Re: [boost] [result_of] Allow result_of to work with C++11 lambdas

Subject: Re: [boost] [result_of] Allow result_of to work with C++11 lambdas
From: Peter Dimov (lists_at_[hidden])
Date: 2013-04-21 21:35:26

Jeffrey Lee Hellrung, Jr. wrote:
> On Sun, Apr 21, 2013 at 7:48 AM, Daniel Walker 
> <daniel.j.walker_at_[hidden]>wrote:
> > This is due to a dark corner of TR1: the result and result_type members
> > are consulted in non-nullary functors but nullary functors always 
> > generate
> > void. (See the section on known difference between boost::result_of and 
> > TR1
> > in the documentation.)
>
> The rationale for this always confused me, with the end result (it seems)
> that it makes TR1 result_of basically unusable for nullary function calls.

result_type is consulted. result<> is not. type is void (instead of a 
compile error) when there is no result_type.

The reason for the type being void is that in

template<class T> class reference_wrapper
{
private:

    T & t_;

public:

    typename result_of<T()>::type operator()() const
    {
        return t_();
    }
};

operator()() is not a template. If result_of<T()>::type wasn't present or 
generated an error when T doesn't have result<>, reference_wrapper<int> 
would have generated an error when instantiated.

In C++11, you can make operator()() a template. In C++03, you can't.