Constructor and anonymous union with const members

Question

Is it possible to have an anonymous union with const members? I have the following:

struct Bar {
  union {
    struct { const int x, y; };
    const int xy[2];
  };
  Bar() : x(1), y(2) {}
};

With G++ 4.5 I get the error:

error: uninitialized member ‘Bar::<anonymous union>::xy’ with ‘const’ type ‘const int [2]’

`struct { const int x, y; };` is not valid C++. Are you asking whether you can do something with GCC? — Johannes Schaub - litb, Jun 30 '11 at 23:21
Why is it not valid? Does it need a constructor with an initialisation list? — user2023370, Jul 02 '11 at 18:40
The GCC -pedantic switch seems useful. "ISO C++ prohibits anonymous structs" it says. If I remove the `const`s above, I get the same warning. — user2023370, Jul 02 '11 at 23:28

score 3 · Accepted Answer · answered Mar 06 '12 at 14:05

This was a problem in GCC that was fixed in version 4.6. Your code now works fine.

It still depends on a GCC extension because it uses an anonymous struct, but most compilers support them now. Also, the following code now builds properly with -pedantic:

struct Bar {
  union {
    const int x;
    const int y;
  };
  Bar() : x(1) {}
};

That code is also accepted by Clang and Visual Studio 2010 (but fails with 2008).

score 0 · Answer 2 · edited May 23 '17 at 12:06

0

Yes. Its possible but you've to initialized it when its constructed. You cannot leave it uninitialized. But in this particular case, I don't think its possible, since you cannot initialize an array in the initialization-list.

By the way, have a look at this interesting topic:

An union with a const and a nonconst member?

edited May 23 '17 at 12:06

Community

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answered Jun 30 '11 at 23:06

Nawaz

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score 0 · Answer 3 · answered Jul 02 '11 at 23:31

0

No. Try using GCC's -pedantic switch:

  warning: ISO C++ prohibits anonymous structs

The example is therefore also illegal with the consts removed.

answered Jul 02 '11 at 23:31

user2023370

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I am afraid this does not answer the question. Anonymous structs are forbidden, but not anonymous unions. – sam hocevar Mar 06 '12 at 13:59
@SamHocevar Thankyou! I appreciate your contribution, but surely the answer is technically still no: I was looking for legal C++. – user2023370 Mar 06 '12 at 21:21
Maybe you could edit the question, then? It states _“Is it possible to have an anonymous union with const members“_ to which the answer appears to be yes. You seem to have a different problem with anonymous `struct`s. – sam hocevar Mar 06 '12 at 21:37

Constructor and anonymous union with const members

3 Answers3