how to declare a reference to a boost shared pointer?

[eric]

Can I declare a pure virtual member function which accepts as input a
boost shared pointer to an object of a base class, such that

- concrete implementations of the function can redirect the pointer to
a new object

- clients of the function can pass in as input a boost shared pointer
to an object of a derived class

?



Consider the following classes in the problem domain:



class Serializable {};

class Foo : public Serializable {};

class Bar : public Foo {};



Here is the pure virtual member function I would like to declare:



class Serializer {

public:

virtual void serializeObject(boost::shared_ptr < Serializable >
&) = 0;

}



And the relevant subset of one of its concrete implementations:



class XmlReader : public Serializer {

virtual void serializeObject(boost::shared_ptr < Serializable >
&p) {

p = boost::shared_ptr < Serializable > (new Bar());

}

}



The client would call the function like this:



boost::shared_ptr < Foo > foo;

Serializer *s = new XmlReader();

s->serializeObject(foo);



The above code doesn't compile, the error message from VC6 is



test.cpp(72) : error C2664: 'serializeObject' : cannot convert
parameter 1 from 'class boost::shared_ptr<class Foo>' to 'class
boost::shared_ptr<class Serializable> &'

A reference that is not to 'const' cannot be bound to a
non-lvalue



The closest I can get is to declare the function like this:



class XmlReader : public Serializer {

virtual boost::shared_ptr < Serializable > p serializeObject()
{

boost::shared_ptr < Serializable > ret;

ret = boost::shared_ptr < Serializable > (new Bar());

return ret;

}

}



And call it like this:



foo = boost::dynamic_pointer_cast< Foo >(s->serializeObject());



But that's not ideal for my purposes, I would prefer to have a function
which modifies its inputs and returns void.



I can't implement the function as a template because I need it to be
virtual.



Any assistance gratefully received.



Regards,

Eric