How do you exchange private data with Pimpl without exposing your internals?

Question

If you have an object B that needs a copy of a private member of an object A, and the private member is hidden by a Pimpl, how do you make it happen without exposing your internals? // Foo.h

class Foo
{
private :
  struct impl ;
  impl * pimpl ;
};

// Foo.cpp
struct impl { std::string data; }

//main.cpp
Foo A;
Foo B;
// I want A::pimpl->data copied to B::pimpl->data and I don't want std::string exposed in my Foo header.

Answer 1

// header
class Foo
{
    public:
       void Copy( const Foo & );
    private :
       struct impl ;
       impl * pimpl ;

};

//cpp file
struct impl {std::string data; }

void Foo::Copy( const Foo & f ) {
      pimpl->data = f.pimpl->data;
}