Struct alignment vs alignment of fields

[via Digitalmars-d-learn]

On Friday, 8 August 2014 at 18:20:41 UTC, ketmar wrote:
> yeah, chars (and bytes, and so on) are not aligned. i.e.
>
> align(1) struct B {
>   int qtim;
>   int bid;
>   int ofr;
>   int bidsiz;
>   int ofrsiz;
>   short mode;
>   char ex;
>   byte mmid;
>   char z;
> }
>
> has sizeof == 25. not sure if specs mentions this, but they 
> should.

It's not surprising that `char` and `byte` behave like this, 
because `byte.alignof == 1`. But it's not obvious that this also 
applies to arrays of them. I had expected them to be treated as 
opaque objects of a certain size, and therefore have an alignment 
that corresponds to their size.

     pragma(msg, byte.alignof);
     pragma(msg, (byte[1]).alignof);
     pragma(msg, (byte[2]).alignof);
     pragma(msg, (byte[3]).alignof);
     pragma(msg, (byte[4]).alignof);
     pragma(msg, (byte[5]).alignof);
     pragma(msg, (byte[6]).alignof);
     struct S {
         byte a;
         byte b;
         byte c;
         byte d;
     }
     struct T {
         byte a;
         short b;
         int c;
     }
     pragma(msg, S.alignof);
     pragma(msg, T.alignof);

This outputs "1" for all types except `T`, which has 4. So this 
even applies to structs, not only arrays. Which may make sense, 
because each element will always be accessed with correct 
alignment. However, accessing the aggregate as a whole might 
result in unaligned reads/writes.