std::tr1::function wierdness

[Adrian Harris]

I'm playing around with std::tr1 in gcc 4.1.0 using a piece of code borrowed 
from 'Effective C++ Third Ed' and I cannot figure out the two different error 
messages.

Undoubtedly there is some subtle issue at work, but it escapes me.

Thanks

Code lifted and modified from pp 173..175:
-------
#include <tr1/functional>

class B;
int defaultFn( const B & ) { return 0; }

struct DefaultFnObj
{
     int operator()( const B & ) const { return 0; }
};

class B {
  public:
     typedef std::tr1::function<int ( const B & )> Fn_t;

     explicit B( Fn_t fn = defaultFn ) :
         m_fn( fn ) {}
     int fn() const { return m_fn( *this ); }
  private:
     Fn_t m_fn;
};

int main()
{
     // This works
     DefaultFnObj fn0;
     B b0( fn0 );
     const int rc0 = b0.fn();

     // This doesn't because of the explicit constructor call
     DefaultFnObj fn1();
     B b1( fn1 );
     const int rc1 = b1.fn();

     // This doesn't either (similar to above, but different compiler error)
     B b2( DefaultFnObj() );

     return rc0 + rc1 + b2.fn();
}
------