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: Daniel Walker (daniel.j.walker_at_[hidden])
Date: 2013-04-21 21:56:57

On Apr 21, 2013, at 9:35 PM, "Peter Dimov" <lists_at_[hidden]> wrote:

> 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.
> 

Thanks Peter. I forgot that you can still use result_type with nullary functors. I feel like I haven't used TR1 in a while...

- Daniel