make_map<int>(make_vector()) cannot be compiled

[skhrshin]

I'm about to make a boost::fusion::map<boost::fusion::pair<int, boost::fusion::vector<>>> object, but the following code won't be compiled.

#include <boost/fusion/include/make_map.hpp>
#include <boost/fusion/include/make_vector.hpp>

int main() {
    boost::fusion::make_map<int>(boost::fusion::make_vector());
}

You can try it here: https://wandbox.org/permlink/9t5X0yHFBW5eZ2xC

Since boost::fusion::map looked to be doing something special if passed data is sequence, I took a look at the document but could find nothing about it. What's wrong?

[djowel]

It looks like a bug with make_map. In the meantime, a workaround not using make_map, but directly using map. Something like:

#include <boost/fusion/include/make_map.hpp>
#include <boost/fusion/include/make_vector.hpp>
#include <string>

using namespace boost::fusion;

typedef map<
    pair<int, vector<>> >
map_type;

map_type m(make_pair<int>(vector<>()));

A PR is very welcome if you can investigate some more on fixing this.

[skhrshin]

Thanks for telling me a workaround! I could make my own make_map function using map, pair and it is working fine with make_vector.

The original make_map doesn't pass pair object to the ctor of map. That is the cause of this issue where any object whose traits::is_sequence<T>::value will be true cannot be passed. Modifying arg to fusion::pair<Key, typename detail::as_fusion_element<T>::type>(arg) will fix it, but now I wonder why arg is lvalue reference rather than forwarding reference. Maybe it's related with some compatibility issues? I haven't understood much enough to make a PR for it yet.

[djowel]

I wonder why arg is lvalue reference rather than forwarding reference

Well, actually fusion predates c++11. There are still some things need to be reworked, but care must be taken because fusion still has to support older compilers, although I am starting to think that maybe it's time to move on.