Re: [Boost-users] [thread] Ambiguity in variadic wait_for_all

Subject: Re: [Boost-users] [thread] Ambiguity in variadic wait_for_all
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2017-02-07 23:09:25

On 8/02/2017 06:51, Vicente J. Botet Escriba wrote:
> You have made two changes IIUC, boost version and compiler version, so I
> don't know if this is a regression in boost 1.53  or a behavior that
> different between c++02 and C++11. I suspect that it  is the change to
> C++11.

Sorry, I gave a misleading impression there.  Both Boost versions were 
tested with the same compiler (VS2015), so the issue was introduced by 
the Boost upgrade only.

Explicitly disabling the variadic version by defining 
BOOST_NO_CXX11_VARIADIC_TEMPLATES also avoids the issue, so I'm pretty 
sure it's just the introduction of that which did it.

> Curiously I added enable if only for the function with two arguments,
> but not for the others.

That's correct -- only the two-argument form is ambiguous with the 
iterator overload.

Although having said that, the iterator form takes a single template 
parameter and the future form takes two template parameters, so the 
compiler ought to be able to tell them apart (since the template 
parameters were explicitly provided), unless it's trying to infer the 
second parameter.

> Please, could you try to add the enable_if to the variadic template and
> tell me if this would fix your issue?
>
>     template<typename F1, typename... Fs>
>     *typename boost::enable_if<is_future_type<F1>,void>::type*
> wait_for_all(F1& f1, Fs&... fs)

Yes, making that change resolves the ambiguity and the original code 
compiles again.