Why no offsetof for static struct?

[Ali Çehreli via Digitalmars-d-learn]

On 07/10/2017 02:14 PM, FoxyBrown wrote:
 > On Monday, 10 July 2017 at 20:13:46 UTC, Adam D. Ruppe wrote:
 >> On Monday, 10 July 2017 at 20:01:39 UTC, FoxyBrown wrote:
 >>> Cannot get the offset of static members of a struct
 >>
 >> That's because static members do not have an offset. They are not part
 >> of the struct in memory, just in name.
 >>
 >>> We can clearly get a pointer to the static struct X
 >>
 >> There's barely any such thing as a static struct. That's just a struct
 >> that stands without outside context.... which is almost all structs,
 >> actually, so the term isn't special.
 >>
 >>> since &X.x is effectively the address of X(regardless nomenclature
 >>> and terminology issues in D trying to hide this).
 >>
 >> No, it isn't. Static members are stored in an entirely different place
 >> than non-static members. They are really just global variables in
 >> memory with their in-source name being nested somewhere else.
 >
 >
 > This is not true, just because there isn't any official statement, or
 > even if denied, does not make it true. I have proof:
 >
 > auto GetStaticAddress(T)()
 > {
 >     mixin("auto p = cast(T*)&T."~__traits(allMembers, T)[0]~";");
 >     return p;
 > }
 >
 >
 > Returns the address of a struct's static members.

Yes but that address is not offset from the beginning of any struct 
object. What would be its relationship to the following two objects?

import std.stdio;

auto GetStaticAddress(T)()
{
     mixin("auto p = cast(T*)&T."~__traits(allMembers, T)[0]~";");
     return p;
}

struct S {
     static int s;
     int m;
}

void main() {
     writeln("S.s is at ", GetStaticAddress!S);
     auto a = S();
     auto b = S();
     writeln("a   is at ", &a);
     writeln("b   is at ", &b);
}

Prints

S.s is at 7F6D68484710
a   is at 7FFEADF41B68
b   is at 7FFEADF41B6C

So there is no offset of S.s to speak of (i.e. number of bytes from the 
beginning of) neither from a nor b.

 > It's pretty obvious, the compiler seems to instantiate the static
 > members simply as a sort of "singleton".

Makes sense.

 > They are laid out with the same
 > relative offsets as the struct(which is obvious, as that is the most
 > natural way). That is all that is needed to treat the memory the static
 > variables use as the struct in which they are part of. No need to make
 > it any more complex than that.

S.s is sitting in memory all by itself without are relation to any other 
S members.

 > Just because D obfuscates that static structs have addresses, doesn't
 > mean they don't.

I assume you mean "static members". Then, yes, of course they have 
addresses. And you don't even need a function like GetStaticAddress():

     writeln("S.s is at ", &S.s);

 > No need to perpetuate a misnomer. Static structs and
 > classes have addresses.

Again, I think you mean "static members of" or "instances of" structs 
and classes.

Ali