C++11 "In class initialization" feature is not working for unions

Question

Minimal code example:

struct B { 
  union U {
    struct S {} s;
    int i = 100;
  }
  u;  
};

Now if we declare a B obj; then the obj.u.i is assigned a garbage value instead of 100. See the demo here. (The garbage value differs based on optimization flags etc.).

Is the "In class initialization" feature supposed to work with unions.

If yes, then what is the correct syntax? Or is this a g++ bug?
If not then what int i = 100; does?

[clang](http://coliru.stacked-crooked.com/view?id=6ab1a0f46b2229dfc46dfa6f4f5e3163-6e9f1f680880347f6708b805c806db62) seems happy with your code. — Praetorian, Jul 22 '13 at 15:06
@Praetorian, Thanks. However, you haven't put the output statements. So here is the correct [clang output](http://coliru.stacked-crooked.com/view?id=542928082d0e3f7d6edd525a7fcfac44-6e9f1f680880347f6708b805c806db62). — iammilind, Jul 22 '13 at 15:50

JohannesD · Accepted Answer · 2013-07-22T15:09:39.223

This looks like a GCC bug. The standard says (9.5p2):

At most one non-static data member of a union may have a brace-or-equal-initializer.

Otherwise, the rules are the same as for a regular class.

EDIT: In addition, 12.6.2p8:

In a non-delegating constructor, if a given non-static data member or base class is not designated by a mem-initializer-id (including the case where there is no mem-initializer-list because the constructor has no ctor-initializer) and the entity is not a virtual base class of an abstract class (10.4), then

if the entity is a non-static data member that has a brace-or-equal-initializer, the entity is initialized as specified in 8.5;

otherwise, if the entity is a variant member (9.5), no initialization is performed;

otherwise, the entity is default-initialized (8.5).

Presumably the implicitly defined default constructor counts here. The i member meets the criteria in the first bullet point, so it's initialized like it were a regular class member. The s member matches the second bullet point, so it's left uninitialized.

It does not compile on VS 2010 & 2012, not even the first version, with the error: only static const integral data members can be initialized within a class. Or both compilers have a bug or there is something more... — joy, Jul 22 '13 at 15:05
@neagoegab Neither of those VS versions support in-class initialization of data members. [clang](http://coliru.stacked-crooked.com/view?id=6ab1a0f46b2229dfc46dfa6f4f5e3163-6e9f1f680880347f6708b805c806db62) is compiling the code with no errors. — Praetorian, Jul 22 '13 at 15:07

score 2 · Answer 2 · edited Jul 22 '13 at 14:55

2

I think it is like that because a union reunites more than one element. Below is the work around syntax:

struct B { 
  union U {
    int i;
  }
  u {100};  
};

int main () {
  B obj;
  std::cout << "obj.u.i = " << obj.u.i << "\n";
}

edited Jul 22 '13 at 14:55

iammilind

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answered Jul 22 '13 at 14:39

joy

1,569
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Please stop guessing. Post an answer which you *know*, and can suppport. – Nawaz Jul 22 '13 at 14:42
@Nawaz, I belive that I have supported my answer. The alternative is to find it in the standard, but at that you are faster than me :D, but I will update the answer. – joy Jul 22 '13 at 14:47
This solution doesn't work with my actual problem, just add `struct S {} s;` inside the `union U` and the program gives [compiler error](http://ideone.com/5tS7q9). I have updated the question accordingly. – iammilind Jul 22 '13 at 14:51
@iammilind The workaround is to define a default constructor for `U`: `U() : i(100) {}` instead of using in-class member initialization. – Casey Jul 22 '13 at 15:18

C++11 "In class initialization" feature is not working for unions

2 Answers2