On Thursday, 7 August 2014 at 17:22:15 UTC, Marc Schütz wrote:
(Original discussion:
http://forum.dlang.org/thread/fckwpddiwxonabqaf...@forum.dlang.org#post-pskjgieddhpntzaokohj:40forum.dlang.org)
I would expect `B` to have a gap between `ex` and `mmid`. AFAIK
the outer `align(1)` only applies to the struct in its
entirety, not to the individual fields. However for both DMD
git and LDC 0.14.0-alpha1 (based on DMD 2.065), `A` and `B`
have the same size.
After some thinking, I believe this is because arrays inherit
the alignment of their element types. Is this correct? If yes,
where is this documented? I had expected `char[4]` to be
aligned at a 4-byte boundary.
I'm not sure about all the latest compiler changes, but let's
try and answer some of this.
TDPL pg. 268-269 explains this (although could be out of date
with recent changes). I'll copy what's relevant..
7.1.11.1 The align Attribute
If you want to override the compiler's choice of alignment, which
influences the padding inserted. You could use an align
modifier... etc..
class A {
char a;
align(1) int b;
char c;
}
With the specification above the fields of A are laid out
without gaps between them.
You may use align with an entire class definition:
align(1) struct S {
char a;
int b;
char c
}
...
Align is not suppose to be used with pointers and references...
Back to the question. Most fields will be aligned on 4-byte
boundaries, or 8-byte depending on if it's 32/64 bit. This is
mostly for performance reasons, but also with addresses it may
affect the GC. Overriding the compiler is mostly going to be more
useful when working against C/C++ structures where they are also
forcibly aligned for space.
So assuming we have the above struct.
S[2] s;
It's probably going to be aligned on 4's for the first one; But
it doesn't have to be. But the others? I'm not so sure... The
inner alignment and padding is 4 per, so if the struct S has a
size of 12, then it's still going to be aligned by 4's by
default... i think? Maybe the alignment has to do if it's
inserted into another object. On the stack it probably ignores
the alignment attribute...
struct B {
char a;
S s; //infers align(1) by it's definition?
}
Well regardless, unless you're overriding both, you're probably
going to get some form of alignments of 4, be it for arrays or
for speed... I hope this isn't confusing.
