C++: unions with methods?

问题:

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)

回答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.

回答2:

How could you possibly implement such a thing? Here's a pointer to a union, hope you don't mind that you have no idea which variables are safe to use and which aren't.

Unions are a dead language feature really anyway- they've been totally superseded by library-based methods like boost::variant or boost::any. Kind of similar to the void* and functional macros - they're very rarely useful in C++ compared to other options.