Why is sizeof giving wrong answer - c++11

Question

ok I've come across a weirdness, maybe someone can explain it.
Source code is (c++ 11) :
‪#‎include‬ <stdio.h>
struct xyz_ {
float xyz[3];
float &x = xyz[0];
float &y = xyz[1];
float &z = xyz[2];
};
int main(int argc, char *argv[])
{
xyz_ xyz;
xyz.x = 0;
xyz.y = 1;
xyz.z = 2;
xyz.xyz[1] = 1;
printf("as array %f %f %f\n",xyz.xyz[0],xyz.xyz[1],xyz.xyz[2]);
printf("as elements %f %f %f\n",xyz.x,xyz.y,xyz.z);
int sizexyz = sizeof(xyz);
int sizefloat = sizeof(float);
printf("float is %d big, but xyz is %d big\n",sizefloat,sizexyz);
return 0;
}
output is:
as array 0.000000 1.000000 2.000000
as elements 0.000000 1.000000 2.000000
float is 4 big, but xyz is 24 big
So the structure works as I would expect, but the size is twice as large as it should be. Using chars instead of float in the structure gives a segfault when run.
I wanted to use struct xyz_ as either an array of floats or individual float elements.

Answer 1

It is unspecified whether references require storage. In this case your output suggests that your compiler has decided to use storage to implement the references x, y and z.

Answer 2

Suppose you add another constructor:
struct xyz_ {
float xyz[3];
float &x = xyz[0];
float &y = xyz[1];
float &z = xyz[2];
xyz_()
{
}
xyz_(float& a, float& b, float& c)
: x(a), y(b), z(c)
{
}
};
It should be clear that now the three x, y and z members may be bound to the array elements or may be bound to something else.
Looks like what you are looking for is
union P3d {
float xyz[3];
struct {
float x, y, z;
};
};
Unfortunately for some strange reasons (apparently mostly political) this is not supported in the standard (despite compilers do actually support it).

Answer 3

How about this:
struct xyz_
{
float xyz[3];
float &x() {return xyz[0];}
float &y() {return xyz[1];}
float &z() {return xyz[2];}
};
Not as beautiful or elegant, but might reduce the size a bit, though I think the this pointer might occupy additional space, not sure...
Of course you would have to use x(), y() and z().

Answer 4

What would be the size of xyz_ if it's declared like this?
struct xyz_ {
float xyz[3];
float *x = &xyz[0];
float *y = &xyz[1];
float *z = &xyz[2];
};
The reference also needs it's own space to store the information where it is pointing at.

Answer 5

In C you could do the following, but it's not legal in C++11.
union xyz_ {
float xyz[3];
struct { float x, y, z; };
};