Question

struct test{
    unsigned long int asd[][3][6];
};

sizeof(struct test) returns 0. So, if that is an exact alias of

struct test{
    unsigned long int asd[0][3][6];
};

is there any practical use for such a field declaration? You may consider also the template metaprogramming stuff, which is always surprising.

Answer 1

The first example demonstrates the use of a flexible array member, a feature of C99. However in order to get that snippet to compile, you need to have another member in your struct, ie:

struct test{
    int a;
    unsigned long int asd[][3][6];
};

This documentation on gcc tells you why sizeof evaulates to zero, and the syntactical difference of a normal array:

In ISO C90, you would have to give contents a length of 1, which means
either you waste space or complicate the argument to malloc.

In ISO C99, you would use a flexible array member, which is slightly
different in syntax and semantics:

• Flexible array members are written as contents[] without the 0.
• Flexible array members have incomplete type, and so the sizeof operator may not be applied. As a quirk of the original implementation
  of zero-length arrays, sizeof evaluates to zero.
• Flexible array members may only appear as the last member of a struct that is otherwise non-empty.
• A structure containing a flexible array member, or a union containing such a structure (possibly recursively), may not be a
  member of a structure or an element of an array. (However, these uses
  are permitted by GCC as extensions.)