Re: [boost] boost::result_of and C++03

From: Peter Dimov (pdimov_at_[hidden])
Date: 2008-04-05 21:09:24

Eric Niebler:
...

> make_plus is trickier. It must be optimal and safe. When passed lvalues,
> it should return a new node that holds the children by reference. When
> passed rvalues, it should return a new node that hold the children by 
> value.
>
> == Problem ==
>
> Here's the thing ... I have no idea how to implement make_plus as a TR1
> function object.

I think that you are right; you can't implement make_plus as a C++03 
function object. result_of is a red herring. The missing part is &&, not 
decltype (which result_of is supposed to approximate).

You can do this:

> struct make_plus
> {
>   template<typename Sig> struct impl;
>
>   template<typename This, typename Left, typename Right>
>   struct impl< This(Left, Right) >
>   {
>     // Left and Right are allowed to be reference types
>     typedef plus<Left, Right>::type result_type;
>
>     result_type operator()(
>       typename add_const_ref<Left>::type left
>      ,typename add_const_ref<Right>::type right
>     ) const
>     {
>       return result_type(left, right);
>     }
>   };
> };

but it's not a function object.