Address of the union (11 Jan 2006)

The following C++ code will be rejected by both Visual C++ and gcc:

  class BOX {
  public:
    BOX() {}
  };

  union {
    void *pointers[8];
    BOX box;
  };

gcc says something like "error: member `BOX ::box' with constructor not allowed in union"; Visual C++ reports a compiler error C2620.

Now that is too bad, because in my particular case, I really needed both a union (to save memory in a critical area of the code) and that box member with a default constructor! Now I'm sure that all those CEOs around the world who are currently sacking people in the thousands would readily agree that union members aren't constructive enough, but why even turn this into a C++ language rule?

I have a workaround for this now, but I'm still a little puzzled about the compiler restriction. My guess is that the compiler is trying to avoid intialization ambiguity in a scenario like this:

  class FOO {
    int foo;
  public:
    FOO() : foo(42) {}
  };

  class BAR {
    int bar;
  public:
    BAR() : bar(4711) {}
  };

  union {
    FOO foo;
    BAR bar;
  };

Which constructor "wins" here? But then, C++ isn't exactly over-protective in other areas, either, so if I want to shoot myself into the foot, get out of my way, please. Or is there another reason? Hints most welcome.

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