Union in c++ are they feasible

Question

Can a union in C++ have a member function? How do union with data members and member functions exist if an object is created?

If I suppose yes, then are they feasible an

Answer 1

The union is a C-structure, and does not work well with C++ types (there are a number of caveats actually). However there already exist a C++ equivalent, which effectively work with all C++ classes and user-defined classes and is even safer than the union!

Behold Boost.Variant!

You can define a boost::variant and it'll make sure:

• that the appropriate constructor/destructor/assignment operator is run, when required
• that you only access the lastest value that was set

And it even comes with the excellent: boost::static_visitor which let's you apply a method on the union regardless of its type and provide compile-time checking to warn you whenever you have forgotten one of the possible types!

class MyVisitor: boost::static_visitor
{
public:
  int operator()(std::string const& s) const {
    return boost::lexical_cast(s);
  }

  int operator()(Foo const& f) const { return f.getAsInt(); }

  int operator()(char c) const { return c; }
};

typedef boost::variant MyVariant;

int main(int argc, char* argv[]) {
  MyVariant v; // the std::string is constructed

  if (argc % 2) { v = Foo(4); }
  if (argc % 3) { v = argv[1][0]; }
  if (argc % 5) { v = argv[1]; }

  std::cout << boost::apply_visitor(MyVisitor(), v) << '\n';
  return 0;
}

Also... it's as efficient (fast) as a union, and does not involve any dynamic look-up like Boost.Any would.