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: 2012-09-04 22:08:08

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.

- Daniel