"Paul Mensonides" <pmenso57@comcast.net> writes:
[mailto:boost-bounces@lists.boost.org] On Behalf Of Larry Evans
Regarding what the generated code is doing (as opposed to what the generator code is doing), the major problem with forwarding without language support is that it is a combinatorial problem--rather than linear. I.e. you really need various combinations of const and-or [snip]
Ouch! Do you know of anyone working on a compiler to solve this forwarding problem?
I know this is one of the things being tossed around in the committee, but I don't know the likelihood that of acceptance or if it has already been accepted. Dave would know more.
It's solved by rvalue references, which, IIRC, are in a late stage of review for inclusion into the next standard. http://open-std.org/JTC1/SC22/WG21/docs/papers/2006/n2027.html
One problem of forwarding is arity (which can be more-or-less solved with code generation). The more stubborn problem with forwarding is temporaries. They cannot be bound to a non-const reference, and the template mechanism cannot deduce T as const U in a parameter of type T&. OTOH, non-temporaries don't have any problem deducing T as U or const U in a parameter of type T&. Unfortunately, the only way to handle it transparently is to overload via cv-qualifier--which leads the the combinatorial explosion when multiple parameters are involved.
Strictly speaking, it's not even transparent if you use the combinatorial explosion because you lose rvalue-ness, which is detectable today (at least for mutable rvalues, which are the ones that usually matter). You will also probably force the compiler to make more copies than it would if you were passing parameters by value in some cases, which is sorta the same issue. -- Dave Abrahams Boost Consulting www.boost-consulting.com