gcc 4.9 bug in structures initialization?

Question

I have the code:

struct A {
    int a;
};

struct B {
    int b;
    const A a[2];
};

struct C {
    int c;
    const B b[2];
};

const C test = {0, {}};

int main()
{
    return test.c;
}

I have gcc 4.8.2 and 4.9.2. It can be compiled just fine with:

g++-4.9 -Wall test.cpp -o test
g++-4.8 -std=c++11 -Wall test.cpp -o test
g++-4.8 -Wall test.cpp -o test

However, it can't be compiled with:

g++-4.9 -std=c++11 -Wall test.cpp -o test

And the compiler output is:

test.cpp:15:22: error: uninitialized const member ‘B::a’
 const C test = {0, {}};
                      ^
test.cpp:15:22: error: uninitialized const member ‘B::a’

Is this a bug or I just don't understand something?

And [compiles on GCC trunk](http://melpon.org/wandbox/permlink/bXwXkocS3OKbRzUA). Looks like a bug that has been fixed. — T.C., Apr 07 '15 at 08:57
@T.C. please add that as an answer, if you don't want to rep then make it CW. As we have seen with many meta questions comments are not meant to stick around and I have seen important comments removed before. So important details do not belong in comments. — Shafik Yaghmour, Apr 07 '15 at 09:21

score 3 · Accepted Answer · edited Apr 07 '15 at 22:03

This is a bug that essentially reduces to GCC complaining about not-explicitly-initialized const data members in aggregate initialization. E.g.

struct {const int i;} bar = {};

Fails as there is no initializer-clause for i in bar's initializer. However, the standard specifies in §8.5.1/7 that

If there are fewer initializer-clauses in the list than there are members in the aggregate, then each member not explicitly initialized shall be initialized from its brace-or-equal-initializer or, if there is no brace-or-equal-initializer, from an empty initializer list (8.5.4).

Thus the code initializes i (as if by = {}), and GCCs complaint is incorrect.

In fact, this bug has already been reported four years ago as #49132, and is fixed in GCC 5.

gcc 4.9 bug in structures initialization?

1 Answers1