On Mon, 14 Jan 2019 at 20:51, John Spicer <j...@edg.com> wrote: > I agree with your conclusion. > > A question came up recently about whether nested inline namespaces > (P1094R2) needed a feature test macro. In the CWG discussion Alisdair > said there was intentionally no feature test macro. > > It might be helpful for proposals to that do not have a feature test macro > to say so explicitly so that we know whether it was by design or an > oversight. > > It might also be useful to write up the "if you have to write the alternative code anyway, you might as well just do that" principle in SD-6, so we can point people to it.
Assign that task to me if you like, although I might not be able to get around to it for a while. > John. > > > On Jan 11, 2019, at 10:31 AM, Jonathan Wakely <c...@kayari.org> wrote: > > > > A colleague queried the absence of a feature-test macro for > > https://wg21.link/p0458 "Checking for Existence of an Element in > > Associative Containers". > > > > I couldn't find any record of discussion in LWG review or on this > > mailing list to indicate whether the omission was intentional or an > > oversight. > > > > IMHO there's not much need for a macro, based on the "if you have to > > write the alternative code anyway, you might as well just do that" > > principle. i.e. the macro allows you to write: > > > > #if __cpp_lib_assoc_contains > > if (cont.contains(key)) > > #else > > if (cont.find(key) != cont.end()) > > #endif > > { > > // ... > > } > > > > But the performance characteristics of the two forms are identical, so > > checking the macro just obfuscates the code. If you need to support > > old compilers, just write the code the old way. > > > > The new contains member is more efficient than a badly-written check > > for multimaps and multisets that uses "count": > > > > #if __cpp_lib_assoc_contains > > if (cont.contains(key)) > > #else > > if (cont.count(key) != 0) > > #endif > > { > > // ... > > } > > > > But this code should be fixed anyway. > > > > Does SG10 agree that we don't need a macro for p0458? > > _______________________________________________ > > Features mailing list > > Features@isocpp.open-std.org > > http://www.open-std.org/mailman/listinfo/features > >
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