Why int **m its not equal to int m[][]?

Question

I'd like to clarify a concept that I haven't been able to fully understand.

Why in C is the type int **m compatible with int *m[N], but not with int m[N][N]? An 2D-array can be seen as a pointer to an array of integers, so int (*m)[N]. However, it could also be seen as an array of pointers to integers, so int *m[N]. What I don't understand is why int **m cannot be seen as int m[][] but as int *M[], while int m[][] can be seen as int (*m)[] but not as int **m.

I apologize if this explanation seems a bit confusing.

Answer 1

int m[X][Y] couldn't possibly decay into an int ** since m contains no pointers to which the int ** could point.

Array T m[X] decays into a pointer to its first element (&m[0]) when used as a pointer. Since the element has type T, this pointer will obviously have the type T *.

Example m	T, the type of m[0]	T *, the type of &m[0]
int m[X]	int	int *
int m[X][Y]	int[Y]	int (*)[Y]

Here's how you could pass a 2d array (so to speak) to a function:

void f( size_t x, size_t y, int (*m)[y] ) {
   // ...
}

int m[X][Y];
f( X, Y, m );