Claus Brod: Address of the union (11 Jan 2006)

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---+++ [[BlogOnSoftware20060111][Address of the union]] (11 Jan 2006)

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

<pre>
  class BOX {
  public:
    BOX() {}
  };

  union {
    void *pointers[8];
    BOX box;
  };
</pre>

gcc says something like "error: member `BOX <anonymous union>::box' with constructor not
allowed in union"; Visual C++ reports a compiler error [[http://msdn.microsoft.com/library/en-us/vccore/html/C2620.asp][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? :D

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:

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

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

  union {
    FOO foo;
    BAR bar;
  };
</pre>

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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