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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::tuples::cons<std::pair<int,std::string_view> &,boost::tuples::cons<int

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?

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