RE: packing fun

My problem was that it's not documented what the compiler exactly does wrt
to packing for inheritance with VC++ (my last post contained the
confirmation from Microsoft that it's doing what you would expect with no
nasty surprises) and that the variant of gcc 3.1 we used for Mac and Linux64
versions of UT2003 packed in the strange way I mentioned in a previous post.
This seemed to have been as a regression as 3.3.2 packs like VC++ again.

Our scripting language requires properties to match up exactly with auto-
generated C++ headers so knowing exactly how different compilers pack (and
how to get them all to pack the same way) is vital for us :)

-- Daniel, Epic Games Inc.

> -----Original Message-----
> From: [email protected] 
> [mailto:[email protected]] On 
> Behalf Of Scoubidou944
> Sent: Tuesday, December 30, 2003 1:09 PM
> To: [email protected]
> Subject: Re: [GD-General] packing fun
> 
> Don't sure about what your problem is on padding.
> 
> #pragma pack(push)
> #pragma pack(4)
> struct Alice
> {
>     int Integer1;
>     char Char1;
>     // HERE COMPILER WILL ADD PADDING BYTES TO 0
>     char u8Padding1[3];  // == 0
> };
> 
> struct Bob : Alice
> {
>     char Char2;
>     // HERE COMPILER WILL ADD PADDING BYTES TO 0
>     char u8Padding2[3];  // == 0
> };
> #pragma pack(pop)
> 
> So in memory, we will have 2 structure which are both aligned 
> on 4 bytes :
> Integer1    Char1    u8Padding1    Char2    u8Padding2
> |     4             1              3           |     1               3
> |
> +-------------------------------+---------------------+
> |                  Alice                       |              Bob
> |
> 
> If you don't want to loose space in memory (is there any 
> warrior of low
> memory using in C++ coding ;p) push & pop the stack to fit to 
> the minimum
> size required or reduced size of variables like this :
> 
> struct Alice
> {
>     int         i24Integer1 : 24;           // Will have 
> previous Integer1
> which fit in 24 bits instead 32
>     int         i8Integer1   : 8;            // Will have 
> previous Char1
> };
> ==> sizeof (Alice) = 4 instead 8 and we keep 4 bytes alignment.
> 
> Vincent
> 
> 
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