Using back_insert_iterator in C++

c++, istream, iterator

Solution

You don't want to implement the function

std::istream& readResults(
    std::istream& is,
    std::back_insert_iterator<std::vector<Result>> insertIt)

What you want to do is instead implement the template function

template<class InsertIterator>
std::istream& readResults(std::istream& is, InsertIterator insertIt)

When you pass `std::back_inserter(vec)`, the template will automatically be instantiated to the right type.

The code for this function will essentially be

template<class InsertIterator>
std::istream& readResults(std::istream& is, InsertIterator insertIt) {
  std::string studentId;
  size_t grade;
  while (is >> studentId >> grade) {
    Result new_result = {studentId, grade};
    *(insertIt++) = new_result;
  }
}

`std::copy` copies from one iterator to another. But you're not copying from an iterator. You're inserting elements one at a time, and you do this by dereferencing the iterator, as though it's a pointer, and then incrementing it when you're done.

Problem

Original source