Re: [boost] [result_of] now uses decltype on release branch

Subject: Re: [boost] [result_of] now uses decltype on release branch
From: Daniel Walker (daniel.j.walker_at_[hidden])
Date: 2012-09-05 01:46:32

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