Question

What’s a simple but non-trivial example of class defined using union in C++?

• the class should be defined using union
• the class should have constructor(s) and destructor explicitly declared and implemented
• the class should have methods that demonstrate the difference of such a class from a class defined using class key word

Answer 1

Maybe something like this:

#include <new>

enum EActive { ActiveNone, ActiveS, ActiveT };
struct S;
struct T;

union Foo
{
   struct TagOnly { EActive current; TagOnly() : current(ActiveNone) { } } tag;
   S a;
   T b;

   void destroy()
   {
     if      (tag.current == ActiveT) b.~T();
     else if (tag.current == ActiveS) a.~S();
   }

   void setS(S const & s)
   {
     destroy();
     S * const p = ::new (static_cast<void*>(&a)) S(s);
     tag.current = p ? ActiveS : ActiveNone;
   }

   // similar for setT(T const & t)

   ~Foo() { destroy(); }

   Foo() : tag() { }

};

I’m assuming here that TagOnly, S and T share the common initial segment EActive. (It might be smarter to stick S and T into wrapper classes <typename U> struct UnionWrapper { EActive tag; U u; /* ...*/ };.)