[Boost-users] Re: bind (excessive?) argument copying

Hi Peter, Peter Dimov wrote: <snip>

...

Four copies - I had hoped to only see a single copy. Logically, there doesn't seem to be a need for more than one copy. Is there an implementation issue that requires this many copies?

Not really, but eliminating the unnecessary copies is (relatively) hard, at least with the current code base (it has been developed with portability in mind and supports VC6, among others). The specification requires one copy. One more is almost unavoidable. The other two can be eliminated, in theory. But...

Ok, thanks. Out of interest, is there a simple explanation for why one more is almost unavoidable. <snip>

... please note that the copies occur at bind time. In a typical scenario you bind once and call O(N) times (or bind and store in a function<> with the associated dynamic memory allocation), so this isn't usually a problem. And, as others have pointed out, there's always ref(). ;-)

Yes I understand. I guess I'm not using bind typically ;-) FYI and for those suggesting boost::ref, the actual case is using bind for message passing between threads. I have a polymorphic wrapper for bind something like this: class bind_wrapper { public: virtual void execute() = 0; }; template <typename binder_t> class bind_wrapper_impl : public bind_wrapper { public: bind_wrapper_impl(const binder_t &binder_a) : binder_m(binder_a) { } virtual void execute() { binder_m(); } private: binder_t binder_m; }; template <typename binder_t> std::auto_ptr make_bind_wrapper(const binder_t &binder_a) { return std::auto_ptr( new bind_wrapper_impl(binder_a)); } Now the idea is that certain functions transparently transfer work to a worker thread. So for such a function "foo", there would be an internal implementation function: void foo_impl(const std::string &string_a) { // do stuff } and a public proxy that requests foo_impl to be called on the worker thread: future foo_async(const std::string &arg_a) { std::auto_ptr message_l( make_bind_wrapper(boost::bind(&foo_impl, arg_a))); // add message_l to message queue and return future to allow // rendezvous ... } The worker thread will collect this message and call execute(), something like: void worker_thread() { // retrieve from message queue and execute std::auto_ptr message_l = ... message_l->execute(); } So, in this case: 1. Yes, a copy is required - boost::ref is not appropriate. 2. Each bind corresponds to a single call (not O(N) calls) We have a home grown solution (less general than bind of course) requiring only a single copy. I was hoping to replace it with bind but it's hard to justify adding the extra argument copies. Thanks for the info though, Cheers, Trent.