On 7 April 2017 at 15:00, Agustín Bergé <agustinberge_at_[hidden]> wrote:

> On 4/7/2017 5:19 PM, Richard Smith wrote:
> > On 30 March 2017 at 19:14, Agustín Bergé <agustinberge_at_[hidden]
> > <mailto:agustinberge_at_[hidden]>> 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.