What happens if I define a 0-size array in C/C++?

Question

An array cannot have zero size.

ISO 9899:2011 6.7.6.2:

If the expression is a constant expression, it shall have a value greater than zero.

The above text is true both for a plain array (paragraph 1). For a VLA (variable length array), the behavior is undefined if the expression's value is less than or equal to zero (paragraph 5). This is normative text in the C standard. A compiler is not allowed to implement it differently.

gcc -std=c99 -pedantic gives a warning for the non-VLA case.

As per the standard, it is not allowed.

However it's been current practice in C compilers to treat those declarations as a flexible array member (FAM) declaration:

C99 6.7.2.1, §16: As a special case, the last element of a structure with more than one named member may have an incomplete array type; this is called a flexible array member.

The standard syntax of a FAM is:

struct Array {
  size_t size;
  int content[];
};

The idea is that you would then allocate it so:

void foo(size_t x) {
  Array* array = malloc(sizeof(size_t) + x * sizeof(int));

  array->size = x;
  for (size_t i = 0; i != x; ++i) {
    array->content[i] = 0;
  }
}

You might also use it statically (gcc extension):

Array a = { 3, { 1, 2, 3 } };

This is also known as tail-padded structures (this term predates the publication of the C99 Standard) or struct hack (thanks to Joe Wreschnig for pointing it out).

However this syntax was standardized (and the effects guaranteed) only lately in C99. Before a constant size was necessary.

• 1 was the portable way to go, though it was rather strange.
• 0 was better at indicating intent, but not legal as far as the Standard was concerned and supported as an extension by some compilers (including gcc).

The tail padding practice, however, relies on the fact that storage is available (careful malloc) so is not suited to stack usage in general.

In Standard C and C++, zero-size array is not allowed..

If you're using GCC, compile it with -pedantic option. It will give warning, saying:

zero.c:3:6: warning: ISO C forbids zero-size array 'a' [-pedantic]

In case of C++, it gives similar warning.

It's totally illegal, and always has been, but a lot of compilers neglect to signal the error. I'm not sure why you want to do this. The one use I know of is to trigger a compile time error from a boolean:

char someCondition[ condition ];

If condition is a false, then I get a compile time error. Because compilers do allow this, however, I've taken to using:

char someCondition[ 2 * condition - 1 ];

This gives a size of either 1 or -1, and I've never found a compiler which would accept a size of -1.