Struggling trying to use opCast to solve the the interface impl VS the class impl

[user1234]

On Saturday, 8 November 2025 at 10:04:45 UTC, user1234 wrote:
> Following recent discussion in the General group, I had the 
> idea to use `opCast` in order to return a specially cooked 
> interface instance.
>
> That would not be a decent solution because for example 
> `opCast` template func is not virtual but nonetheless I'd like 
> to know if this could work...
>
> Only problem is that I cant manage to build the custom 
> interface, or more exactly, it is not binary compatible:

Slightly less C-ish and eliminating the recursive opCast, but 
still not working (valgrind reports memory errors)

```d
import core.stdc.stdio, core.memory;

interface CanStart
{
     void start();
}

class Base : CanStart
{
     public void start()
     {
         puts("object start()");
     }

     public void interfaceStart()
     {
         puts("itf start()");
     }

     T opCast(T)()
     {
         static if (is(T == CanStart))
         {
             struct S
             {
                 Base b;
                 void* f;
             }

             auto s = cast(S*) GC.malloc(size_t.sizeof*2);
             s.b = this;  // likely the issue
             s.f = (&interfaceStart).ptr;

             auto result = *cast(CanStart*) s;

             return result;
         }
         assert(0);
     }
}

void main()
{
     auto b = new Base;
     b.start();

     auto cs = cast(CanStart) b;
     cs.start();
}
```

Anyone ?