Statically get offset of member variable

First of, as requested, you're goal is not achievable as the type of a impacts the offst of a inside B:

struct B
{
    int some_variables[256];
    A</* offset of a inside B */> a;
};

This is alignment.

---

You could use the standard macro offsetof. This implies two things:

1. Since offsetof(type, member) is well-defined only for standard-layout types, the enclosing type must be standard-layout,
2. and since offsetof can only be "called" on complete types, its statically-computed result can only be set to the subobject dynamically; it canoot be a template non-type parameter, but can be a constructor argument.

Full program

#include <cassert>
#include <cstdint>
#include <cstddef>

struct Location
{
    Location(std::size_t offset) : offset_(offset) {}
    std::size_t offset_;
    operator std::intptr_t () const { return reinterpret_cast<std::intptr_t>(this) - offset_; }
};

struct SomeType
{
    int some_variables[256];
    Location location = offsetof(SomeType, location);
};

int main()
{
    SomeType obj;
    assert(reinterpret_cast<std::intptr_t>(&obj) == obj.location); // does pass
}

live demo

But as you commented, this is quite useless as Location could be simply defined as

template<class T>
struct Location
{
    Location(T* location) : location_(location) {}
    T* location_;
    operator T* () const { return location; }
};

and initialized with Location location = this;.