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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 reasons)
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A union is a user-defined type similar to structs in C except for one key difference. Structures allocate enough space to store all their members, whereas unions can only hold one member value at a time.
A union can have member functions (including constructors and destructors), but not virtual functions. A union cannot have base classes and cannot be used as a base class.
C unions allow data members which are mutually exclusive to share the same memory. This is quite important when memory is valuable, such as in embedded systems. Unions are mostly used in embedded programming where direct access to the memory is needed.
Here, are cons/drawbacks for using union: You can use only one union member at a time. All the union variables cannot be initialized or used with varying values at a time. Union assigns one common storage space for all its members.
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.
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