ambiguity and 0

[it . damn]

I would like to understand what g++ is doing in the following case a
little better, in the hope of not beating my head into a wall for 2
days again.

=======Start of sample code===================
#include <iostream>

class A
{
public:
   int foo(unsigned int x) const
   {
      std::cout << "foo(unsigned int x) const\n";
      return 1;
   }

   int foo(void* x)
   {
      std::cout << "foo(void* x)\n";
      return 1;
   }
};

int main()
{
   A a;

   // calls  foo(void* x)
   a.foo(0);

   // calls  foo(unsigned int x) const
   unsigned int x = 0;
   a.foo(x);

   // the following will trip an ambiguity error
   //a.foo((unsigned int)0);

   // calls foo(unsigned int x) const
   a.foo(1);
   a.foo(2);

   return 0;
}
======================================
Output:
foo(void* x)
foo(unsigned int x) const
foo(unsigned int x) const
foo(unsigned int x) const
======================================

My main question has to be why 0 is treated as void* but any other
number won't be.  Is this because 0 is NULL and a pointer is basically
an unsigned int?  (or am I completely off base with this)

The second would be why the cast causes an ambiguity but not a.foo(x).