Optimal struct layout template?

[Sergey Gromov]

Thu, 18 Dec 2008 00:21:58 +0300, Denis Koroskin wrote:

> On Thu, 18 Dec 2008 00:12:18 +0300, Sergey Gromov <snake.scaly at gmail.com>  
> wrote:
> 
>> Tue, 16 Dec 2008 10:09:41 +0100, Don wrote:
>>
>>> Example 2:
>>> struct Foo { double, int } // 12 byte size, wants 8-byte alignment.
>>
>> Foo.sizeof is 16.
>>
> 
> It is 12 with align(1) (I see no problem with it).

With align(1) the Foo.alignof is 1 so there is no problem to talk about.
The problem is when you have elements with non-trivial alignment
requirements and want to pack them as tightly as possible.

>> The thing is, array of Foo's must be properly aligned, and &foo[1] must
>> be cast(byte*) &foo[0] + Foo.sizeof.  This means that Foo.sizeof must be
>> itself 8-aligned.  The solution is { Foo, Foo, int, int }.
> 
> Stop right there. You can't access &foo[1] no matter what align or  
> structure layout is used. Example:
> 
> align(default):
> struct Data
> {
>      int i1;
>      short s2;
>      char c1;
>      int 12;
> }
> 
> What is &i1[1]?

i1 is not an array.

Sorry I wasn't clear enough.  I was talking about an imaginary array of
Foo's:

  Foo[] foo;

You must admit that foo[1] and &foo[1] makes perfect sense for such
construct.  And that

  cast(byte*) &foo[1] - cast(byte*) &foo[0] == Foo.sizeof

must hold.