How do boost::variant and boost::any work?

Question

How do variant and any from the boost library work internally? In a project I am working on, I currently use a tagged union. I want to use something else, because unions in

Answer 1

If you read the boost::any documentation they provide the source for the idea: http://www.two-sdg.demon.co.uk/curbralan/papers/ValuedConversions.pdf

It's basic information hiding, an essential C++ skill to have. Learn it!

Since the highest voted answer here is totally incorrect, and I have my doubts that people will actually go look at the source to verify that fact, here's a basic implementation of an any like interface that will wrap any type with an f() function and allow it to be called:

struct f_any
{
   f_any() : ptr() {}
   ~f_any() { delete ptr; }
   bool valid() const { return ptr != 0; }
   void f() { assert(ptr); ptr->f(); }

   struct placeholder
   {
     virtual ~placeholder() {}
     virtual void f() const = 0;
   };

   template < typename T >
   struct impl : placeholder
   {
     impl(T const& t) : val(t) {}
     void f() const { val.f(); }
     T val;
    };
   // ptr can now point to the entire family of 
   // struct types generated from impl
   placeholder * ptr;

   template < typename T >
   f_any(T const& t) : ptr(new impl(t)) {}

  // assignment, etc...
};

boost::any does the same basic thing except that f() actually returns typeinfo const& and provides other information access to the any_cast function to work.