Re: [Patch] Use variadic templates for tr1::is_function

[Doug Gregor <dgregor at cs dot indiana dot edu>]

On Apr 10, 2007, at 1:34 PM, Paolo Carlini wrote:

> Hi again, Doug,
>
> for the first time I'm looking into some details of result_of...  
> and I have some basic curiosities. For example, is not at all clear  
> to me why it doesn't work at all with plain functions (vs function  
> pointers, etc.). Can you do a bit of popularization and explain why  
> this kind of code path cannot be followed, in principle:
>
> #include <type_traits>
> #include <cassert>
>
> template<typename>
>  struct result_of;
>
> template<typename _Res, typename... _ArgTypes>
>  struct result_of<_Res(_ArgTypes...)>
>  { typedef _Res type; };
>
> int main()
> {
>  using std::is_same;
>  assert( (is_same<result_of<int (double)>::type, int>::value) );
> }
>
> ???

That's not a valid use of result_of.

result_of<F(A1, A2, ..., AN)>::type is the return type when calling a  
function object of type F with arguments a1, a2, ..., aN of types A1,  
A2, ..., AN, respectively. In your example use, F is "int", which is  
not a valid function object.

Now, you could do something like:

	result_of<int(double)(double)>::type

That would be int.

	Cheers,
	Doug