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digitalmars.D - Enums with Inline Subclass

reply mark_mcs <mark mnetcs.com> writes:
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?
Nov 13 2019
next sibling parent Bastiaan Veelo <Bastiaan Veelo.net> writes:
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.
Nov 13 2019
prev sibling next sibling parent Bastiaan Veelo <Bastiaan Veelo.net> writes:
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).


You might be interested in 
https://code.dlang.org/packages/state-machine

Bastiaan.
Nov 14 2019
prev sibling parent Suleyman <sahmi.soulaimane gmail.com> writes:
On Wednesday, 13 November 2019 at 20:46:25 UTC, mark_mcs wrote:

 [...]
 My question is: should this be possible? Is this a grammar bug 
 or a codegen bug?


You should open an issue at https://issues.dlang.org/.

This works:
```
class I {}

static I obj;
static this()
{
     obj = new class I {};
}

void main() {}
```

But this doesn't:
```
class I {}
static immutable I obj = cast(immutable) new class I {};

void main() {}
```
Nov 14 2019