Re: Callbacks Again

From: William Kempf (sirwillard_at_[hidden])
Date: 2000-11-17 18:07:52

--- In boost_at_[hidden], Jesse Jones <jejones_at_a...> wrote:
> >On Fri, 17 Nov 2000 20:54:33 -0000
> >"William Kempf" <sirwillard_at_m...> wrote:
> >
> >> --- In boost_at_[hidden], Douglas Gregor <gregod_at_r...> wrote:
> >> > On Fri, 17 Nov 2000 19:40:05 -0000
> >> > 1) I use one "Callback" class that takes any number of 
parameters
> >> >   instead of having many Callback0, Callback1, ..., CallbackN
> >> >   classes.
> >>
> >> In theory I really like this.  Unfortunately, it requires partial
> >> template specialization and so may hinder or prevent 
implementation
> >> on VC++, which I think is a major problem.  It's definately an 
idea
> >> worth looking into, however.
> >
> >I believe that we need partial specialization in any case. Even if 
we use
> >Callback0, Callback1, etc. classes, we still (might) need a void 
(partial)
> >specialization. e.g.,
> >
> >template<typename Arg1, typename Arg2>
> >struct Callback2<void, Arg1, Arg2> { ... };
> 
> I tried to use something like the following:
> 
> // force a compile error if the function returns an incompatible 
type
> template <typename RETURN_TYPE>
> struct ReturnValue {
>    template <typename T>
>    RETURN_TYPE Cast(T value)   {return value;}
> };
> 
> // for void callbacks we'll eat the return value
> template <>
> struct ReturnValue<void> {
>    template <typename T>
>    void Cast(T)            {}
> };
> 
> virtual R Call() const  {return ReturnValue<R>().Cast(mFunctor());}
> 
> but I couldn't get it to work with CW 5.3. Perhaps it will work 
with MSVC...

Check out the threads on simulating partial specialization.  This 
particular issue was even coded for using VC++ for a bind() function 
by Dave Abrahams, I believe.  The general idea is to use full 
specialization instead of partial specialization and to just 
eliminate the return all together for void return types (as opposed 
to trying to cast them).  After all, VC++ won't work with this code:

void func()
{
}

template <typename T>
void bar(T* p)
{
   return func();
}

Bill Kempf