c++ - boost::range::join for multiple ranges

Question

I want to do the following:

std::vector<int> a = {1,2,3}, b = {4,5,6}, c = {7,8,9};

for(auto&& i : join(a,b,c)) {
  i += 1
  std::cout << i;  // -> 2345678910
}

I tried using boost::range::join, this works fine:

auto r = boost::join(a,b);
for(auto&& i : boost::join(r,c)) {
  i += 1;
  std::cout << i;  // -> 2345678910
}

Chaining joins, reading operations work:

for(auto&& i : boost::join(boost::join(a,b),c))
  std::cout << i;  // -> 123456789

However, writing doesn't work:

for(auto&& i : boost::join(boost::join(a,b),c)) {
  i += 1; // Fails  :(
  std::cout << i;  
}

My variadic join has the same problem, i.e. works for reading but not for writing:

template<class C> C&& join(C&& c) { return c; }

template<class C, class D, class... Args>
auto join(C&& c, D&& d, Args&&... args)
-> decltype(boost::join(boost::join(std::forward<C>(c), std::forward<D>(d)),
                     join(std::forward<Args>(args)...))) {
return boost::join(boost::join(std::forward<C>(c), std::forward<D>(d)),
                     join(std::forward<Args>(args)...));
}

Mehrdad gave the solution in the comments

template<class C>
auto join(C&& c)
-> decltype(boost::make_iterator_range(std::begin(c),std::end(c))) {
return boost::make_iterator_range(std::begin(c),std::end(c));
}

template<class C, class D, class... Args>
auto join(C&& c, D&& d, Args&&... args)
-> decltype(boost::join(boost::join(boost::make_iterator_range(std::begin(c),std::end(c)),
                                 boost::make_iterator_range(std::begin(d),std::end(d))),
                     join(std::forward<Args>(args)...))) {
  return boost::join(boost::join(boost::make_iterator_range(std::begin(c),std::end(c)),
                                 boost::make_iterator_range(std::begin(d),std::end(d))),
                     join(std::forward<Args>(args)...));
}

Answer 1

There are two overloads of boost::join

template<typename SinglePassRange1, typename SinglePassRange2>
joined_range<const SinglePassRange1, const SinglePassRange2>
join(const SinglePassRange1& rng1, const SinglePassRange2& rng2)

template<typename SinglePassRange1, typename SinglePassRange2>
joined_range<SinglePassRange1, SinglePassRange2>
join(SinglePassRange1& rng1, SinglePassRange2& rng2);

When you do this

for(auto&& i : boost::join(boost::join(a,b), c)) {
           //  ^^^^        ^^^^ temporary here
           //   ||
           //  calls the const ref overload

You get a temporary joined_range and as those can only bind to const references, the first overload is selected which returns a range that doesn't allow modifying.

You can work around this if you avoid temporaries:

#include <boost/range.hpp>
#include <boost/range/join.hpp>

int main()
{
    std::vector<int> a = {1,2,3}, b = {4,5,6}, c = {7,8,9};
    auto range = boost::join(a,b);

    for(int& i : boost::join(range,c)) {
        i += 1;
        std::cout << i;
    }
}

Live demo.

I haven't looked into your variadic functions, but the problem is likely similar.