Why is std::back_inserter_iterator not WeaklyIncrementable in RangeV3?

Question

To my surprise this Concept-like assertion fails in RangeV3.

#include
#include
int main(){
         


        

Answer 1

It looks like there are some unexpected (to me) requirements need by ranges::copy. So RangesV3 provides a drop in replacement ranges::back_inserter that works.

However there are many other iterators in the standard that do not work for the same reason but to which there is no drop-in replacements, so it can get ugly.

For example, I had to adapt a new iterator to replace std::ostream_iterator creating some artificial functions, including a default constructor:

template
struct ranges_ostream_iterator : std::ostream_iterator{
    using std::ostream_iterator::ostream_iterator;
    ranges_ostream_iterator() : std::ostream_iterator{std::cout}{} // I have to put something here
    ranges_ostream_iterator& operator++(){std::ostream_iterator::operator++(); return *this;}
    ranges_ostream_iterator& operator++(int){return operator++();}      
    using difference_type = int;
    int operator-(ranges_ostream_iterator const&){return 0;}
};

With this, ranges::copy(first, last, ranges_ostream_iterator(std::cout)) work, whereas ranges::copy(first, last, std::ostream_iterator(std::cout)) doesn't.