Streambuf extending C++

MyStreamBuf.cpp

#include <streambuf>
#include <array>
#include <algorithm>
#include <iostream>
#include <sstream>

template <typename CharT>
class MyStreamBuf : public std::basic_streambuf<CharT> {
  public:
  using char_type = CharT;
  using traits_type = typename std::basic_streambuf<CharT>::traits_type;
  using int_type = typename traits_type::int_type;

  MyStreamBuf() {
    setg(&output_buf_.front(), &output_buf_.front(), &output_buf_.front());
  }

  void str(const std::string& s) {
    if (s.size() < output_buf_.size()) {
      std::copy(s.cbegin(), s.cend(), output_buf_.begin());
      setg(&output_buf_.front(), &output_buf_.front(), &output_buf_.front() + s.size());
    }
    else {
      std::copy_n(s.cbegin(), output_buf_.size(), output_buf_.begin());
      setg(&output_buf_.front(), &output_buf_.front(), &output_buf_.back());
    }
  }

  protected:
  int_type underflow() {
    std::cout << "MyStreamBuf underflow!" << std::endl;
    return traits_type::eof();
  }

  private:
  std::array<char_type, 128> output_buf_;
};

using CMyStreamBuf = MyStreamBuf<char>;

int main() {
  CMyStreamBuf msb;
  const std::string s{ "Hello" };

  msb.str(s);
  for (std::size_t i = 0; i < s.size() + 1; ++i) {
    std::cout << static_cast<char> (msb.sbumpc()) << std::endl;
  }

  for (std::size_t i = 0; i < s.size(); ++i) {
    std::cout << static_cast<char> (msb.sbumpc()) << std::endl;
  }

  return 0;
}