[Boost-users] [Boost.Range] combine, native range-for, and destructuring

Subject: [Boost-users] [Boost.Range] combine, native range-for, and destructuring
From: John M. Dlugosz (ngnr63q02_at_[hidden])
Date: 2018-04-14 10:34:30

I want to go through two vectors at the same time, and the way to do this is to "zip" the 
items from each into a tuple.  boost::combine is a function for that.

     for (auto&[def,residue] : boost::combine(playdef, residues)) {

gives me an error

1>e:\scratch\c++code\scratchproject2\fizzbuzz\fizzbuzz.cpp(66): error C2440: 
'initializing': cannot convert from

'boost::tuples::cons<std::pair<int,std::string_view> &,boost::tuples::cons<int 
&,boost::fusion::detail::build_tuple_cons<boost::fusion::single_view_iterator<Sequence,boost::mpl::int_<1>>,Last,true>::type>>' 
to

'boost::tuples::cons<std::pair<int,std::string_view> &,boost::tuples::cons<int 
&,boost::fusion::detail::build_tuple_cons<boost::fusion::single_view_iterator<Sequence,boost::mpl::int_<1>>,Last,true>::type>> 
&'

The only difference is in the final `&` character.

Trying to explore the behavior in pieces, I try:

     for (auto row : boost::combine(playdef, residues)) {
             auto&[def, residue] = row;  // line 1
             row.get<1>() = 17;  // line 2
             residue= 42;  // line 3

Now line2 does work, and I note that assignment goes through and updates the original 
`residues` vector.

The destructuring on line 1 does compile now, being on a separate line.
But line 3 gives me an error about not being able to assign to a crazy complicated type.

What's going on?  Is Boost.Range zipped ranges simply not compatible with the native 
language features?