Recently I got my first surprise with our use of D. The symptom was that two local variables in two different functions appeared to be sharing data.

A simplified example is shown below (the original was machine translated from Pascal and involved templates and various levels of indirection). What I did not know is that the initial value of struct members is a compile time feature, apparently. What I suspect is happening is that the array lives in the static data segment (or is created in the module constructor?) and that the slices inside arr1 and arr2 get initialised to point to that same array.

I could use some help in rewriting the code below so that arr1 and arr2 each have their own data; ideally with minimal changes so that I can make the transcompiler do the right thing.

Thanks!
Bastiaan.

void main()
{
	import std.stdio;

	WrapIntegerArray arr1;
	arr1[0] = 42;

	WrapIntegerArray arr2;

	writeln(arr2[0]); // 42, not 0.
	writeln("arr1.wrap.arr.ptr = ", arr1.wrap.arr.ptr);
	writeln("arr2.wrap.arr.ptr = ", arr2.wrap.arr.ptr); // identical
	assert(arr2[0] == 0); // fails
}

struct IntegerArray
{
	int[] arr;
	alias arr this;
	this(int l)
	{
		arr = new int[l];
	}
}

struct WrapIntegerArray
{
	auto wrap = IntegerArray(5); // This is CTFE! :-(
	alias wrap this;
}

On Monday, 11 November 2019 at 19:17:37 UTC, Bastiaan Veelo wrote:
> Recently I got my first surprise with our use of D. The symptom was that two local variables in two different functions appeared to be sharing data.
>
> A simplified example is shown below (the original was machine translated from Pascal and involved templates and various levels of indirection). What I did not know is that the initial value of struct members is a compile time feature, apparently. What I suspect is happening is that the array lives in the static data segment (or is created in the module constructor?) and that the slices inside arr1 and arr2 get initialised to point to that same array.
>
> I could use some help in rewriting the code below so that arr1 and arr2 each have their own data; ideally with minimal changes so that I can make the transcompiler do the right thing.
>
> Thanks!
> Bastiaan.
>
> void main()
> {
> 	import std.stdio;
>
> 	WrapIntegerArray arr1;
> 	arr1[0] = 42;
>
> 	WrapIntegerArray arr2;
>
> 	writeln(arr2[0]); // 42, not 0.
> 	writeln("arr1.wrap.arr.ptr = ", arr1.wrap.arr.ptr);
> 	writeln("arr2.wrap.arr.ptr = ", arr2.wrap.arr.ptr); // identical
> 	assert(arr2[0] == 0); // fails
> }
>
> struct IntegerArray
> {
> 	int[] arr;
> 	alias arr this;
> 	this(int l)
> 	{
> 		arr = new int[l];
> 	}
> }
>
> struct WrapIntegerArray
> {
> 	auto wrap = IntegerArray(5); // This is CTFE! :-(
> 	alias wrap this;
> }


Defining and using a constructor for WrapIntegerArray seems to work:

void main()
{
	import std.stdio;

	WrapIntegerArray arr1 = WrapIntegerArray(5);
	arr1[0] = 42;

	WrapIntegerArray arr2 = WrapIntegerArray(5);

	writeln(arr2[0]); // 42, not 0.
	writeln("arr1.wrap.arr.ptr = ", arr1.wrap.arr.ptr);
	writeln("arr2.wrap.arr.ptr = ", arr2.wrap.arr.ptr); // identical
	assert(arr2[0] == 0); // fails
}

struct IntegerArray
{
	int[] arr;
	alias arr this;
	this(int l)
	{
		arr = new int[l];
	}
}

struct WrapIntegerArray
{
        this (int v) {
	  wrap = IntegerArray(5);
        }

        IntegerArray wrap;
	alias wrap this;
}

Hope this helps.
Antonio

On Monday, November 11, 2019 12:17:37 PM MST Bastiaan Veelo via Digitalmars- d-learn wrote:
> Recently I got my first surprise with our use of D. The symptom was that two local variables in two different functions appeared to be sharing data.
>
> A simplified example is shown below (the original was machine translated from Pascal and involved templates and various levels of indirection). What I did not know is that the initial value of struct members is a compile time feature, apparently. What I suspect is happening is that the array lives in the static data segment (or is created in the module constructor?) and that the slices inside arr1 and arr2 get initialised to point to that same array.
>
> I could use some help in rewriting the code below so that arr1 and arr2 each have their own data; ideally with minimal changes so that I can make the transcompiler do the right thing.
>
> Thanks!
> Bastiaan.
>
> void main()
> {
>   import std.stdio;
>
>   WrapIntegerArray arr1;
>   arr1[0] = 42;
>
>   WrapIntegerArray arr2;
>
>   writeln(arr2[0]); // 42, not 0.
>   writeln("arr1.wrap.arr.ptr = ", arr1.wrap.arr.ptr);
>   writeln("arr2.wrap.arr.ptr = ", arr2.wrap.arr.ptr); // identical
>   assert(arr2[0] == 0); // fails
> }
>
> struct IntegerArray
> {
>   int[] arr;
>   alias arr this;
>   this(int l)
>   {
>       arr = new int[l];
>   }
> }
>
> struct WrapIntegerArray
> {
>   auto wrap = IntegerArray(5); // This is CTFE! :-(
>   alias wrap this;
> }

All struct and class members which are directly initialized must have their values known at compile-time. For structs, that's what goes in the init value for the type. A side effect of this is that it's usually a bad idea to directly initialize dynamic arrays which are member variables. You need to do the initialization in a constructor. And for structs, if you need a no-arg constructor, then you'll need to use a factory function (since structs can't have no-arg constructors). e.g.

struct WrapIntegerArray
{
    IntegerArray wrap;
    alias wrap this;

    this(int len)
    {
        wrap = IntegerArray(len);
    }
}

or

struct WrapIntegerArray
{
    IntegerArray wrap;
    alias wrap this;

    static make()
    {
        WrapIntegerArray retval;
        retval.wrap = IntegerArray(5);
        return retval;
    }
}

So, you could then have something like

    auto arr1 = WrapIntegerArray(5);
    arr1[0] = 42;

    or

    auto arr1 = WrapIntegerArray.make();
    arr1[0] = 42;

but if you use the init value (which is what you get if you let the type be default-initialized), then you'll have to first do something to allocate the dynamic array if you want to be able to index it, since if you don't give it a value at compile-time, it's null (and you don't want to give it a value at compile-time, because then every default-initialized struct of that type will refer to the same dynamic array). Of course, you could always just append values, and the dynamic array will be allocated and grow accordingly, but that's obviously not the same as allocating it up front to have a specific length.

- Jonathan M Davis

On Monday, 11 November 2019 at 20:05:11 UTC, Antonio Corbi wrote:
> Defining and using a constructor for WrapIntegerArray seems to work:
>
> void main()
> {
> 	import std.stdio;
>
> 	WrapIntegerArray arr1 = WrapIntegerArray(5);
> 	arr1[0] = 42;
>
> 	WrapIntegerArray arr2 = WrapIntegerArray(5);
>
> 	writeln(arr2[0]); // 42, not 0.
> 	writeln("arr1.wrap.arr.ptr = ", arr1.wrap.arr.ptr);
> 	writeln("arr2.wrap.arr.ptr = ", arr2.wrap.arr.ptr); // identical
> 	assert(arr2[0] == 0); // fails
> }
>
> struct IntegerArray
> {
> 	int[] arr;
> 	alias arr this;
> 	this(int l)
> 	{
> 		arr = new int[l];
> 	}
> }
>
> struct WrapIntegerArray
> {
>         this (int v) {
> 	  wrap = IntegerArray(5);
>         }
>
>         IntegerArray wrap;
> 	alias wrap this;
> }
>
> Hope this helps.
> Antonio

Thanks, Antonio. My problem is that the length of the array should be a built-in property of WrapIntegerArray (immutable in this case); what I'd actually want is a constructor without arguments. Jonathan's suggestion of using a factory function comes closest to that.

Bastiaan.

On Tuesday, 12 November 2019 at 07:59:39 UTC, Bastiaan Veelo wrote:
> On Monday, 11 November 2019 at 20:05:11 UTC, Antonio Corbi wrote:
>> [...]
>
> Thanks, Antonio. My problem is that the length of the array should be a built-in property of WrapIntegerArray (immutable in this case); what I'd actually want is a constructor without arguments. Jonathan's suggestion of using a factory function comes closest to that.
>
> Bastiaan.

I'm curious to know what is the equivalent in Pascal that your transpiler fails to translate since Pascal records don't have constructors at all. Maybe you used an old school 'Object' ?

On Monday, 11 November 2019 at 21:52:12 UTC, Jonathan M Davis wrote:
> On Monday, November 11, 2019 12:17:37 PM MST Bastiaan Veelo via Digitalmars- d-learn wrote:

[...]

>> I could use some help in rewriting the code below so that arr1 and arr2 each have their own data; ideally with minimal changes so that I can make the transcompiler do the right thing.
>>
>> Thanks!
>> Bastiaan.
>>
>> void main()
>> {
>>   import std.stdio;
>>
>>   WrapIntegerArray arr1;
>>   arr1[0] = 42;
>>
>>   WrapIntegerArray arr2;
>>
>>   writeln(arr2[0]); // 42, not 0.
>>   writeln("arr1.wrap.arr.ptr = ", arr1.wrap.arr.ptr);
>>   writeln("arr2.wrap.arr.ptr = ", arr2.wrap.arr.ptr); // identical
>>   assert(arr2[0] == 0); // fails
>> }
>>
>> struct IntegerArray
>> {
>>   int[] arr;
>>   alias arr this;
>>   this(int l)
>>   {
>>       arr = new int[l];
>>   }
>> }
>>
>> struct WrapIntegerArray
>> {
>>   auto wrap = IntegerArray(5); // This is CTFE! :-(
>>   alias wrap this;
>> }
>
> All struct and class members which are directly initialized must have their values known at compile-time. For structs, that's what goes in the init value for the type. A side effect of this is that it's usually a bad idea to directly initialize dynamic arrays which are member variables. You need to do the initialization in a constructor. And for structs, if you need a no-arg constructor, then you'll need to use a factory function (since structs can't have no-arg constructors). e.g.
>
> struct WrapIntegerArray
> {
>     IntegerArray wrap;
>     alias wrap this;
>
>     this(int len)
>     {
>         wrap = IntegerArray(len);
>     }
> }
>
> or
>
> struct WrapIntegerArray
> {
>     IntegerArray wrap;
>     alias wrap this;
>
>     static make()
>     {
>         WrapIntegerArray retval;
>         retval.wrap = IntegerArray(5);
>         return retval;
>     }
> }
>
> So, you could then have something like
>
>     auto arr1 = WrapIntegerArray(5);
>     arr1[0] = 42;
>
>     or
>
>     auto arr1 = WrapIntegerArray.make();
>     arr1[0] = 42;
>
> but if you use the init value (which is what you get if you let the type be default-initialized), then you'll have to first do something to allocate the dynamic array if you want to be able to index it, since if you don't give it a value at compile-time, it's null (and you don't want to give it a value at compile-time, because then every default-initialized struct of that type will refer to the same dynamic array). Of course, you could always just append values, and the dynamic array will be allocated and grow accordingly, but that's obviously not the same as allocating it up front to have a specific length.
>
> - Jonathan M Davis

Thank you Jonathan. A factory function seems to be what I need, then. It'll be an interesting challenge to have my transpiler detect when a factory function is needed, and making sure that they are called at the right places. But this might escalate since structs can have members that are structs that have a factory function. So the enclosing struct also needs a factory function. And so forth.

My problem is the translation of so called schematic arrays, a type that I have only seen in Extended Pascal. A schematic array is a type in which the length of the array is a parameter of the type. Extended Pascal does not have dynamic arrays, so schematic arrays are its attempt at improving a bit on just plain static arrays. The length parameter can be constant, in which it behaves akin to templates (which it doesn't have either), but it can also be set at run time during initialization (so I can't translate it to a template).

Maybe I can omit the translation of schematic arrays (IntegerArray in this case) altogether and instead detect where and how they are instantiated. Then replace them with a static array whenever the length is known at compile time, and a dynamic array otherwise. A static array is not nice, but at least they can be members of structs without a factory function. Either way, my transpiler needs to smarten up...

Bastiaan.

On Tuesday, 12 November 2019 at 08:15:20 UTC, Basile B. wrote:
>
> I'm curious to know what is the equivalent in Pascal that your transpiler fails to translate since Pascal records don't have constructors at all. Maybe you used an old school 'Object' ?

Note that Extended Pascal is not exactly Pascal. An example:

  TYPE Ints(upperBound) = ARRAY [1 .. upperBound] of Integer;
       MyRecord = RECORD
                      integers : Ints(5);
                  END;
       SchematicRecord(num) = RECORD
                                  integers : Ints(num);
                              END;
       IntsPtr = ^Ints;

  PROCEDURE myProcedure(PROTECTED VAR someArr : Ints);
  BEGIN
      writeln(someArr.upperBound);
  END;

  PROCEDURE proc;
  VAR dynamicInts : IntsPtr;
      rec : MyRecord;
  BEGIN
      dynamicInts = new(10);
      myProcedure(dynamicInts^);
      myProcedure(rec.integers);
  END;

In this case, only the upper bound of Ints is parameterized, but the lower bound could be parameterized as well. Records can also be schematized. Procedure arguments can take schemas that are undiscriminated, they carry their schemaparameters as properties.

Bastiaan.