Enums with Inline Subclass

[Bastiaan Veelo]

On Wednesday, 13 November 2019 at 20:46:25 UTC, mark_mcs wrote:
> I spend most of my time in java land but try to sneak in some D 
> code whenever possible. One pattern that comes in handy is to 
> use Java enums as states in a finite state machine (good for 
> XML parsers). Given that D supports enums with custom base 
> types, I was curious to see if this pattern would translate.
>
> The code looks like this:
>
>
> interface A
> {
>     void process();
> }
>
> enum Test : A
> {
>     Value1 = new class A {
>         override void process() { writeln("Value1"); }
>     },
>     Value2 = new class A {
>         override void process() { writeln("Value2"); }
>     }
> }
>
> void main()
> {
>     Test.Value1.process();
> }
>
>
> It's syntactically valid but you get linker errors. Seems the 
> compiler doesn't generate the vtable for the inline classes:
>
> undefined reference to `_D3app4Test12__anonclass16__vtblZ'
> undefined reference to `_D3app4Test12__anonclass17__ClassZ
>
> My question is: should this be possible? Is this a grammar bug 
> or a codegen bug?

That is some odd code you got there. I would not have expected it 
to work. Normally references to objects cannot be stored in an 
enum. But if you change the interface into a class, and define 
the anonymous classes with a name, then it works:

import std;

class I
{
     void process() {}
}

class C : I
{
     override void process()
     {
         writeln("C");
     }
}

class D : I
{
     override void process()
     {
         writeln("D");
     }
}

// enum e = new C; // Unable to initialize enum with class or 
pointer to struct. Use static const variable instead.

enum e3 : I
{
     val1 = new C,
     val2 = new D,
     val3 = new class I
     {
         override void process()
         {
             writeln("E");
         }
     }
};

void main()
{
     e3.val1.process;
     e3.val2.process;
     // e3.val3.process; // link error
}


Bastiaan.