Re: [boost] [result_of] ambiguous instantiation for function pointers

From: Douglas Gregor (doug.gregor_at_[hidden])
Date: 2007-03-05 10:26:35

On Mon, 2007-03-05 at 13:12 +0900, shunsuke wrote:
> Eric Niebler wrote:
> > shunsuke wrote:
> >> Hi,
> >>
> >> CVS HEAD can't compile this under maybe any conforming compiler.
> >> (RC_1_34 is ok.)
> >>
> >>
> >> 	#include <boost/utility/result_of.hpp>
> >>
> >> 	int my_fun(int) { return 0; }
> >>
> >> 	template< class F >
> >> 	void test(F )
> >> 	{
> >> 	    typedef typename
> >> 	        boost::result_of<F()>::type
> >> 	    result_t; // shall be 'void'.
> >> 	}
> >>
> >> 	int main()
> >> 	{
> >> 	    ::test(&my_fun);
> >> 	}
> >>
> >>
> >> Specializations are not enough?
> > 
> > 
> > I'm a bit confused. The type of F is int(*)(int). You're asking what the 
> > return type is if you call it with no parameters. But you can't do that! 
> > It takes an int. result_of<F(int)>::type compiles fine. I'm not sure 
> > this is a bug. Can you justify why you think this code should compile?
> 
> AFAIK, nullary instantiations of 'result_of' must always succeed; for 
> function forwarding.
> See the Rationale at
>   http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1454.html
> 
> According to 20.5.4, I was slightly wrong.
> 'result_t' shall be 'int'? (I'm sure nobody cares, though.)

Yes, result_t should be 'int'. I've committed a fix to CVS; the patch I
committed follows.

  Cheers,
  Doug

Index: boost/utility/result_of.hpp
===================================================================
RCS file: /cvsroot/boost/boost/boost/utility/result_of.hpp,v
retrieving revision 1.9
diff -r1.9 result_of.hpp
17a18,19
> #include <boost/mpl/if.hpp>
> #include <boost/mpl/bool.hpp>
57a60,65
> template<typename FArgs>
> struct is_function_with_no_args : mpl::false_ {};
> 
> template<typename F>
> struct is_function_with_no_args<F(void)> : mpl::true_ {};
> 
59,60c67
< struct result_of_impl<F, FArgs, false>
<   : F::template result<FArgs>
---
> struct result_of_nested_result : F::template result<FArgs>
63,65c70,74
< template<typename F>
< struct result_of_impl<F, F(void), false>
<   : result_of_void_impl<F>
---
> template<typename F, typename FArgs>
> struct result_of_impl<F, FArgs, false>
>   : mpl::if_<is_function_with_no_args<FArgs>,
>            result_of_void_impl<F>,
>            result_of_nested_result<F, FArgs> >::type