Aliasing struct and array the C++ way

Question

This is a C++ followup for another question of mine

In the old days of pre-ISO C, the following code would have surprised nobody:

struct Point {
             


        

Answer 1

Use an constexpr array of pointer-to-member:

#include <math.h>

struct Point {
    double x;
    double y;
    double z;
};

double dist(struct Point *p1, struct Point *p2) {
    constexpr double Point::* coords[3] = {&Point::x, &Point::y, &Point::z};

    double d2 = 0;
    for (int i=0; i<3; i++) {
        double d = p1->*coords[i] - p2->*coords[i];
        d2 += d * d;
    }
    return sqrt(d2);
}

Answer 2

You could use the fact that casting a pointer to intptr_t doing arithmetic and then casting the value back to the pointer type is implemetation defined behavior. I believe it will work on most of the compilers:

template<class T>
T* increment_pointer(T* a){
  return reinterpret_cast<T*>(reinterpret_cast<intptr_t>(a)+sizeof(T));
  }

This technic is the most efficient, optimizers seems not to be able to produce optimal if one use table look up: assemblies-comparison

Answer 3

IMHO the easiest way is to just implement operator[]. You can make a helper array like this or just create a switch...

struct Point
{
    double const& operator[] (std::size_t i) const 
    {
        const std::array coords {&x, &y, &z};
        return *coords[i];
    }

    double& operator[] (std::size_t i) 
    {
        const std::array coords {&x, &y, &z};
        return *coords[i];
    }

    double x;
    double y;
    double z;
};

int main() 
{
    Point p {1, 2, 3};
    std::cout << p[2] - p[1];
    return 0;
}

Answer 4

struct Point {
  double x;
  double y;
  double z;
  double& operator[]( std::size_t i ) {
    auto self = reinterpret_cast<uintptr_t>( this );
    auto v = self+i*sizeof(double);
    return *reinterpret_cast<double*>(v);
  }
  double const& operator[]( std::size_t i ) const {
    auto self = reinterpret_cast<uintptr_t>( this );
    auto v = self+i*sizeof(double);
    return *reinterpret_cast<double const*>(v);
  }
};

this relies on there being no packing between the doubles in your `struct. Asserting that is difficult.

A POD struct is a sequence of bytes guaranteed.

A compiler should be able to compile [] down to the same instructions (or lack thereof) as a raw array access or pointer arithmetic. There may be some problems where this optimization happens "too late" for other optimzations to occur, so double-check in performance sensitive code.

It is possible that converting to char* or std::byte* insted of uintptr_t would be valid, but there is a core issue about if pointer arithmetic is permitted in this case.