Thread: Iterator quandary

I am trying to implement a reasonable iterator interface, but the STL has me a little confused. Let's say we have defined the following (somewhat nonsensical) function:

Code:

#include <cstdlib>

template <typename ForwardIterator>
ForwardIterator write_char(int ch, std::size_t count, ForwardIterator next, ForwardIterator eof) {
  while (count--) {
    if (next == eof)
      return ForwardIterator();
    *next++ = ch;
  }
  return next;
}

The iteration policy seems pretty reasonable to me. Compare with operator ==, dereference once, and return the updated position. In the case where iterator "hits the wall", return a default-initialized instance of the iterator type.

Then we invoke it like this:

Code:

#include <iostream>

using namespace std;

int main() {
  char buf[5] = {0};
  char* end = buf + sizeof(buf);
  char* pos = write_char('1', 3, buf, end);
  cout << "buf: '" << buf << "' (" << (pos - buf) << " chars written)" << endl;
  char* uho = write_char('!', 3, pos, end);
  if (uho == NULL)
    cout << "Error: buffer is full!" << endl;
}

So far, so good...or so I thought. Let's try it with "std::back_inserter":

Code:

#include <iostream>
#include <string>
#include <iterator>

using namespace std;

int main() {
  string buf;
  auto pos = write_char('?', 3, back_inserter(buf), back_inserter());
}

Well that doesn't compile. There is no default constructor for "back_inserter" and you can't even compare them with each other either. I am obviously using the wrong STL object here. But what to use? Surely there is a way to put this all together into a generic interface?