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Subject: Re: [boost] [paired ptr] Proposing a new smart pointer object formanaging bidirectional relationships between objects
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-05-04 06:52:50

Dan Walters wrote:
> Rob Stewart wrote:
> >That's an MSVC-specific warning. Use pragmas push, disable,
> > and pop, conditionally compiled for MSVC, to manage that warning
> For MSVC, warning 4355 is disabled in the paired_ptr header.

Did you use #pragma warning (push) and #pragma warning (pop) to control its scope?

> >Using Boost.Function makes your callback support more
> > flexible, which makes it more useful to users.
> Callback functions are now of type boost::function1<paired_ptr<T,U>*>
> where T is the owner type and U is the pointed to owner type. This

As I said before, do not assume that the object type for the member function pointer is the owner type. A user may wish to invoke a member function on a different object.

> works well as users can generate global functions to handle the
> callbacks, use static member functions, and even static member
> functions from other classes. Hence for member usage:
> class dog
> {public:
> static void connect_to_cat(paired_ptr<dog,cat>* p_paired_ptr) //

The argument type should be a reference since you'll ensure it is never null. Don't make the library user check that for every callback.

> paired_cat.set_connect_callback(&dog::connect_to_cat);

No, it should be

   paired_cat.connect_callback(this, &dog::connect_to_cat);

Notice that the object type and the member function type are supplied. The implementation of connect_callback() can create a boost::function, binding the this pointer, and then forward to the boost::function-based overload.

That member function pointer overload isn't strictly necessary, but it is a nice convenience. That overload must be a member function template since the object pointer's type can differ from the owner type:

template <class T, U>
class paired_ptr
   template <class V>
   connect_callback(V *, void (V::*)(paired_ptr<T,U> &));

You should support this overload, too:

   template <class V>
   connect_callback(V const *, void (V::*)(paired_ptr<T,U> &) const);

> >I fail to understand the problem. In each case you are
> > invoking a functor with a reference to a paired_ptr. (You
> > could even support overloading for const and non-const
> > access.) What you need is type erasure to make the callback
> > invocation ignorant of how the underlying function is
> > invoked. IOW, you want a member function pointer invocation,
> > via Boost.Function, say, to look just like calling a
> > non-member function pointer or function object.
> The problem is that to call a normal function, the arguments would be:
> void callback_function(<paired_ptr<T,U>* p_ptr);
> For a member function it would be:
> void callback_function(T* this, <paired_ptr<T,U>* p_ptr);

Of course, but that's why I pointed out that you'd use type erasure via Boost.Function and Boost.Bind (or std::bind1st).

> Obviously, the this pointer is available in member functions and so is
> passed invisibly to the function. So the parameter lists are different
> and not inter-operable between global functions and member functions.
> This can be overcome using std::bind1st.


> I opt for settling for global functions and static member functions,
> as it is a simple and flexible solution.

You don't need to leave out non-static member functions with the approach I suggested.

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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