[Boost-users] how to declare a reference to a boost shared pointer?

From: eric (spamsink42_at_[hidden])
Date: 2005-11-29 03:53:07

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

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

- concrete implementations of the function can redirect the pointer to
a new object of an (even more) derived class

?

Consider the following class hierarchy 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

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

The best solution I have found so far is to declare the function like this:

    class XmlReader : public Serializer {

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

            boost::shared_ptr < Serializable > 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