Joel de Guzman wrote:
David Abrahams wrote:
Joel de Guzman <joel@boost-consulting.com> writes:
In the extension example, I see repeatedly the same patter transferring the cv-ref qualification from one type to another. Can't that be made simpler for extenders?
typedef typename mpl::if_< is_const<Sequence>, std::string const&, std::string&>::type type; Can you point me to an example in the library, please?
This isn't from the library code. I copied it directly from the docs at libs/fusion/doc/html/fusion/extension.html
Ah, ok. I'll let Dan reply then. He wrote that part of the docs.
There is a common theme here when you need to return refs into the container (from deref, at, at_key etc.) you need to const qualify the result if the container is const. There is similiar logic in the library proper, for example in the vector deref_impl we have: typedef typename mpl::eval_if< is_const<vector> , fusion::detail::cref_result<element> , fusion::detail::ref_result<element> >::type type; The ref/cref generation is currently in detail, so library private implementation, and the is_const switch is repeated as with my example. So yes this is a recurring theme, we could provide a utility to do the necessary, but that does not seem to be a feature specific to fusion (as opposed to any other lib that needs to make similar decisions, I believe there is similar code in phoenix for example). Possibly we could make this decision at a higher level, in deref proper for example, but I'm not sure the logic is appropriate in all cases, such as transform views, were it may be appropriate to return non-const refs from a deref of a const container. Cheers Dan