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

Subject: Re: [Boost-users] [thread] Ambiguity in variadic wait_for_all
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2017-02-07 17:51:38

Le 03/02/2017 à 02:38, Gavin Lambert a écrit :
> A colleague recently ran into an interesting issue when upgrading some 
> working code from Boost 1.55 to 1.60/1.63 (and from C++03 to C++11).
>
> The exact details are unimportant, but it was trying to get the 
> address of boost::wait_for_all(Iterator, Iterator), for use in a 
> Boost.Bind call.
>
> In 1.55, this worked fine, although of course it had to supply 
> template parameters that proved the parameters were not futures (thus 
> causing enable_if<is_future_type<F1>> to fail, disabling the 
> two-future-args overload of the function.
>
> In 1.60/1.63, the variadic method is encountered instead, which lacks 
> any enable_if condition and thus the ambiguity between the two remains 
> and the compiler errors out.  (When calling it directly, the variadic 
> one manages to get disabled as long as the iterator type lacks a 
> wait() method, but this doesn't apply when just getting the method 
> address, as it's not part of the signature.)
>
> Should the variadic form also have enable_if?  Or is this considered 
> user error for trying to use Bind in a variadic-compatible compiler 
> instead of using a lambda?
Hi,

I will suggest you to replace the call to bind by lambdas whenever you can.

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.

Curiously I added enable if only for the function with two arguments, 
but not for the others.
     template<typename F1,typename F2>
     typename boost::enable_if<is_future_type<F1>,void>::type 
wait_for_all(F1& f1,F2& f2)

When I added the C++11 version I surely forget to add the enable_if.

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)

Best,
Vicente