Re: [boost] Re: Callbacks Again

From: Douglas Gregor (gregod_at_[hidden])
Date: 2000-11-19 19:22:57

On Sun, 19 Nov 2000 09:22:52 -0800
Karl Nelson <kenelson_at_[hidden]> wrote:

> 5) Generate readable errors.  
> 
> Okay this was the tall order.  Gtk-- is not supposed to
> be hyper advanced C++.  It was meant for people constructing 
> GUIs who used Qt.  They may be experts but more likely 
> they are a bit green.  In the design of the library, I
> wanted to isolate the errors.  Thus rather the spitting out...
> 
>   on line  (From boost/Callback)
>      s=mul_to_int();  // where mul_to_int doesn't take right arguments
> 
> boost/callback.hpp: In method `int boost::detail::functor_callback_impl<2>::base<mul_to_int,int,float,float,boost::detail::Unused>::operator ()(float, float)':
> boost/callback.hpp:461:   instantiated from here
> boost/callback.hpp:256: no match for call to `(mul_to_int) (float &, float &)'
> callback1.cpp:13: candidates are: int mul_to_int::operator ()(float, float, bool
> 
> 
> SigC spits out 
> 
>   on line
>     s=slot(&mul_to_int);
> 
> test.cc:8: no match for `SigC::Slot2<int,float,float> & = SigC::Slot3<int,float,float,bool>'
> /usr/local/include/sigc++/slot.h:489: candidates are: class SigC::Slot2<int,float,float> & SigC::Slot2<int,float,float>::operator =(const SigC::Slot2<int,float,float> &)
> 
> Likely this last one isn't an issue to you.  But to my users it
> really is important. 

I consider it an issue, but I know it is also solvable. I've just uploaded an updated version of a callback library which emits more readable error messages, e.g., the following code

struct hello_world {
  int operator() { return 5; }
};
// ...
callback<void> cb1;
cb1 = hello_world();

Will return this error message with g++-2.95.2:

callback1.cpp:22:   instantiated from here
../../boost/callback.hpp:236: `function_call_operator_signatures_are_not_compatible_with_callback' is not a member of type `boost::INVALID_CALLBACK_TARGET<hello_world>'

More metaprogramming could yield more accurate error messages, but VC++ has become the limiting factor.

        Doug Gregor