Enum that can be 0 or null

[Alex Parrill via Digitalmars-d-learn]

On Tuesday, 7 June 2016 at 04:31:56 UTC, ParticlePeter wrote:
> On Monday, 6 June 2016 at 20:32:23 UTC, Alex Parrill wrote:
>> They'd be the same type, since you would define the vulkan 
>> functions to take these structures instead of pointer or 
>> integer types.
>>
>> It relies on a lot of assumptions about the ABI that make a 
>> raw pointer work the same as a structure containing just one 
>> pointer, which is why I did not give it much consideration.
>
> Is there a way to use opCast (just an idea, not working code) ?
>
> private struct VK_HANDLE_HELPER {
> 	const void * handle = null;
> 	alias handle this;
> 	T opCast(T)() if( is( T == uint64_t )) {
> 		return 0uL;
> 	}
> }
> const VK_NULL_HANDLE = VK_HANDLE_HELPER();


I don't think opCast gets called for implicit conversions; it 
only gets called for explicit casts. I'll test it later.