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

From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-03-25 17:41:30

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<void (Packet const& p)>
>
> 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<WalkPacket const&>( 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<class F, class T>
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<arg_type*>(&u)) {
            f(*arg);
        }
    }
    F f;
};
template<class T, class F>
downcaster<F, T> make_downcaster(F f) {
    return(downcaster<F, T>(f));
}

In Christ,
Steven Watanabe