On 6 October 2017 at 01:04, Richard Smith <rich...@metafoo.co.uk> wrote: >> One of us is missing something. Why does that not work with >> the non-guaranteed-copy-elision implementation? > > I can't write > NoCopyNorMove x = factory(); > in C++14. It's ill-formed. Same goes for > auto x = factory(); > > > The macro does not help you with that. It does not magically make a C++17 > feature appear in C++14 mode.
I can avoid compiling that when I know it won't work, for which I can use the macro. I can compile that when I know it'll work, for which the macro tells me that it'll work. > > >> >> Furthermore, if I decide that I really >> hate having to use those references, I will disable the whole type if >> guaranteed copy elision is not available. >> >> >> Do you anticipate actually wanting to do that? It seems like an odd choice >> to permit the reference declaration only if guaranteed copy elision is >> available. > > The choice is that rather than performing tricks with having to > befriend every possible factory function > and forcing users to use lifetime-extending references, such RAII > handles are provided only when copy > elision is available, and otherwise completely different tools need to be > used. > > > I think we're still missing an example of that. That would be wrapping the whole type in #ifdef __cpp_mandatory_copy_elision, and providing a different type that doesn't rely on copy elision, like a type that is movable and has a different name. _______________________________________________ Features mailing list Features@isocpp.open-std.org http://www.open-std.org/mailman/listinfo/features
