Article: The Lost Art of Structure Packing

[Nick Treleaven]

On Monday, 2 November 2026 at 12:22:38 UTC, Ogion wrote:
> Maybe we need some way to allow the compiler rearrange the 
> order of struct fields to optimally pack them, like it does 
> with classes?
>
> ```D
> struct S {
> align (auto):
>     byte a;    // placed at offset 0
>     int b;     // placed at offset 4
>     byte[3] c; // placed at offset 1
> }
> ```

BTW making `S` a class doesn't pack its fields like that with dmd 
AFAICT. I get field offsets 16, 20, 24. However, if `a` and `c` 
are declared next to each other then it does. In fact, if you do 
that then the struct will get packed the same as well.

> I’d even argue that auto-packing should be the default option.

That might be optimal for memory size, but not necessarily for 
speed AIUI. However, having the option makes sense, assuming a 
compiler does do this for classes already.