Hi all,
I am writing some code which uses MPL and fusion together, and in
general I am happy with their interoperability. However, I have
stumbled on a few areas in which functionality I expected was missing,
or where expressions I expected to work did not. All of these issues
have workarounds, but I'd appreciate some input on whether the problem
is with the libraries, or with my expectations.
1. mpl::pair and fusion::pair
Fusion sequences are conforming MPL sequences, and MPL sequences can
be transmuted into fusion sequences. However, the same is not true of
their pair classes. I feel that fusion::pair ought to conform to some
"MPL pair" concept such that:
- I can access its types by the same 'first' and 'second' members as
in mpl::pair
- mpl::first and mpl::second metafunctions should work against it
For instance, I'd like to be able to extract a sequence of key types from a map:
// OK!
typedef mpl::map, mpl::pair > m1;
typedef mpl::transform<
m1,
mpl::firstmpl::placeholders::_1,
mpl::back_inserter > >::type keys1;
// Not OK!
typedef fusion::map, fusion::pair > m2;
typedef mpl::transform<
m2,
mpl::firstmpl::placeholders::_1,
mpl::back_inserter > >::type keys2;
// OK, but requires different code for mpl and fusion maps
typedef mpl::transform<
m2,
fusion::result_of::firstmpl::placeholders::_1,
mpl::back_inserter > >::type keys3;
Also, I feel there should be a canned way to translate mpl::pair into
fusion::pair. fusion::result_of::as_pair?
2. mpl::map and fusion::map
This code works nicely:
typedef mpl::vector v1;
typedef fusion::result_of::as_vector<v1>::type v2;
v2 v(1, 'a', false);
However, this does not:
typedef mpl::map, mpl::pair > m1;
typedef fusion::result_of::as_map<m1>::type m2;
m2 m('a', 3);
The type 'm2' ends up as fusion::map,
mpl::pair >, which clearly is no good. I feel that
as_map should automatically translate mpl::pair to fusion::pair so
that the result is a usable container.
Please let me know if I am barking up the wrong tree, or whether these
seem like useful features to anyone else. If there is consensus that
they are useful, I would be interested in developing patches for them.
Thanks,
-Gabe
