----- Original Message ----- From: "Joel de Guzman" <joel@boost-consulting.com> To: <boost-users@lists.boost.org> Sent: Tuesday, January 06, 2009 11:26 PM Subject: Re: [Boost-users] [fusion] const and transform algorithm
vicente.botet wrote:
Hi,
What is the reason fusion::transform taskes a const Sequence? template< typename Sequence, typename F > typename result_of::transform<Sequence const, F>::type transform( Sequence const& seq, F f);
Does this means that the transform do not change the sequence itself o also its elements?
Yes. Fusion transform returns a lazy (transform) view. This view is non-mutating. It is not like STL's transform which mutates the target container in place. Fusion's is purely functional and lazily evaluated.
Can for_each mutate the sequence elements? if yes, would the following get a transformation I'm locking for? vector<shared_pair<shared_future<int>,int>,shared_pair<shared_future<char>,int> > t; boost::fusion::for_each(t, intrusive_get()); vector<int, char> res=unzip<1>(boost::fusion::for_each(t, intrusive_get())); where shared_pair would be a sequence something like a shared_ptr<pair<A,B> and unzip would be the reverse function of zip. BTW do you think that the counterpart function of zip has a place on Fusion?
boost::tuple<int, char> = ??(boost::fusion::transform(t, get())); boost::as_vector? Is there a as_tuple function? Should this force the evaluation of the get function?
As I said, Fusion transform is purely functional. The result of a transform is a view. The function will only be called when you iterate over the view. The transformation does not change/mutate the sequence itself. If that's what you want, you can assign the view back to the original container. For example:
vector<int, char> v; v = transform(v, f);
I was lock for in the documentation and I have not found this assignation. Where in the documentation can I found that I can do that? If for_each can change/mutate the sequence itself, why don't have a mutating_transform that takes a Sequence& and not a Sequence const& Just a last question. How move semantics maps with purely functional Sequences? Do you plan to add move semantics for the elements of the tuple? The concept of MovableSequence has a sens for you? Thanks, Vicente