I noticed a lot of the "Primary section" column is wrong in the C++17
feature table. Several sections got renumbered recently, so the
features that were in there a few months ago have the wrong number
now. For example, some of the shared_ptr utils are listed as 20.8, but
weak_type is 20.10. In the latest draft they're all 20.11!


On 22 July 2016 at 20:06, Nelson, Clark <clark.nelson_at_[hidden]> wrote:
> What I have done instead is add an indication for every proposed macro for
> which we don't yet have an example -- which is to say, for which we don't
> yet have a rationale. I'd really like to get more examples -- but I don't
> plan to do all the work of inventing them myself.


Example for P0163R0 __cpp_lib_shared_ptr_weak_type:

template<class ObjectType>
    void register_observers(ObjectType& obj)
    {
        auto sptr = obj.shared_from_this();
#if __cpp_lib_shared_ptr_weak_type
        decltype(sptr)::weak_type wptr(sptr);
#else
        // Fails if sptr is not std::shared_ptr
        // (e.g. std::experimental::shared_ptr or boost::shared_ptr)
        auto wptr = weak_ptr<decltype(sptr)::element_type>(sptr);
#endif
        sptr.reset(); // drop the strong reference as soon as possible
        register_observer_1(wptr);
        register_observer_2(wptr);
    }


And similar example for P0033R1 __cpp_lib_enable_shared_from_this:

template<class ObjectType>
    void register_observers(ObjectType& obj)
    {
#if __cpp_lib_enable_shared_from_this
        auto wptr = obj.weak_from_this();
#else
        auto sptr = obj.shared_from_this();
        // Fails if sptr is not std::shared_ptr
        // (e.g. std::experimental::shared_ptr or boost::shared_ptr)
        auto wptr = weak_ptr<decltype(sptr)::element_type>(sptr);
        sptr.reset(); // drop the strong reference as soon as possible
#endif
        register_observer_1(wptr);
        register_observer_2(wptr);
    }

I tried to come up with one for P0074R0, but I don't think this is
very good because if you're going to write the fallback anyway then
the macro doesn't help much:

#if __cpp_lib_transparent_operators >= 201510
using generic_owner_less = std::owner_less<>;
#else
struct generic_owner_less {
  template<typename T, typename U>
    bool
    operator()(const std::shared_ptr<T>& p1, const std::shared_ptr<U>& p2) const
    { return p1.owner_before(p2); }
};
#endif