boost::range::join for multiple ranges

Question

I want to do the following:

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

for(auto&& i : join         


        

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.