C++: unions with methods?

Question

Is there anything wrong with a union having one or more methods? Or anything to watch out for? (I can see constructors/destructors being problematic for schizophrenic reason

Answer 1

From the C++03 & C++0x (Draft N3092) standards:

9.5 Unions
A union can have member functions (including constructors and destructors), but not virtual (10.3) functions. A union shall not have base classes. A union shall not be used as a base class.

Initializing the union using the aggregate initializer syntax (U u = { 42 };) or setting a member afterwards (U u; u.i = 42;) is not "problematic". And neither is initializing it using a constructor (U u( 42 );).
The only "catch" is that you cannot use the aggregate initializer syntax for a union that has a user defined constructor.