On Saturday, 17 February 2018 at 12:49:07 UTC, Andrei Alexandrescu wrote:
> On 02/16/2018 10:10 PM, rikki cattermole wrote:
>> Could use the name for the field as well. At the minimum useful for debugging purposes.
>
> That would be tricky because fields are decomposed down to primitive types. -- Andrei

That's unfortunate. Not being able to get the names and types together is my main dissatisfaction with the current interface.

On 2/19/18 6:18 AM, Nathan S. wrote:
> On Saturday, 17 February 2018 at 12:49:07 UTC, Andrei Alexandrescu wrote:
>> On 02/16/2018 10:10 PM, rikki cattermole wrote:
>>> Could use the name for the field as well. At the minimum useful for debugging purposes.
>>
>> That would be tricky because fields are decomposed down to primitive types. -- Andrei
> 
> That's unfortunate. Not being able to get the names and types together is my main dissatisfaction with the current interface.

.tupleof does all this:

struct S
{
  int x;
  double y;
}

static foreach(alias v; S.tupleof)
{
  pragma(msg, typeof(v));
  pragma(msg, v.offsetof);
  pragma(msg, __traits(identifier, v));
}

output while compiling:
int
0LU
x
double
8LU
y

It's admittedly an ugly interface...

-Steve

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.

-Steve

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

On Saturday, 17 February 2018 at 00:04:16 UTC, Andrei Alexandrescu wrote:
> I've been long bothered that the builtin .tupleof and our own abstractions Fields and RepresentationTypeTuple in std.traits - all omit the essential information of field offsets. That makes types that use align() to have the same .tupleof, Fields, and RepresentationTypeTuple even though they shouldn't.
>
> The right answer is Layout a tuple of (offset, type) pairs describing entirely the memory layout of a type. We need such for memory allocation, garbage collection, serialization, and more.
>
> The implementation turned out to be quite compact - 81 lines including a compile-time mergesort. Destroy!
>
> https://github.com/dlang/phobos/pull/6192
>
>
> Andrei

how does this deal with static members ?

On Wednesday, February 21, 2018 05:53:31 Basile B. via Digitalmars-d wrote:
> On Saturday, 17 February 2018 at 00:04:16 UTC, Andrei
>
> Alexandrescu wrote:
> > I've been long bothered that the builtin .tupleof and our own abstractions Fields and RepresentationTypeTuple in std.traits - all omit the essential information of field offsets. That makes types that use align() to have the same .tupleof, Fields, and RepresentationTypeTuple even though they shouldn't.
> >
> > The right answer is Layout a tuple of (offset, type) pairs describing entirely the memory layout of a type. We need such for memory allocation, garbage collection, serialization, and more.
> >
> > The implementation turned out to be quite compact - 81 lines including a compile-time mergesort. Destroy!
> >
> > https://github.com/dlang/phobos/pull/6192
> >
> >
> > Andrei
>
> how does this deal with static members ?

It's built on tupleof, so it doesn't do anything with static members. But static members have nothing to do with the layout of a type, so it wouldn't make sense for it to do anything with static members.

- Jonathan M Davis

On 02/21/2018 12:53 AM, Basile B. wrote:
> On Saturday, 17 February 2018 at 00:04:16 UTC, Andrei Alexandrescu wrote:
>> I've been long bothered that the builtin .tupleof and our own abstractions Fields and RepresentationTypeTuple in std.traits - all omit the essential information of field offsets. That makes types that use align() to have the same .tupleof, Fields, and RepresentationTypeTuple even though they shouldn't.
>>
>> The right answer is Layout a tuple of (offset, type) pairs describing entirely the memory layout of a type. We need such for memory allocation, garbage collection, serialization, and more.
>>
>> The implementation turned out to be quite compact - 81 lines including a compile-time mergesort. Destroy!
>>
>> https://github.com/dlang/phobos/pull/6192
>>
>>
>> Andrei
> 
> how does this deal with static members ?

Static members are not part of the layout - aside from lexical scoping, they have the same regime as module-level static data. -- Andrei

On 2/20/18 10:56 PM, Andrei Alexandrescu wrote:
> On 02/20/2018 07:34 AM, Steven Schveighoffer wrote:
>> 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.
> 

[snip]

Yep, this all makes sense, thanks! Sounds very similar to the Strawman structs thing, only probably better thought out ;)

-Steve