa[j].Flag; bb[i].StateEven = aa[j].State; j++; // j = 1, 3, 5..... bb[i].FlagOdd = aa[j].Flag; bb[i].StateOdd = aa[j].State; }

[thestrat]

I think struct is creatin a problem. try doing it by using union inside a struct. i think it will work. tHeStRat

[zakkas2483]

Good Ans.... But i have some confusion. In That code what is the meaning of This BYTE Flag : 1; Line. How we can use one bit from that Byte? i got the error on that each line.. Please solve my doubt. :) hiren Thakkar

[Iain Clarke Warrior Programmer]

If you read the other answers, you'll see someone refers to bitfields. That's the important clue - you can look that up in your c++ book, as the full answer is too long for me to bother typing here! Iain.