Can there be a C++ type that takes 0 bytes

Question

I'm trying to declare a C++ variable that takes up zero bytes. Its in a union, and I started with the type as int[0]. I don't know if that is actually zero bytes (although sizeof(int[0]) was 0). I need a better way to declare a 0 byte type, and hopefully one that can be typedefed to something like nullType or emptyType. The variable is in a union, so in the end memory is reserved anyway. I tried void on the off chance it would work, but C++ complained. I'm using Ubuntu 10.10, with a current version of the kernel, and an up-to-date GCC. Here's the union:

union RandomArgumentTypesFirst
{
    uint uintVal;
    nullType nullVal;
}

And here is the typedef:

typedef int[0] nullType;

The compiler says this about the typedef:

error: variable or field ‘nullVal’ declared voidmake[2]:

When I typed in int[0], it worked. Any suggestions?

EDIT: As @fefe said in the comments, the int[0] may be provided as an extension by the compiler. GCC's website says that the compiler has many extensions by default.

Answer 1

You cannot instantiate any data type in C++ that takes up zero bytes. The Standard dictates than an empty class, such as:

class Empty {};

...will result in the following being true:

Empty emp;
assert( sizeof(emp) != 0 );

The reason for this is so that you can take the address of the object.

EDIT: I originally said the sizeof would be 1, but per @Als' comment, I have found the relevant passage in the Standard, and it is indeed simply non-zero:

[Classes] §9/3

Complete objects and member subobjects of class type shall have nonzero size

Answer 2

The standard explicitly prohibits the existence of an instance of a type with size 0, the reason is that if an object could have size 0, then two different objects could be located at the exact same address. An empty struct, for example, will be forced to have size > 0 to comply with that requirement even if when used as base of a different type, the compiler can have it have size == 0.

What is it that you want to do with an empty class?