[SG10] New is_aggregate trait

Richard Smith richard at metafoo.co.uk
Sat Apr 8 00:17:25 CEST 2017


On 7 April 2017 at 15:00, Agustín Bergé <agustinberge at gmail.com> wrote:

> On 4/7/2017 5:19 PM, Richard Smith wrote:
> > On 30 March 2017 at 19:14, Agustín Bergé <agustinberge at gmail.com
> > <mailto:agustinberge at gmail.com>> wrote:
> >
> >     LWG2911 gave us the new `is_aggregate` trait, but no feature-test
> macro.
> >     I would like to suggest `__cpp_lib_is_aggregate`. What follows is a
> >     motivational use case, courtesy of Jonathan Wakely:
> >
> >          #include <vector>
> >          template<typename T, typename... Args>
> >          T make(Args&&... args)
> >          {
> >          #if __cpp_lib_is_aggregate
> >              if constexpr (std::is_aggregate_v<T>)
> >                  return { std::forward<Args>(args)... };
> >              else
> >          #endif
> >                  return T(std::forward<Args>(args)...);
> >          }
> >          struct Agg { int i; };
> >          int main()
> >          {
> >              auto v = make<std::vector<int>>(1, 2);
> >          #if __cpp_lib_is_aggregate
> >              // make<> only supports aggregates if std::is_aggregate is
> >     available
> >              auto a = make<Agg>(1);
> >
> >
> > What does the #else look like here? I'm not yet seeing how this feature
> > test macro is useful.
> >
> >          #endif
> >          }
>
> There is no #else form here, the example is depicting a program with
> decreased functionality. A user of `make` that depends on the feature
> simply uses it, and it results in a compilation error when the feature
> is not present:
>
>      int main()
>      {
>          auto a = make<Agg>(1);
>      }
>
> Whereas a user of `make` that does not depend on the feature simply does
> not use it, and lack of support for the feature does not result in a
> compilation error:
>
>      int main()
>      {
>          auto v = make<std::vector<int>>(1, 2);
>      }
>
> The feature test macro is needed for the implementation of `make`, in
> order to support both use cases.


This 'make' seems like a terrible idea, but you've convinced me that people
might make use of the feature test macro.
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