I participated in Google's 'Summer of Code' for Boost this year. My project was to port Boost.Fusion to c++0x. The program is over now and I would like to give you a short report on my work and put my code up for comments. In a nutshell, there is rvalue support, native variadic templates are used if possible and the public classes and (meta-) functions are guarded by a set of static asserts that detect most invalid (template) arguments. I also added certain new functionality to provide an easier, faster and more intuitive programming interface. My implementation is pretty much backwards compatible and all test cases pass on gcc 3.4/4.4 and vc 9. Here is a short overview of the most striking changes besides the implementation of raw c++0x support. I removed the deque container and the extension-classes. The deque container is undocumented and, in my opinion, does not go well with the other container due to its intrinsic extension mechanism. There also should not be a performance gain compared to a regular fusion::joint_view in combination with two fusion::vector. The extension classes are pretty much Boost.Proto specific. At the moment they seem to be out of place, especially as they are undocumented, unmaintained and not even part of the regular file structure. I would like to see them either fully documented (with the underlying concept of segmented sequences) or in the detail namespace of Boost.Proto. I added support for associative iterators. An associative forward sequence shall provide an iterator type which implements the extension backend of the intrinsic (meta-)functions fusion::result_of::value_of_data, fusion::deref_data and fusion::result_of::key_of. This has the advantage that all view classes, even fusion::iterator_range, are able to adapt the underlying sequence's associative-ness. fusion::transform_view, as a special case, got an additional template parameter. This parameter specifies whether the view should model either the forward concept only or the associative and the forward concept. This design breaks consistency with Boost.MPL. Unfortunately these changes are crucial and I would rather risk this break with Boost.MPL than missing that important feature. Using the old implementation, there is no way to avoid explicitly converting from a view back to an associative sequence to preserve the associative-ness. This leads to a nasty runtime overhead due to evaluation of every element of the view and copy-/move-constructing it. Besides, other changes include: -If the compiler supports rvalue references, all fusion container have a template constructor (taking either const lvalue or 'templated' rvalue references). -If the compiler supports variadic templates, fusion::cons, fusion::pair and fusion::single_view have a variadic template constructor (taking either const lvalue or 'templated' rvalue references) that allows emplace construction. -The view classes adapt the full category of their underlying sequences. As mentioned above, the associative-ness is preserved, but also, if possible, the random-access or bidirectional trait. -The result_of::-metafunctions accept r- and lvalue references arguments. If an argument is not reference-qualified, it will be considered to be an lvalue reference. All existing extension implementations must be changed to adapt this new behaviour. -The algorithm functions now also take (and correctly forward) non-const reference arguments. -Fusion iterators (with reference compatible sequence types and a very same index) and views (with reference compatible sequence types) have a working operator= that correctly reassigns the underlying sequence references. -Fusion now also adapts MPL iterators. If, and only if, the <boost/fusion/adapted/mpl.hpp>-header is included, mpl sequences and iterators are recognized as fusion sequences and iterators. -There are new headers to adapt std::(tr1::)array or std::(tr1::)tuple. The overall compile-time performance of my code is satisfying. Compiling a compilation unit that includes all possible fusion headers takes on gcc 4.4 slightly more time than the official fusion implementation. If native variadic templates are present, the compilation is a lot faster (usually up to 3 to 6 times), regardless of the value of FUSION_MAX_xxx_SIZE. The compile-time performance of actual fusion code is mixed. The intrinsic container and iterator (meta-)functions compile a little bit slower compared to the official fusion implementation. This is due to explicit removal of the references on the arguments of all result_of::metafunctions and some additional abstraction layer. If the compiler support rvalue references, there has to be a rather ugly and slow type evaluation/transformation to avoid issues with rvalue references on non-array, non-class types (see /support/internal/ref.hpp and 8.5.3p5 of the current c++0x draft for more details). On the other hand, I added several optimizations (such as result preevaluation), and, if variadic templates are present, the actual instantiation of fusion containers is a lot faster. All in all, the compile-time performance is a little bit worse if c++0x features are not present. If c++0x features are present, the code should compile a little bit faster in most cases. For example, the lambda example of the Boost.Proto documentation, which internally heavily relies on Boost.Fusion, takes on my machine 2.6 seconds to compile with the 'old' fusion on gcc 4.4, 3 seconds with my port without -std=c++0x, and 2.1 seconds with c++0x features being present. The overall performance might be even better once Boost.MPL adapts native variadic templates and the compiler supports template aliases. There have been tons of other changes, mostly bugfixes, optimizations (in terms of compile time of course) and removals of redundancy. Whenever I had the choice, I tried to prefer clean, redundancy-free instead of heavily optimized code. The overall performance loss is not noticeable though, and most compile-time optimizations that go beyond the as-little-template-instantiations-as-possible paradigm are pretty much compiler dependent anyway. There is still some work to do. I did not add c++0x-specific test cases and there are several parts of the documentation which need to be updated. By the way, are there any official guidelines for documenting c++03/c++0x-specific differences? For now, I would like to hold off on implementing these changes until Boost.TypeTraits adapts c++0x. Right now I implemented missing or incorrectly behaving trait functions in the fusion::detail-namespace. I would rather like to use the official ones, at least in the documentation and in the test cases. Besides that, I think my implementation is trunk-ready. If there are no objections, I would like to finish the outstanding work as soon as possible, and then merge my code with the trunk. Of course I will provide long-term support for the project. In fact, I would love to become an official fusion maintainer. You can find the code in the boost sandbox (/sandbox/SOC/2009/fusion). If you test the code with a compiler that has decltype support, please use the HEAD of the trunk rather than the official 1.40 distribution package to compile your code. Best regards Christopher