automatic conversion

hi,

The code below fails to compile with my g++ 4.1.1:

class Base
{
public:
   virtual ~Base () = 0;
};
Base::~Base () {}
class Derived : public Base
{
public:
   virtual ~Derived () {}
};

void Do (const Base &base)
{}

class A
{
public:
   operator Derived () const {return Derived ();}
};

int main (int argc, char *argv[])
{
   Derived derived = A ();
   const Base &base = A ();
   Do (A ());

   return 0;
}

with the following error:
test.cc: In function 'int main(int, char**)':
test.cc:27: error: cannot allocate an object of abstract type 'Base'
test.cc:2: note: because the following virtual functions are pure within
'Base':
test.cc:6: note: virtual Base::~Base()

Which points to the line which calls function Do (A ()). I have to confess
that I am a bit fumbled by this: I would have expected the line above
const Base &base = A (); to also fail or to have both lines not fail.

Anyhow, the question then is to know whether there is a way to make this
code to compile and run to allow the compiler a way to guess the right
type conversion when calling Do (const Base &). I could, of course, make A
derive from Base but this (A) class (in my code) is really not expected to
be polymorphic so, it kind of defeats the whole purpose of the exercise.

Any hints ?

regards,
Mathieu

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