Casting (double *) to (double **) allowed?

Question

Because I followed a discussion where was told "Aliasing through incompatible pointer types is undefined behavior" (e.g. double d; int *p = (int *)&d; following question:

Is it allowed to cast an (double *) to (double **), e.g. double *d1; double **d2 = &d2 and using syntax like d2[0][y] expecting to be the same as d1[y]?

I know that it is not exactly aliasing through incompatible pointer types, but however I am not sure. Background is that I want to have a function which operates on 2-dimensional arrays (= images) but I want to be able to pass only a row or column of an image.

Answer 1

double** is incompatible with double*. The conversion is legal, but the only thing you can do with the results is cast it back. This seems more or less obvious: on a 32 bit machine, a double* won't even have the same size as a double.

But your example doesn't convert a double* to double**. It creates a new double**, which points to the double*. This is fine.

Answer 2

Is it allowed to cast an (double ) to (double *), e.g. double *d1; double **d2 = &d2 and using syntax like d2[0][y] expecting to be the same as d1[y]?

Of course not, and it won't even compile.

But if you meant:

double *d1; 
double **d2 = &d2;

then yes, the above is perfectly valid and since d2[0][y] can be seen as (*d2)[y], there's really no problem at all.

Finally, remember that in the above code, you are not casting anything: &d2 is already of type double**. If you have to cast something, please use C++ style casts (static_cast, dynamic_cast, reinterpret_cast, etc..) and not C style casts.