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Subject: [boost] [Fusion] port to c++0x
From: Christopher Schmidt (mr.chr.schmidt_at_[hidden])
Date: 2009-09-22 18:40:56

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)
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

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
-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
-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
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

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