compile time 'address'

[Steven Schveighoffer]

On 11/29/18 12:59 PM, Dominic Jones wrote:
> Hello Steve,
> 
> Thank you for your comments. A couple of replies:
> 
> 
> On Thursday, 29 November 2018 at 16:29:00 UTC, Steven Schveighoffer wrote:
>> 1. No you cannot read c0 or c1 at compile time, because they are 
>> runtime values. You can fix this by changing them to immutable or enum.
> 
> The variables are deliberately 'auto' rather than 'auto constexpr' as 
> per the C++ example, so I do wish to preserve that in the D example. 
> Nevertheless, in C++ querying (at least to perform comparison) the 
> sudo-address, whatever it is (perhaps, behind the scenes it is simply a 
> stack frame offset), is permitted.
> 
> 
>> 2. D doesn't like you taking the address of compile-time values, but 
>> the error message is really bad.
>>
>> If I do this:
>>
>> auto cmp(T, U)(const T t, const U u) // note, no ref
>> {
>>   return t == u; // note, change to value not address
>> }
> 
> Presumably, this is now simply comparing values held by the variables, 
> rather than the sudo-address of the original variables as per the C++ 
> example?

Ah, ok. Essentially you are looking for the layout of the local variables.

One thing that is available for D is the compile-time property offsetof, 
which gives the offset from the start of an aggregate that an item is.

for example:

struct S
{
   int x;
   bool y;
}

static assert(S.y.offsetof == 4);

So while this isn't available for function locals, the idea is similar. 
But I don't think there's an equivalent to what you are able to do here 
in C++.

I don't think it would be a huge stretch to apply this same property to 
function locals, but it would require an enhancement request.

-Steve