Enum that can be 0 or null

[ParticlePeter via Digitalmars-d-learn]

On Saturday, 21 May 2016 at 06:36:53 UTC, tsbockman wrote:
> ...
> As an example, if VK_NULL_HANDLE only ever needs to be assigned 
> to opaque types on the D side (that is, types that serve only 
> as an ID or address for communicating with the C side), you 
> could do this:
>
> private struct VkNullHandle { }
> enum VK_NULL_HANDLE = VkNullHandle.init;
>
> mixin template VkHandle(bool dispatchable) {
> 	static if (dispatchable || (size_t.sizeof == 8))
> 		void* bits = null;
> 	else
> 		ulong bits = 0;
> 	
> 	this(typeof(this) that) {
> 		this.bits = that.bits; }
> 	this(VkNullHandle that) {
> 		this.bits = typeof(this.bits).init; }
>
> 	ref typeof(this) opAssign(typeof(this) that) {
> 		this.bits = that.bits;
> 		return this;
> 	}
> 	ref typeof(this) opAssign(VkNullHandle that) {
> 		this.bits = typeof(this.bits).init;
> 		return this;
> 	}
> }
>
> struct VkDevice { mixin VkHandle!true; }
> struct VkInstance { mixin VkHandle!true; }
>
> struct VkFence { mixin VkHandle!false; }
> struct VkSemaphore { mixin VkHandle!false; }
>
>
> void main() {
> 	VkInstance a = VK_NULL_HANDLE;
> 	VkFence b = VK_NULL_HANDLE;
> }
>
> (DPaste: https://dpaste.dzfl.pl/8f4ce39a907f )
>
> The above is typesafe, and can easily be made compatible with a 
> typical C API, since C does no parameter-related name mangling.

In this case I don't see how it would be possible to use your 
VkDevice, etc. as argument to the C functions, as they are of 
different type, no?