On Tue, Jul 1, 2008 at 6:57 PM, Eric Niebler <eric@boost-consulting.com> wrote:
David Abrahams wrote:
Eric Niebler wrote:
David Abrahams wrote:
I don't suppose it's possible to forward lvalues and const rvalues as lvalues, and mutable rvalues as rvalues? I haven't thought of a way, but that would be as close to "perfect" as we might expect to get in C++0x and it would be close enough for me :-)
We would need 2^N overloads,
That's OK for small N
boost::function and phoenix have argument limits of 10 by default. 2^10 is a big number. Can't wait for variadics and rvalue refs.
I use a wrapper that does perfect forwarding up to 5 arguments (and all ref/all const ref for the remaining 10). I never needed more than that and I do not think it has an important contribution to compile time (that is, heavy lambda usage will kill compile time way more than that).
and it would be unsound because it would incorrectly forward const rvalues as lvalues.
Can you show me an example of a "sound" function that treats a const rvalue differently from an lvalue, please?
fusion::make_vector(), for instance. It returns a fusion::vector<> where the members can be held by value or by reference. Getting it wrong can lead to dangling references. Proto has similar function objects that need to be careful about the lifetime of temporaries.
IMHO you can ever safely hold on a parameter by reference unless you somehow explicitly decorate that parameter (with reference_wrapper for example). I do not see how brute force forwarding or C++0x perfect forwarding change anything. I should probably read again the thread referenced by David Abrahams -- gpd