[Boost-users] Re: Re: Re: Re: Re: Re: result_of tutorial

Peter Dimov wrote:

Ian McCulloch wrote:

...

Thanks Peter, I think I understand how it works now. Unfortunately, the lack of SFINAE is a real showstopper for me. I want to write functions like

template <typename T> typename result_of::type operator-(T const& x) { return negate()(x); }

but the lack of SFINAE here makes boost::result_of essentially useless for this.

Yes, I've encountered this too. The problem is in the unconditional instantiation of the nested ::result<>. Since we don't have a reliable way to detect a nested template yet (requires core language change), this can't be fixed except by removing the ::result<> logic altogether and relying on specializations instead, as in the example I gave.

I don't get it: can't you detect whether nested::result::type exists? As long as the primary template of nested::result is defined as an empty struct this should work fine(?).

This won't help us in general, though. result_of is not supposed to support SFINAE, because it may be implemented with __typeof or __decltype internally.

Do you mean that a substitution failure in a typeof or decltype expression *is* an error? Ouch! Cheers, Ian