Multiple inheritance and the this pointer

Question

Suppose I have this struct:

struct vector_data
{
    double x, y;

    double& operator[](size_t index)
    {
        return * (static_cast<double*>(static_cast<void*>(this)) + index);
    }
};

The operator[] should work as expected, because vector_data is a POD type. The expected behaviour is that vector_data[0] returns x, and vector_data[1] returns y.

Now suppose I have a second struct:

struct more_data
{
    double evil_data;

    // There could be more here, data or functions
};

And derive from both like this:

struct composed : public more_data, public vector_data
{
};

Will this destory the expected behaviour of operator[]? In other words, will the this-pointer of vector_data in the derived struct still point to the vector_data part of the struct, or will it point to the beginning of the derived struct?

If it does destroy operator[], then how can I resolve this problem? I can inherit from vector_data first, but suppose composed contains virtual functions. I know most compilers put the vtable at the end, but this is not guaranteed. What would be the best approach?

Answer 1

Leaving aside the issues of your incorrect pointer arithmetics (the possibility of padding between x and y invalidates your assumption), here is a quick illustration of what's going on with this pointer when you use multiple inheritance:

#include <iostream>
using namespace std;

struct a {
    int aa;
    void showA() {
        cerr << this << endl;
    }
};
struct b {
    int bb;
    void showB() {
        cerr << this << endl;
    }
};
struct c : public a, b {
    int cc;
    void showC() {
        cerr << this << endl;
    }
};
int main() {
    c x;
    x.showA();
    x.showB();
    x.showC();
}

showA and showB print different numbers; showC prints the same number as showA, because a is listed first in the list of bases. If you switch a and b there, then showC and showB would be the same. The "magic" is in the C++ compiler: it is smart enough to give each member function a correct this pointer.

Answer 2

Probably what you want is something like:

struct vector_data
{
   union 
   {
        struct 
        {
            double x, y;
        }; 
        double data[2];
   }; 

   double& operator[](size_t index)
   {
       return data[index];
   }
}