On Sep 4, 2012, at 10:29 PM, Joel de Guzman wrote:
On 9/5/2012 10:08 AM, Daniel Walker wrote:
On Sep 4, 2012, at 8:01 PM, Joel de Guzman wrote:
On 9/5/2012 12:53 AM, Michel Morin wrote:
Joel de Guzman wrote:
However! This makes the current result_of code not an exact replacement to decltype which allows this variation of above:
Right. boost/std::result_of is not an exact replacement to decltype, since decltype allows SFINAE but boost/std::result_of doesn't.
[...]
If Fusion's invoke used decltype instead of result_of, it would have worked.
I tried to compile my test case for fusion::invoke with SFINAE-enabled result_of, but it failed to compile. After adding a "fallback type" to SFINAE-enabled result_of, then the test case runs fine.
The following code (attached) demonstrates the problem of Fusion::invoke with the current decltype based result_of. Comment out the first line for the code to use plain decltype vs. result_of. Notice that because result_of does not allow SFINAE, it barfs when the compiler tries the first overload of invoke (substitution failure). The compiler could have chosen the second overload.
I see no other way to get around this problem of result_of. I am getting inclined to use decltype directly in fusion instead of going through result_of. Problems like this kinda defeats the purpose of decltype-ifying result_of, but heck.
The question is: should we allow SFINAE for result_of. I think now that we should.
Thanks for the sample code, Joel. I don't exactly understand the problem though. Why overload invoke? Isn't this the perfect occasion to use rvalue references? invoke could be defined as follows and everything would work fine.
template <typename F, typename Arg> typename boost::result_of<F(Arg&&)>::type invoke(F f, Arg&& arg) { return f(boost::forward<Arg>(arg)); }
The decltype-based result_of constructs the call-expression using rvalue-references (via boost::decval) for perfect forwarding.
Yes, I agree. This is the perfect occasion to use rvalue references. What you wrote is a plausible solution and one I am looking into right now (not as simple as the test code though).
However... that is pretty much besides the point. And the point is that the current decltype based result_of breaks with overloads.
OK, I think there's something we can do about this. I've attached a modification of your sample code that uses a simple is_callable metafunction predicate with enable_if so that SFINAE can kick in. It's a proof of concept. I'm not familiar with Eric's can_be_called so I don't know if these metafunctions test exactly the same thing. My is_callable tests if a valid call expression can be constructed from a result_of-style signature; i.e. can the signature be used with result_of to get a result type. Now, this could be folded into result_of; e.g. result_of could inherit from enable_if_lazy<is_callable<...>, ...>, or something like that, if you follow me. Is it worth fleshing this out? Or should we just leave it as a known (and soon to be documented) difference in behavior between TR1 and decltype-based result_of? - Daniel