Re: [boost] Re: Callbacks Again

From: Jesse Jones (jejones_at_[hidden])
Date: 2000-11-17 19:48:50

>--- 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).

Despite the name, there's no casting at all in the code I posted.

>After all, VC++ won't work with this code:
>
>void func()
>{
>}
>
>template <typename T>
>void bar(T* p)
>{
>   return func();
>}

Ugh. That seems to be enough to shoot down my alternative on MSVC.

  -- Jesse