On 02/20/2018 07:34 AM, Steven Schveighoffer wrote:
On 2/18/18 4:52 AM, Dmitry Olshansky wrote:
On Saturday, 17 February 2018 at 19:37:12 UTC, Steven Schveighoffer
wrote:
On 2/17/18 9:59 AM, Andrei Alexandrescu wrote:
On 02/17/2018 09:03 AM, Steven Schveighoffer wrote:
I found this also works:
static foreach(alias x; S.tupleof)
{
writeln(x.offsetof);
}
Yes, the implementation uses offsetof.
I guess I'm just confused based on the statement "the builtin
.tupleof ... [omits] the essential information of field offsets."
What is this construct giving us that .tupleof doesn't?
I guess the construct captures offsets as part of type. This is useful
for allocators + 2 things with same Layout can be bitblitted to each
other.
I haven't looked at it in depth, so I didn't know the result of the
abstraction (I thought it was a tuple, or a pair of tuples).
Note, you could do this without the need for a new abstraction, simply
with .tupleof on the type itself.
And static foreach has made this much simpler. But definitely the
interface to getting things from tupleof is not consistent. This reason
alone may be cause to add such a construct.
There's the difference that with inline static foreach you can express
one processing of one layout, whereas with a structured result you can
express the notion of any processing of any layout. The distinction
between an inlined for loop and the map function comes to mind.
In particular, static foreach would be a good implementation device for
Layout. I happened to express it a different way because it seemed
simpler, but contributions welcome.
Next step is to manipulate layouts. I have in mind an algorithm
"eraseNonPointerTypes". It does the following. Consider:
struct A
{
int a, b;
string x;
double c;
int[] y;
}
So the layout is Layout!(0, int, 4, int, 8, string, 24, double, 32,
int[]). Next we want to not care about non-pointers - just merge all
that nonsense together. So eraseNonPointerTypes applied to this layout
would yield Layout!(0, ubyte[8], 8, string, 24, ubyte[8], 32, int[]).
At this point we figure that this partially erased layout is the same as
for this other type:
struct B
{
double a;
string b;
long c;
int[] d;
}
And the ba dum tss of it all is that you can allocate an A, deallocate
it, then allocate a B in the same memory - all in safe code. Even if you
have a dangling pointer to A messing with a B object, the code will be
incorrect but will not lose memory safety.
This is a major part of Alexandru Jercaianu's work on Blizzard - a
memory-safe allocation framework. I am so excited about the impending
release I can't stand myself.
Andrei
