C++ asterisk and bracket operators used together

Question

Sorry for the crappy title, but I don't know how to better describe this problem.

What is the meaning of:

int (*x)[];

and how to initialize it?

I know it isn't int *x[] since:

int a,b;
int (*x)[] = {&a, &b};

won't compile.

Thank you in advance.

Answer 1

Type declarations have an expression-like syntax, so you parse them as you would an expression:

      x       x is
     *x       a pointer
    (*x)[]    to an array of unknown dimensions
int (*x)[]    of int

The precedence rule is that the operators to the right bind tighter than those to the left, in each case, the operator closer to the element binds tighter, and finally, that parentheses can be used to change these bindings, so:

int  *x[];    is an array of pointers,
int *(x[]);   as is this.
int (*x)[];   whereas here, the parentheses change the bindings.

Answer 2

Using cdecl, you can easily determine the type:

declare x as pointer to array of int

So you can initialize x with the address of an array:

int a[]; // conceptual: error if compiled
x = &a;

Note that the size of the array is part of its type, so you cannot assign the address of an array of size N to a pointer to array of size M, where N!=M (this is the reason for the error above: you need to know the size of the array to declare it)

int b[5];
int c[6];
int (*y)[6];
y = &b; // error
y = &c; // OK