Re: [Boost-users] Need boost's help with poor design

25 Mar 2008

      AMDG

Robert Dailey wrote:
...
Hi,
Currently I'm using Boost.Signals to transfer Packet objects to 
subscribers. Right now my Boost.Signal looks like below:
boost::signal
Right now Packet is the base class for several other packet types, 
each having the specific data a subscriber needs access to. Of course, 
given this design, the only way to obtain the data is to perform a 
downcast via static_cast or dynamic_cast:
WalkPacket const& wp = static_cast( p ); // 'p' 
here is a generic Packet object.
This is obviously bad design since it isn't type safe at all. It's 
also repetitive and tedious since each subscriber must duplicate the 
logic to perform the downcast. I'm wondering if there's anything in 
Boost you guys can suggest that I use to make this a little better. I 
would prefer to have a common Signal object that can take differently 
typed Slot objects. For example, I want to pass in a slot that looks 
like: "void (WalkPacket& p)" and have the signal utilize implicit 
conversions or something. Templates aren't even being used here 
because I can't find a way to fit them in, since everything is being 
done at runtime and not compile time.
In this case you want the packets that are not WalkPackets to be ignored?
You could wrap the slots in a class like this: (untested)

template
struct downcaster {
    typedef boost::remove_reference<T> arg_type;
    downcaster(F f) : f(f) {}
    template<class U>
    void operator()(const U& u) const {
        if(arg_type* arg = dynamic_cast(&u)) {
            f(*arg);
        }
    }
    F f;
};
template
downcaster make_downcaster(F f) {
    return(downcaster(f));
}

In Christ,
Steven Watanabe