Re: Unexpected aliasing

[Antonio Corbi via Digitalmars-d-learn]

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