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

On Apr 21, 2013, at 9:35 PM, "Peter Dimov" wrote:

Jeffrey Lee Hellrung, Jr. wrote:

...

On Sun, Apr 21, 2013 at 7:48 AM, Daniel Walker 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::type operator()() const { return t_(); } };

operator()() is not a template. If result_of::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