[SG10] New is_aggregate trait

Agustín Bergé agustinberge at gmail.com
Sat Apr 8 00:00:02 CEST 2017


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.

Regards,
-- 
Agustín K-ballo Bergé
http://talesofcpp.fusionfenix.com


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