Variant parts record problem

[masonwheeler]

Using variant parts in records makes a lot of things easier. For example, I wrote a color structure that looks like this:
TMyColor = record
case boolean of
false: (color: cardinal);
true: (rgba: packed array[1..4] of byte);
end;
It works great! Any changes made to the one interpretation get applied automatically to the other, because they're mapped to the same memory. But trying to set up a bitfield for easy bitwise operations... doesn't seem to work.
TBitfield16 = record
case boolean of
false: (num: word);
true: (bits: packed array[1..16] of boolean);
end;
In this one, changes made to the one value don't seem to affect the other one. Anyone know why, and how I can fix it?

[cairnswm]

A boolean takes 1 byte of memory, not 1 bit



A Boolean variable occupies one byte of memory, a ByteBool variable also occupies one byte, a WordBool variable occupies two bytes (one word), and a LongBool variable occupies four bytes (two words).

[masonwheeler]

Aww crap. I could have sworn that used to work in other versions...

[JSoftware]

If you use FPC then you could do it with the bitpacked keyword:


TBitfield16 = bitpacked record
case boolean of
false: (num: word);
true: (bits: packed array[1..16] of boolean);
end;

[masonwheeler]

Unfortunately, my code won't compile under Lazarus, so that's not an option for the moment.

I rewrote the whole thing using operator overloads, and most of them work... except for one that doesn't. Here's what I've got so far:
interface
type
TBitset16 = record
bits: array[1..16] of boolean;
class operator Implicit(a: word): TBitset16; inline; //works
class operator implicit(a: TBitset16): word; inline; //doesn't work
end;

implementation
{$R-}
class operator TBitset16.Implicit(a: word): TBitset16; //works
var
dummy: word;
i: integer;
begin
dummy := 1;
for I := 1 to 16 do
begin
result.bits[i] := (a and dummy > 0);
dummy := dummy shl 1;
end;
end;

class operator TBitset16.implicit(a: TBitset16): word; //doesn't work
var
dummy: word;
i: integer;
begin
result := 0;
dummy := 1;
for I := 1 to 16 do
begin
if a.bits[i] then
inc(result, dummy);
dummy := dummy shl 1;
end;
end;
{$R+}
This compiles, and I can assign a word to a TBitset16 just fine, but if I try to actually assign a TBitset16 to a word, the compiler says they're incompatible. How do I fix this?

[User137]

If you are noto using FPC, then how class operators are working under delphi? What version it is?

[masonwheeler]

I'm running BDS 2006, which supports class operators for records. But for some reason it doesn't like my assignment operator. Anyone with experience in this know what I'm doing wrong?

[JSoftware]

Are you assigning it from a variable or from a constant?

I think most constant numeric arguments evaluate to longints

[masonwheeler]

Assigning it *from* numbers works fine. (All integer types can be implicitly converted to each other anyway.) It's trying to assign it *to* a Word that doesn't work.


var
bitset: TBitset16;
number: word;
begin
bitset := number; //compiler likes this
bitset := 5; //this works just fine too
number := bitset; //compiler chokes on this one
end;