Declaring empty C++ array with aggregate initializer

Question

In C++, unlike in C, an empty array T name[] is illegal,

the type declared is "array of unknown bound of T", which is a kind of incomplete type.

However, it is legal when

used in a declaration with an aggregate initializer

like T name[] = {val1, val2, ...}, where the array is allocated with the number of elements in the initializer list.

What is the expected behaviour when the aggregate initializer is empty? T name[] = {}

I have tested g++ (version 4.8.4) and clang (version 3.4), neither of which give any error or warning, and seem to allocate 1 element. Is this the defined behaviour? Documentation?

int a[] = {};
int b[] = {};

Results in:

a[0] -> 0x7ffc3de28dd8
a[1] -> 0x7ffc3de28ddc
b[0] -> 0x7ffc3de28ddc
b[1] -> 0x7ffc3de28de0

What versions of gcc and clang? gcc 6.1 and clang 3.8 complain about the array of length 0. — Baum mit Augen
– Baum mit Augen ♦, Commented Jun 28, 2016 at 17:49
The addresses of the arrays still have to be valid, so that's probably why something is allocated. But I think dereferencing is undefined behavior when the size of the array is 0. You can get the size of an array literal with: sizeof(a)/sizeof(a[0]) — E.M.
– E.M., Commented Jun 28, 2016 at 18:03

skypjack · Accepted Answer · 2016-06-29 17:33:41Z

3

From the working draft, [8.5.1/5] (Aggregates):

An empty initializer list {} shall not be used as the initializer-clause for an array of unknown bound.

See also [footnote/105]:

The syntax provides for empty initializer-lists, but nonetheless C++ does not have zero length arrays.

It sounds like an UB.

Note also that this compiles as expected:

template<int N>
void f(int(&)[N]) { }

int main() {
    int v[] = {42};
    f(v);
}

But it doesn't work anymore if you use:

int v[] = {};

Tested with GCC.

edited Jun 29, 2016 at 17:33

answered Jun 28, 2016 at 18:18

skypjack

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AnatolyS · Accepted Answer · 2016-06-28 18:09:01Z

2

According to the C++ standard 1.8.6 (C++ 14):

Two objects that are not bit-fields may have the same address if one is a subobject of the other, or if at least one is a base class subobject of zero size and they are of different types; otherwise, they shall have distinct addresses.4

Compiler must allocate at least one element of your arrays to satisfy the above requirement.

answered Jun 28, 2016 at 18:09

AnatolyS

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