Re: [boost] [result_of] Usage guidelines

Subject: Re: [boost] [result_of] Usage guidelines
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2012-09-04 04:43:42

On Tuesday 04 September 2012 10:06:32 Mathias Gaunard wrote:
> On 04/09/2012 08:59, Andrey Semashev wrote:
> > I suppose, in such cases additional
> > 
> > specializations are still needed:
> >    struct my_foo
> >    {
> >    
> >      template< typename >
> >      struct result;
> >      
> >      template< typename ArgT >
> >      struct result< my_foo(ArgT const&) >;
> 
> I assume you meant
> 
> template< typename ArgT >
> struct result< my_foo(ArgT const&) >
> {
>    typedef ArgT type;
> };
> 
> there?

No, my intent was to disable the const& version, because the operator() 
doesn't support it. However, now that you mentioned it, what I really wanted 
to express is that my operator() requires a non-const lvalue argument and the 
way operator() is written it doesn't communicate that clearly (because ArgT 
can be const).

> >      template< typename ArgT >
> >      struct result< my_foo(ArgT&) >
> >      {
> >      
> >        typedef ArgT type;
> >      
> >      };
> >      
> >      template< typename ArgT >
> >      ArgT operator() (ArgT& arg) const
> >      {
> >      
> >        return arg;
> >      
> >      }
> >    
> >    };
> 
> 1) doesn't support result_of<my_foo(int)>
> 2) not always the same type as what operator() returns (consider a const
> lvalue passed to operator())
> 3) needless repetition
> 
> You probably wanted to write
> 
> struct my_foo
> {
>    template<class Sig>
>    struct result;
> 
>    template<class This, class A0>
>    struct result<This(A0)> : strip<A0> {};
> 
>    template<class A0>
>    typename remove_const<A0>::type operator()(A0& a0) const
>    {
>      return a0;
>    }
> };

No, see my note above. For sake of the example, let's assume my_foo does a 
post-increment on its argument.

    template<class A0>
    typename disable_if< is_const<A0>, A0 >::type operator()(A0& a0) const
    {
      return a0++;
    }

Also, what is strip in your example? I didn't find it in type traits.