Re: [boost] [result_of] now uses decltype on release branch

Subject: Re: [boost] [result_of] now uses decltype on release branch
From: Michel Morin (mimomorin_at_[hidden])
Date: 2012-09-03 23:20:55

[Part 2 of 3]

Daniel Walker wrote:
> 2) Nullary function objects
[...]
> struct functor {
>     template<class> struct result {
>         typedef int type;
>     };
>     int operator()();
> };

There are two types of misusage of result_of:

  A. Wrong definition of return types
  B. Return type evaluation with uncallable signatures

I think the above code (i.e. the code with nullary function objects that
do not specialize boost::result_of/tr1_result_of class template)
belongs to A.

The consequence of "TR1-style result_of has the default type `void`
for nullary calls" is related to B.

B is allowed with TR1-style result_of in many cases, since
* Callability is not checked for function pointers.
* Callability is not checked for function objects having `result_type`.
* Nested class template `result` is often partially specialized
  (in a way that uncallable signatures are allowed).
* Nullary calls are always allowed.
With decltype-based result_of, B is not allowed at all.

TR1-style result_of has the default type `void` to allow
    template <typename F>
    struct FuncWrapper
    {
        typename boost::result_of<F()>::type operator()();

        /* ... */
    };
for function objects that are not nullary-callable.

But, with decltype-based result_of, this code fails to compile for
non-nullary function objects. I think this is an important
breaking change. We can avoid compiler errors using variadic templates.
But, IIRC, VC++ 11 does not implement variadic templates.

Regards,
Michel