compile time 'address'

[Dominic Jones]

Hello,

I would like to capture the "address" of variables at compile 
time. Something not far from this can be done in C++, namely 
addresses can be compared but not captured (then compared, or 
whatever).

I was hoping that it could be done in D, so I wrote something 
similar to test. It seems that D is more restrictive than C++ in 
this matter, as even comparison cannot be performed at compile 
time, as far as I can tell.

What I wish to do may in principle not be possible, but I am not 
familiar enough with compilers to know for sure. Also, I am not 
so familiar with D, so a better transliteration from the C++ 
example maybe possible and yield something on par or better.

The reason for wanting to do all this is to find some way of 
'baking in' the name of a variable (or some locally unique 
identifier) into an expression tree node type, facilitating 
certain expression tree transforms.

The closest I can get to what I want makes use of mixins, but is 
not ideal (https://run.dlang.io/is/Fr2b3f).


C++ (at: https://godbolt.org/z/CLPzGq)

template<typename T, typename U>
auto constexpr cmp(T const &t, U const &u)
{
/* Preferable, but not possible:
      auto constexpr ta = &t; // capture
      auto constexpr ua = &u;
      return ta == ua;
*/
   return &t == &u;
}

int main()
{
   auto c0 = 1;
   auto c1 = 2;

   static_assert(cmp(c0, c0));
   static_assert(!cmp(c0, c1));
}


D (at: https://run.dlang.io/is/rf5azp)

auto cmp(T, U)(const ref T t, const ref U u)
{
   return &t == &u;
}

void main()
{
   auto c0 = 1;
   auto c1 = 2;

   // error: "variable c0 cannot be read at compile time"
   static assert(cmp(c0, c0));
   static assert(!cmp(c0, c1));
}