How to align a struct member in D?

Question

I tried this

struct Foo(T)
{
    align(8) void[T.sizeof] data;
}

but

static assert(Foo!(int).data.alignof == 8);

fails, telling me the alignment is still 1 instead of 8.

Why is this, and how do I fix it, so that it works for any arbitrary alignment that is a power of 2 (not just 8)?

score 5 · Accepted Answer · answered May 19 '12 at 11:49

Browsing the DMD source, it looks like alignof doesn't take into account align attributes.

Here is where it is handled:

... if (ident == Id::__xalignof)
{
    e = new IntegerExp(loc, alignsize(), Type::tsize_t);
}

This converts a .alignof expression into a size_t expression with value alignsize(), so let's look at alignsize() for a static array:

unsigned TypeSArray::alignsize()
{
    return next->alignsize();
}

It just gets the alignment of the element type (void) in your case.

void is handled by TypeBasic::alignsize(), which just forwards to TypeBasic::size(0)

switch (ty)
{
    ...
    case Tvoid:
        size = 1;
        break;
    ...
}

Looking at how other types handle alignof, it doesn't look like align attributes are taken into account at all, but I could be wrong. It may be worth testing the alignment manually.

lol wait so what does `align` do? Is it just there for decoration? xD — user541686, May 19 '12 at 19:11
I think `align` works, but `alignof` just gets the alignment of the type, so they are separate things. — Peter Alexander, May 19 '12 at 19:26
Weird, I was running into problems that led me to check `alignof`... gotta double-check though.. — user541686, May 19 '12 at 19:38

How to align a struct member in D?

1 Answers1